SC16C850IET ,2.5 V to 3.3 V UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA), and 16 mode or 68 mode parallel bus interfaceFeatures and benefits Single channel high performance UART Intel or Motorola bus interface select ..
SC16C850IET ,2.5 V to 3.3 V UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA), and 16 mode or 68 mode parallel bus interfaceGeneral descriptionThe SC16C850 is a 2.5 V to 3.3 V, low power, single channel Universal Asynchrono ..
SC16C850LIBS ,1.8 V single UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA) and 16 mode or 68 mode parallel bus interfaceFeatures and benefits Single channel high performance UART Intel or Motorola bus interface select ..
SC16C850LIET ,1.8 V single UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA) and 16 mode or 68 mode parallel bus interfaceGeneral descriptionThe SC16C850L is a 1.8 V, low power, single channel Universal Asynchronous Recei ..
SC16C850SVIBS ,1.8 V single UART, 20 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA), and XScale VLIO bus interfaceFeatures and benefits Single channel high performance UART 1.8 V operation Advanced package: HVQ ..
SC16C850VIBS ,1.8 V single UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA), and XScale VLIO bus interfaceFeatures and benefits Single channel high performance UART 1.8 V operation Advanced package: HVQ ..
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SC16C850IBS-SC16C850IET
2.5 V to 3.3 V UART, 5 Mbit/s (max.) with 128-byte FIFOs, infrared (IrDA), and 16 mode or 68 mode parallel bus interface
1. General descriptionThe SC16C850 is a 2.5 V to 3.3 V, low power, single channel Universal Asynchronous
Receiver and Transmitter (UART) used for serial data communications. Its principal
function is to convert parallel data into serial data and vice versa. The UART can handle
serial data rates up to 5 Mbit/s. The SC16C850 is functionally (software) compatible with
the SC16C650B. SC16C850 can be programmed to operate in extended mode (see
Section 6.2) where additional advanced UART features are available. The SC16C850
UART provides enhanced UART functions with 128-byte FIFOs, modem control interface,
and IrDA encoder/decoder. On-board status registers provide the user with error
indications and operational status. System interrupts and modem control features may be
tailored by software to meet specific user requirements. An internal loopback capability
allows on-board diagnostics.
The SC16C850IBS with Intel (16 mode) or Motorola (68 mode) bus host interface
operates at 2.5 V to 3.3 V and is available in a very small (Micro-UART) HVQFN32
package.
The SC16C850IET with Intel (16 mode) bus host interface operates at 2.5 V to 3.3 V and
is available in a very small TFBGA36 package.
2. Features and benefits Single channel high performance UART Intel or Motorola bus interface selectable using 16/68 pin 2.5 V to 3.3 V operation Up to 5 Mbit/s data rate 128-byte transmit FIFO to reduce the bandwidth requirement of the external CPU 128-byte receive FIFO with error flags to reduce the bandwidth requirement of the
external CPU 128 programmable Receive and Transmit FIFO interrupt trigger levels 128 Receive and Transmit FIFO reporting levels (level counters) Automatic software (Xon/Xoff) and hardware (RTS/CTS or DTR/DSR) flow control Industrial temperature range (−40 °C to +85 °C) 128 hardware and software trigger levels Automatic 9-bit mode (RS-485) address detection Automatic RS-485 driver turn-around with programmable delay UART software reset High resolution clock prescaler, from 0 to 15 with granularity of 1 ⁄16 to allow
non-standard UART clock to be used
SC16C850
2.5 V to 3.3 V UART, 5 Mbit/s (max.) with 128-byte FIFOs,
infrared (IrDA), and 16 mode or 68 mode parallel bus interface
Rev. 2 — 11 November 2010 Product data sheet
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder Programmable Xon/Xoff characters Software selectable baud rate generator Support IrDA version 1.0 (up to 115.2 kbit/s) Standard modem interface or infrared IrDA encoder/decoder interface Enhanced Sleep mode and low power feature Modem control functions (CTS, RTS, DSR, DTR, RI, CD) Independent transmitter and receiver enable/disable Pb-free, RoHS compliant package offered
3. Ordering information[1] SC16C850IBS/Q900 is AEC-Q100 compliant. Contact i2c.support for PPAP.
Table 1. Ordering informationSC16C850IBS HVQFN32 plastic thermal enhanced very thin quad flat package; no leads; terminals; body 5×5× 0.85 mm
SOT617-1
SC16C850IBS/Q900 HVQFN32 plastic thermal enhanced very thin quad flat package; no leads; terminals; body 5×5× 0.85 mm
SOT617-1
SC16C850IET TFBGA36 plastic thin fine-pitch ball grid array package; 36 balls;
body 3.5×3.5× 0.8 mm
SOT912-1
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
4. Block diagramNXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
5. Pinning information
5.1 PinningNXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
5.2 Pin description
Table 2. Pin description16/68 -2 I
Bus select. Intel or Motorola bus select.When 16/68 pin is at logic 1 or left unconnected (internally pulled-up) the
device will operate in Intel bus (16 mode) type of interface.
When 16/68 pin is at logic 0, the device will operate in Motorola bus (68 mode)
type of interface. C1 19 I
Address 0 select bit. Internal register address selection. C3 18 I
Address 1 select bit. Internal register address selection. B1 17 I
Address 2 select bit. Internal register address selection. E3 26 I
Carrier Detect (active LOW). A logic 0 on this pin indicates that a carrier has
been detected by the modem. Status can be tested by reading MSR[7]. B6 8 I
Chip Select (active LOW). In 16 mode or 68 mode, this input is chip select for
the UART.
CTS D3 24 I
Clear to Send (active LOW). A logic 0 on the CTS pin indicates the modem
or data set is ready to accept transmit data from the SC16C850. Status can be
tested by reading MSR[4].
DSR F2 25 I
Data Set Ready (active LOW). A logic 0 on this pin indicates the modem or
data set is powered-on and is ready for data exchange with the UART. Status
can be tested by reading MSR[5].
DTR E1 22 O
Data Terminal Ready (active LOW). A logic 0 on this pin indicates that the
SC16C850 is powered-on and ready. This pin can be controlled via the
modem control register. Writing a logic 1 to MCR[0] will set the DTR output to
logic 0, enabling the modem. This pin will be a logic 1 after writing a logic 0 to
MCR[0], or after a reset.
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder F4 29 I/O
Data bus (bidirectional). These pins are the 8-bit, 3-state data bus for transferring information to or from the controlling CPU. D0 is the least
significant bit and the first data bit in a transmit or receive serial data stream.D1 E4 30 I/O F5 31 I/O E5 32 I/O F6 1 I/O E6 3 I/O D6 4 I/O D5 5 I/O
INT
(IRQ) 20 O When 16/68 pin is at logic 1 or unconnected, this output becomes active HIGH
interrupt output. The output state is defined by the user through the software
setting of MCR[5]. INT is set to the active mode when MCR[5] is set to a
logic 1. INT is set to the open-source mode when MCR[5] is set to a logic0.
When 16/68 pin is at logic 0, this output becomes device interrupt output
(active LOW, open-drain). An external pull-up resistor to VDD is required.
INT D1 - O
Interrupt output (active HIGH). The output state is defined by the user through the software setting of MCR[5]. INT is set to the active mode when
MCR[5] is set to a logic 1. INT is set to the open-source mode when MCR[5] is
set to a logic0.
IOR
(VDD)
-14 I When 16/68 pin is at logic 1, this input becomes the read strobe (active LOW).
When 16/68 pin is at logic 0, this input pin is not used and should be
connected to VDD.
IOR A3 - I
Read strobe (active LOW). IOW
(R/W) 12 I When 16/68 pin is at logic 1 or unconnected, this input becomes the write
strobe (active LOW).
When 16/68 pin is at logic 0, this input becomes read strobe when it is at logic
HIGH, and write strobe when it is at logic LOW.
IOW B4 - I
Write strobe (active LOW). LOWPWR B5 9 I
Low Power. When asserted (active HIGH), the device immediately goes into low power mode. The oscillator is shut-off and some host interface pins are
isolated from the host’s bus to reduce power consumption. The device only
returns to normal mode when the LOWPWR pin is de-asserted. On the
negative edge of a de-asserting LOWPWR signal, the device is automatically
reset and all registers return to their default reset states. This pin has a 22 kΩ
internal pull-down resistor, therefore, it can be left unconnected (refer to
Section 6.12 “Low power feature”).
RESET
(RESET)
-23 I
Master Reset. When 16/68 pin is at logic 1 or unconnected, this input
becomes the RESET pin (active HIGH).
When 16/68 pin is at logic LOW, this input pin becomes RESET (active LOW).
(See Section 7.23 “SC16C850 external reset condition and software reset” for
initialization details.)
RESET F1 - I
Reset input (active HIGH). See Section 7.23 “SC16C850 external reset condition and software reset” for initialization details. F3 27 I
Ring Indicator (active LOW). A logic 0 on this pin indicates the modem has
received a ringing signal from the telephone line. A logic 1 transition on this
input pin will generate an interrupt if modem status interrupt is enabled. Status
can be tested by reading MCR[6].
Table 2. Pin description …continued
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder[1] HVQFN32 package die supply ground is connected to both VSS pin and exposed center pad. VSS pin must be connected to supply
ground for proper device operation. For enhanced thermal, electrical, and board level performance, the exposed pad needs to be
soldered to the board using a corresponding thermal pad on the board and for proper heat conduction through the board, thermal vias
need to be incorporated in the PCB in the thermal pad region.
RTS D2 21 O
Request to Send (active LOW). A logic 0 on the RTS pin indicates the
transmitter has data ready and waiting to send. Writing a logic 1 in the modem
control register MCR[1] will set this pin to a logic 0, indicating data is available.
After a reset this pin will be set to a logic1. C6 6 I
UART receive data. The RX signal will be a logic 1 during reset, idle (no
data), or when not receiving data. During the local loopback mode, the RX
input pin is disabled and TX data is connected to the UART RX input,
internally. C5 7 O
UART transmit data. The TX signal will be a logic 1 during reset, idle (no
data), or when the transmitter is disabled. During the local loopback mode, the
TX output pin is disabled and TX data is internally connected to the UART RX
input.
VDD A1, D4 28 I
Power supply input.VSS C2, C4 13[1] I
Signal and power ground.XTAL1 A6 10 I
Crystal or external clock input. Functions as a crystal input or as an external clock input. A crystal can be connected between this pin and XTAL2 to form an
internal oscillator circuit. Alternatively, an external clock can be connected to
this pin to provide custom data rates (see Section 6.9 “Programmable baud
rate generator”). See Figure8.
XTAL2 A5 11 O
Output of the crystal oscillator or buffered clock. (See also XTAL1.) Crystal oscillator output or buffered clock output. Should be left open if an
external clock is connected to XTAL1.
Table 2. Pin description …continued
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6. Functional descriptionThe SC16C850 provides serial asynchronous receive data synchronization,
parallel-to-serial and serial-to-parallel data conversions for both the transmitter and
receiver sections. These functions are necessary for converting the serial data stream into
parallel data that is required with digital data systems. Synchronization for the serial data
stream is accomplished by adding start and stop bits to the transmit data to form a data
character (character orientated protocol). Data integrity is ensured by attaching a parity bit
to the data character. The parity bit is checked by the receiver for any transmission bit
errors. The electronic circuitry to provide all these functions is fairly complex, especially
when manufactured on a single integrated silicon chip. The SC16C850 represents such
an integration with greatly enhanced features. The SC16C850 is fabricated with an
advanced CMOS process.
The SC16C850 is an upward solution to the SC16C650B that provides a single UART
capability with 128 bytes of transmit and receive FIFO memory, instead of 32 bytes for the
SC16C650B and 16 bytes in the SC16C550B. The SC16C850 is designed to work with
high speed modems and shared network environments that require fast data processing
time. Increased performance is realized in the SC16C850 by the transmit and receive
FIFOs. This allows the external processor to handle more networking tasks within a given
time. In addition, the four selectable receive and transmit FIFO trigger interrupt levels are
provided in 16C650 mode, or 128 programmable levels are provided in the extended
mode for maximum data throughput performance especially when operating in a
multi-channel environment (see Section 6.2 “Extended mode (128-byte FIFO)”). The FIFO
memory greatly reduces the bandwidth requirement of the external controlling CPU and
increases performance. A low power pin (LOWPWR) is provided to further reduce power
consumption by isolating the host bus interface.
The SC16C850 is capable of operation up to 5 Mbit/s with an external 80 MHz clock. With
a crystal, the SC16C850 is capable of operation up to 1.5 Mbit/s.
The rich feature set of the SC16C850 is available through internal registers. These
features are: selectable and programmable receive and transmit FIFO trigger levels,
selectable TX and RX baud rates, and modem interface controls, and are all standard
features. Following a power-on reset, an external reset, or a software reset, the
SC16C850 is software compatible with the previous generation, SC16C550B, and
SC16C650B.
6.1 UART selectionThe UART provides the user with the capability to bidirectionally transfer information
between an external CPU, the SC16C850 package, and an external serial device. A
logic 0 (LOW) on chip select pin CS allows the user to configure, send data, and/or
receive data via the UART. Refer to Table 3 and Table4.
Table 3. Serial port selection (Intel interface)H = HIGH; L = LOW.=H none=L UART select
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.2 Extended mode (128-byte FIFO)The device is in the extended mode when any of these four registers contains any value
other than 0: FLWCNTH, FLWCNTL, TXINTLVL, RXINTLVL.
6.3 Internal registersThe SC16C850 provides a set of 25 internal registers for monitoring and controlling the
functions of the UART. These registers are shown in Table5.
Table 4. Serial port selection (Motorola interface)H = HIGH; L = LOW.
CS = H none
CS = L UART select
Table 5. Internal registers decoding
General register set (THR/RHR, IER/ISR, MCR/MSR, FCR, LCR/LSR, EFCR, SPR)[1]000Receive Holding Register Transmit Holding Register
001Interrupt Enable Register Interrupt Enable Register
010Interrupt Status Register FIFO Control Register
011Line Control Register Line Control Register
100Modem Control Register Modem Control Register
101Line Status Register Extra Feature Control Register (EFCR)
110Modem Status Register n/a
111Scratchpad Register Scratchpad Register
Baud rate register set (DLL/DLM)[2]000LSB of Divisor Latch LSB of Divisor Latch
001MSB of Divisor Latch MSB of Divisor Latch
Second special register set (TXLVLCNT/RXLVLCNT)[3]011Transmit FIFO Level Count n/a
100Receive FIFO Level Count n/a
Enhanced feature register set (EFR, Xon1/Xon2, Xoff1/Xoff2)[4]010Enhanced Feature Register Enhanced Feature Register
100Xon1 word Xon1 word
101Xon2 word Xon2 word
110Xoff1 word Xoff1 word
111Xoff2 word Xoff2 word
First extra feature register set (TXINTLVL/RXINTLVL, FLWCNTH/FLWCNTL)[5]010Transmit FIFO Interrupt Level Transmit FIFO Interrupt Level
100Receive FIFO Interrupt Level Receive FIFO Interrupt Level
110Flow Control Count High Flow Control Count High
111Flow Control Count Low Flow Control Count Low
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder[1] These registers are accessible only when LCR[7] is a logic0.
[2] These registers are accessible only when LCR[7] is a logic1.
[3] Second Special registers are accessible only when EFCR[0]=1.
[4] Enhanced Feature Registers are only accessible when LCR= 0xBF.
[5] First Extra Feature Registers are only accessible when EFCR[2:1]= 01b.
[6] Second Extra Feature Registers are only accessible when EFCR[2:1]= 10b.
6.4 FIFO operation
6.4.1 32-byte FIFO modeWhen all four of these registers (TXINTLVL, RXINTLVL, FLWCNTH, FLWCNTL) in the
‘first extra feature register set’ are empty (0x00) the transmit and receive trigger levels are
set by FCR[7:4]. In this mode the transmit and receive trigger levels are backward
compatible to the SC16C650B (see Table 6), and the FIFO sizes are 32 entries. The
transmit and receive data FIFOs are enabled by the FIFO Control Register bit 0 (FCR[0]).
It should be noted that the user can set the transmit trigger levels by writing to the FCR,
but activation will not take place until EFR[4] is set to a logic 1. The receiver FIFO section
includes a time-out function to ensure data is delivered to the external CPU (see
Section 6.8). Please refer to Table 11 and Table 12 for the setting of FCR[7:4].
6.4.2 128-byte FIFO modeWhen either TXINTLVL, RXINTLVL, FLWCNTH or FLWCNTL in the ‘first extra feature
register set’ contains any value other than 0x00, the transmit and receive trigger levels are
set by TXINTLVL and RXINTLVL registers. TXINTLVL sets the trigger levels for the
transmit FIFO, and the transmit trigger levels can be set to any value between 1 and 128
with granularity of 1. RXINTLVL sets the trigger levels for the receive FIFO, the receive
trigger levels can be set to any value between 1 and 128 with granularity of 1.
When the effective FIFO size changes (that is, when FCR[0] toggles or when the
combined content of TXINTLVL, RXINTLVL, FLWCNTH and FLWCNTL changes between
equal and unequal to 0x00), both RX FIFO and TX FIFO will be reset (data in the FIFO will
be lost).
Second extra feature register set (CLKPRES, RS485TIME, AFCR2, AFCR1)[6] 1 0 Clock Prescaler Clock Prescaler
100RS-485 turn-around Timer RS-485 turn-around Timer
110Additional Feature Control Register 2 Additional Feature Control Register 2
111Additional Feature Control Register 1 Additional Feature Control Register 1
Table 5. Internal registers decoding …continued
Table 6. Interrupt trigger level and flow control mechanism 00 8 16 8 0 01 16 8 16 7 10 24 24 24 15 11 28 30 28 23
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.5 Hardware flow controlWhen automatic hardware flow control is enabled, the SC16C850 monitors the CTS pin
for a remote buffer overflow indication and controls the RTS pin for local buffer overflows.
Automatic hardware flow control is selected by setting EFR[6] (RTS) and EFR[7] (CTS) to
a logic 1. If CTS transitions from a logic 0 to a logic 1 indicating a flow control request,
ISR[5] will be set to a logic 1 (if enabled via IER[7:6]), and the SC16C850 will suspend TX
transmissions as soon as the stop bit of the character in process is shifted out.
Transmission is resumed after the CTS input returns to a logic 0, indicating more data
may be sent.
When AFCR1[2] is set to logic 1 then the function of CTS pin is mapped to the DSR pin,
and the function of RTS is mapped to DTR pin. DSR and DTR pins will behave as
described above for CTS and RTS.
With the automatic hardware flow control function enabled, an interrupt is generated when
the receive FIFO reaches the programmed trigger level. The RTS (or DTR) pin will not be
forced to a logic 1 (RTS off), until the receive FIFO reaches the next trigger level.
However, the RTS (or DTR) pin will return to a logic 0 after the receive buffer (FIFO) is
unloaded to the next trigger level below the programmed trigger level. Under the above
described conditions, the SC16C850 will continue to accept data until the receive FIFO is
full.
When the TXINTLVL, RXINTLVL, FLWCNTH and FLWCNTL in the ‘first extra feature
register set’ are all zeroes, the hardware and software flow control trigger levels are set by
FCR[7:4]; see Table6.
When the TXINTLVL, RXINTLVL, FLWCNTH or FLWCNTL in the ‘first extra feature
register set’ contain any value other than 0x00, the hardware and software flow control
trigger levels are set by FLWCNTH and FLWCNTL. The content in FLWCNTH determines
how many bytes are in the receive FIFO before RTS (or DTR) is de-asserted or Xoff is
sent. The content in FLWCNTL determines how many bytes are in the receive FIFO
before RTS (or DTR) is asserted, or Xon is sent.
In 128-byte FIFO mode, hardware and software flow control trigger levels can be set to
any value between 1 and 128 in granularity of 1. The value of FLWCNTH should always
be greater than FLWCNTL. The UART does not check for this condition automatically, and
if this condition is not met, spurious operation of the device might occur. When using
FLWCNTH and FLWCNTL, these registers must be initialized to proper values before
hardware or software flow control is enabled via the EFR register.
6.6 Software flow controlWhen software flow control is enabled, the SC16C850 compares one or two sequentially
received data characters with the programmed Xon or Xoff character value(s). If the
received character(s) match the programmed Xoff values, the SC16C850 will halt
transmission (TX) as soon as the current character(s) has completed transmission. When
a match occurs, ISR bit 4 will be set (if enabled via IER[5]) and the interrupt output pin (if
receive interrupt is enabled) will be activated. Following a suspension due to a match of
the Xoff characters’ values, the SC16C850 will monitor the receive data stream for a
match to the Xon1/Xon2 character value(s). If a match is found, the SC16C850 will
resume operation and clear the flags (ISR[4]).
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoderReset initially sets the contents of the Xon/Xoff 8-bit flow control registers to a logic0.
Following reset, the user can write any Xon/Xoff value desired for software flow control.
Different conditions can be set to detect Xon/Xoff characters and suspend/resume
transmissions (see Table 24). When double 8-bit Xon/Xoff characters are selected, the
SC16C850 compares two consecutive receive characters with two software flow control
8-bit values (Xon1, Xon2, Xoff1, Xoff2) and controls TX transmissions accordingly. Under
the above described flow control mechanisms, flow control characters are not placed
(stacked) in the receive FIFO. When using software flow control, the Xon/Xoff characters
cannot be used for data transfer.
In the event that the receive buffer is overfilling, the SC16C850 automatically sends an
Xoff character (when enabled) via the serial TX output to the remote UART. The
SC16C850 sends the Xoff1/Xoff2 characters as soon as the number of received data in
the receive FIFO passes the programmed trigger level. To clear this condition, the
SC16C850 will transmit the programmed Xon1/Xon2 characters as soon as the number of
characters in the receive FIFO drops below the programmed trigger level.
6.7 Special character detectA special character detect feature is provided to detect an 8-bit character when EFR[5] is
set. When an 8-bit character is detected, it will be placed on the user-accessible data
stack along with normal incoming RX data. This condition is selected in conjunction with
EFR[3:0] (see Table 24). Note that software flow control should be turned off when using
this special mode by setting EFR[3:0] to all zeroes.
The SC16C850 compares each incoming receive character with Xoff2 data. If a match
occurs, the received data will be transferred to the FIFO, and ISR[4] will be set to indicate
detection of a special character. Although Table 8 “SC16C850 internal registers” shows
Xon1, Xon2, Xoff1, Xoff2 with eight bits of character information, the actual number of bits
is dependent on the programmed word length. Line Control Register bits LCR[1:0] define
the number of character bits, that is, either 5 bits, 6 bits, 7 bits or 8 bits. The word length
selected by LCR[1:0] also determines the number of bits that will be used for the special
character comparison. Bit 0 in Xon1, Xon2, Xoff1, Xoff2 corresponds with the LSB bit for
the received character.
6.8 Interrupt priority and time-out interruptsThe interrupts are enabled by IER[7:0]. Care must be taken when handling these
interrupts. Following a reset, if Interrupt Enable Register (IER) bit1= 1, the SC16C850
will issue a Transmit Holding Register interrupt. This interrupt must be serviced prior to
continuing operations. The ISR indicates the current singular highest priority interrupt
only. A condition can exist where a higher priority interrupt masks the lower priority
interrupt(s) (see Table 13). Only after servicing the higher pending interrupt will the lower
priority interrupt(s) be reflected in the status register. Servicing the interrupt without
investigating further interrupt conditions can result in data errors.
Receive Data Ready and Receive Time-Out have the same interrupt priority (when
enabled by IER[0]), and it is important to serve these interrupts correctly. The receiver
issues an interrupt after the number of characters have reached the programmed trigger
level. In this case, the SC16C850 FIFO may hold more characters than the programmed
trigger level. Following the removal of a data byte, the user should re-check LSR[0] to see
if there are any additional characters. A Receive Time-Out will not occur if the receive
FIFO is empty. The time-out counter is reset at the center of each stop bit received or
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decodereach time the Receive Holding Register (RHR) is read. The actual time-out value is character time, including data information length, start bit, parity bit, and the size of stop
bit, that is, 1×, 1.5×, or 2× bit times.
6.9 Programmable baud rate generatorThe SC16C850 UART contains a programmable rational baud rate generator that takes
any clock input and divides it by a divisor in the range between 1 and (216− 1). The
SC16C850 offers the capability of dividing the input frequency by rational divisor. The
fractional part of the divisor is controlled by the CLKPRES register in the ‘first extra feature
register set’.
(1)
where:
N is the integer part of the divisor in DLL and DLM registers;
M is the fractional part of the divisor in CLKPRES register;
fXTAL1 is the clock frequency at XTAL1 pin.
Prescaler= 1 when MCR[7] is set to 0.
Prescaler = 4 when MCR[7] is set to 1.
A single baud rate generator is provided for the transmitter and receiver. The
programmable Baud Rate Generator is capable of operating with a frequency of up to MHz. To obtain maximum data rate, it is necessary to use full rail swing on the clock
input. The SC16C850 can be configured for internal or external clock operation. For
internal clock operation, an industry standard crystal is connected externally between the
XTAL1 and XTAL2 pins (see Figure 7). Alternatively, an external clock can be connected
to the XTAL1 pin (see Figure 8) to clock the internal baud rate generator for standard or
custom rates (see Table7).
The generator divides the input 16× clock by any divisor from 1 to (216− 1). The
SC16C850 divides the basic external clock by 16. The baud rate is configured via the
CLKPRES, DLL and DLM internal register functions. Customized baud rates can be
achieved by selecting the proper divisor values for the MSB and LSB sections of the baud
rate generator.
baud rate f XTAL1
MCR7[] 16 N M------+ ⎝⎠⎛⎞××
-------------------------------------------------------------------=
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoderProgramming the baud rate generator registers CLKPRES, DLM (MSB) and DLL (LSB)
provides a user capability for selecting the desired final baud rate. The example in Table7
shows the selectable baud rate table available when using a 1.8432 MHz external clock
input when MCR[7]= 0, and CLKPRES = 0x00.
Table 7. Baud rate generator programming table using a 1.8432 MHz clock when
MCR[7]= 0 and CLKPRES[3:0]=0 2304 900 09 00 1536 600 06 00
110 1047 417 04 17
150 768 300 03 00
300 384 180 01 80
600 192 C0 00 C0
1.2k 96 60 00 60
2.4k 48 30 00 30
3.6k 32 20 00 20
4.8k 24 18 00 18
7.2k 16 10 00 10
9.6k 12 0C 00 0C
19.2k 6 06 00 06
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.10 Loopback modeThe internal loopback capability allows on-board diagnostics. In the Loopback mode, the
normal modem interface pins are disconnected and reconfigured for loopback internally
(see Figure 9). MCR[3:0] register bits are used for controlling loopback diagnostic testing.
In the Loopback mode, the transmitter output (TX) and the receiver input (RX) are
disconnected from their associated interface pins, and instead are connected together
internally. The CTS, DSR, CD, and RI are disconnected from their normal modem control
input pins, and instead are connected internally to RTS, DTR, MCR[3] (OP2) and MCR[2]
(OP1). Loopback test data is entered into the transmit holding register via the user data
bus interface, D[7:0]. The transmit UART serializes the data and passes the serial data to
the receive UART via the internal loopback connection. The receive UART converts the
serial data back into parallel data that is then made available at the user data interface
D[7:0]. The user optionally compares the received data to the initial transmitted data for
verifying error-free operation of the UART TX/RX circuits.
In this mode, the interrupt pin is 3-stated, therefore, the software must use the polling
method (see Section 7.2.2) to send and receive data.
38.4k 3 03 00 03
57.6k 2 02 00 02
115.2k 1 01 00 01
Table 7. Baud rate generator programming table using a 1.8432 MHz clock when
MCR[7]= 0 and CLKPRES[3:0]= 0 …continued
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.11 Sleep modeSleep mode is an enhanced feature of the SC16C850 UART . It is enabled when EFR[4],
the enhanced functions bit, is set and when IER[4] bit is set.
6.11.1 Conditions to enter Sleep modeSleep mode is entered when:
Modem input pins are not toggling.
The serial data input line, RX, is idle for 4 character time (logic HIGH) and AFCR1[4]
is logic 0. When AFCR1[4] is logic 1 the device will go to sleep regardless of the state
of the RX pin (see Section 7.21 for the description of AFCR1 bit 4).
The TX FIFO and TX shift register are empty.
There are no interrupts pending.
The RX FIFO is empty.
In Sleep mode, the UART clock and baud rate clock are stopped. Since most registers are
clocked using these clocks, the power consumption is greatly reduced.
Remark: Writing to the divisor latches, DLL and DLM, to set the baud clock, must not be done during Sleep mode. Therefore, it is advisable to disable Sleep mode using IER[4]
before writing to DLL or DLM.
6.11.2 Conditions to resume normal operationSC16C850 resumes normal operation by any of the following:
Receives a start bit on RX pin.
Data is loaded into transmit FIFO.
A change of state on any of the modem input pins
If the device is awakened by one of the conditions described above, it will return to the
Sleep mode automatically after all the conditions described in Section 6.11.1 are met. The
device will stay in Sleep mode until it is disabled by setting any channel’s IER bit 4 to a
logic0.
When the SC16C850 is in Sleep mode and the host data bus (D[7:0], A[2:0], IOW, IOR,
CS) remains in steady state, either HIGH or LOW, the Sleep mode supply current will be
in the μA range as specified in T able 36 “Static characteristics”. If any of these signals is
toggling or floating then the sleep current will be higher.
6.12 Low power featureA low power feature is provided by the SC16C850 to prevent the switching of the host
data bus from influencing the sleep current. When the pin LOWPWR is activated (logic
HIGH), the device immediately and unconditionally goes into Low power mode. All clocks
are stopped and most host interface pins are isolated to reduce power consumption. The
device only returns to normal mode when the LOWPWR pin is de-asserted. The pin can
be left unconnected because it has an internal pull-down resistor.
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.13 RS-485 features
6.13.1 Auto RS-485 RTS controlNormally the RTS pin is controlled by MCR[1], or if hardware flow control is enabled, the
logic state of the RTS pin is controlled by the hardware flow control circuitry. AFCR2[4] will
take the precedence over the other two modes; once this bit is set, the transmitter will
control the state of the RTS pin. The transmitter automatically asserts the RTS pin
(logic 0) once the host writes data to the transmit FIFO, and de-asserts RTS pin (logic 1)
once the last bit of the data has been transmitted.
To use the auto RS-485 RTS mode the software would have to disable the hardware
flow control function.
6.13.2 RS-485 RTS inversionAFCR2[5] reverses the polarity of the RTS pin if the UART is in auto RS-485 RTS mode.
When the transmitter has data to be sent it will de-asserts the RTS pin (logic 1), and when
the last bit of the data has been sent out the transmitter asserts the RTS pin (logic0).
6.13.3 Auto 9-bit mode (RS-485)AFCR2[0] is used to enable the 9-bit mode (Multi-drop or RS-485 mode). In this mode of
operation, a ‘master’ station transmits an address character followed by data characters
for the addressed ‘slave’ stations. The slave stations examine the received data and
interrupt the controller if the received character is an address character (parity bit = 1).
To use the automatic 9-bit mode, the software would have to disable the hardware and
software flow control functions.
6.13.3.1 Normal Multi-drop modeThe 9-bit Mode in AFCR2[0] is enabled, but not Special Character Detect (EFR[5]). The
receiver is set to Force Parity 0 (LCR[5:3]= 111) in order to detect address bytes.
With the receiver initially disabled, it ignores all the data bytes (parity bit= 0) until an
address byte is received (parity bit= 1). This address byte will cause the UART to set the
parity error. The UART will generate a line status interrupt (IER[2] must be set to ‘1’ at this
time), and at the same time puts this address byte in the RX FIFO. After the controller
examines the byte it must make a decision whether or not to enable the receiver; it should
enable the receiver if the address byte addresses its ID address, and must not enable the
receiver if the address byte does not address its ID address.
If the controller enables the receiver, the receiver will receive the subsequent data until
being disabled by the controller after the controller has received a complete message
from the ‘master’ station. If the controller does not disable the receiver after receiving a
message from the ‘master’ station, the receiver will generate a parity error upon receiving
another address byte. The controller then determines if the address byte addresses its ID
address, if it is not, the controller then can disable the receiver. If the address byte
addresses the ‘slave’ ID address, the controller takes no further action, and the receiver
will receive the subsequent data.
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.13.3.2 Auto address detectionIf Special Character Detect is enabled (EFR[5] is set and the Xoff2 register contains the
address byte) the receiver will try to detect an address byte that matches the programmed
character in the Xoff2 register. If the received byte is a data byte or an address byte that
does not match the programmed character in the Xoff2 register, the receiver will discard
these data. Upon receiving an address byte that matches the Xoff2 character, the receiver
will be automatically enabled if not already enabled, and the address character is pushed
into the RX FIFO along with the parity bit (in place of the parity error bit). The receiver also
generates a line status interrupt (IER[2] must be set to ‘1’ at this time). The receiver will
then receive the subsequent data from the ‘master’ station until being disabled by the
controller after having received a message from the ‘master’ station.
If another address byte is received and this address byte does not match the Xoff2
character, the receiver will be automatically disabled and the address byte is ignored. If
the address byte matches the Xoff2 character, the receiver will put this byte in the RX
FIFO along with the parity bit in the parity error bit (LSR bit 2).
7. Register descriptionsTable 8 details the assigned bit functions for the SC16C850 internal registers. The
assigned bit functions are more fully defined in Section 7.1 through Section 7.23.
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NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
ble
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NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoderhe va
lue shown
repre
ent
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register
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SC1
C85
in
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NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.1 Transmit (THR) and Receive (RHR) Holding RegistersThe serial transmitter section consists of an 8-bit Transmit Hold Register (THR) and
Transmit Shift Register (TSR). The status of the THR is provided in the Line Status
Register (LSR). Writing to the THR transfers the contents of the data bus (D7to D0) to the
transmit FIFO. The THR empty flag in the LSR will be set to a logic 1 when the transmit
FIFO is empty or when data is transferred to the TSR.
The serial receive section also contains an 8-bit Receive Holding Register (RHR) and a
Receive Serial Shift Register (RSR). Receive data is removed from the SC16C850
receive FIFO by reading the RHR. The receive section provides a mechanism to prevent
false starts. On the falling edge of a start or false start bit, an internal receiver counter
starts counting clocks at the 16× clock rate. After 71 ⁄2 clocks, the start bit time should be
shifted to the center of the start bit. At this time the start bit is sampled, and if it is still a
logic 0 it is validated. Evaluating the start bit in this manner prevents the receiver from
assembling a false character. Receiver status codes will be posted in the LSR.
7.2 Interrupt Enable Register (IER)The Interrupt Enable Register (IER) masks the interrupts from receiver ready, transmitter
empty, line status and modem status registers. These interrupts would normally be seen
on the INT output pin.
Table 9. Interrupt Enable Register bits description IER[7] CTS interrupt.
logic0= disable the CTS interrupt (normal default condition)
logic1= enable the CTS interrupt. The SC16C850 issues an interrupt when
the CTS pin transitions from a logic 0 to a logic1. IER[6] RTS interrupt.
logic0= disable the RTS interrupt (normal default condition)
logic1= enable the RTS interrupt. The SC16C850 issues an interrupt when
the RTS pin transitions from a logic 0 to a logic1. IER[5] Xoff interrupt.
logic0= disable the software flow control, receive Xoff interrupt (normal default
condition)
logic1= enable the receive Xoff interrupt IER[4] Sleep mode.
logic0= disable Sleep mode (normal default condition)
logic1= enable Sleep mode IER[3] Modem Status Interrupt. This interrupt will be issued whenever there is a modem
status change as reflected in MSR[3:0].
logic0= disable the modem status register interrupt (normal default condition)
logic1= enable the modem status register interrupt IER[2] Receive Line Status interrupt. This interrupt will be issued whenever a receive
data error condition exists as reflected in LSR[4:1].
logic0= disable the receiver line status interrupt (normal default condition)
logic1= enable the receiver line status interrupt
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.2.1 IER versus Transmit/Receive FIFO interrupt mode operationWhen the receive FIFO is enabled (FCR[0]= logic 1), and receive interrupts
(IER[0]= logic 1) are enabled, the receive interrupts and register status will reflect the
following:
The receive RXRDY interrupt (Level 2 ISR interrupt) is issued to the external CPU
when the receive FIFO has reached the programmed trigger level. It will be cleared
when the receive FIFO drops below the programmed trigger level.
Receive FIFO status will also be reflected in the user accessible ISR register when
the receive FIFO trigger level is reached. Both the ISR register receive status bit and
the interrupt will be cleared when the FIFO drops below the trigger level.
The receive data ready bit (LSR[0]) is set as soon as a character is transferred from
the shift register (RSR) to the receive FIFO. It is reset when the FIFO is empty.
When the Transmit FIFO and interrupts are enabled, an interrupt is generated when
the transmit FIFO is empty due to the unloading of the data by the TSR and UART for
transmission via the transmission media. The interrupt is cleared either by reading the
ISR, or by loading the THR with new data characters.
7.2.2 IER versus Receive/Transmit FIFO polled mode operationWhen FCR[0]= logic 1, setting IER[3:0] puts the SC16C850 in the FIFO polled mode of
operation. In this mode, interrupts are not generated and the user must poll the LSR
register for TX and/or RX data status. Since the receiver and transmitter have separate
bits in the LSR either or both can be used in the polled mode by selecting respective
transmit or receive control bit(s).
LSR[0] will be a logic 1 as long as there is one byte in the receive FIFO.
LSR[4:1] will provide the type of receive errors, or a receive break, if encountered.
LSR[5] will indicate when the transmit FIFO is empty.
LSR[6] will indicate when both the transmit FIFO and transmit shift register are empty.
LSR[7] will show if any FIFO data errors occurred. IER[1] Transmit Holding Register interrupt. In the non-FIFO mode, this interrupt will be
issued whenever the THR is empty, and is associated with LSR[5]. In the FIFO
modes, this interrupt will be issued whenever the FIFO is empty.
logic0= disable the Transmit Holding Register Empty (TXRDY) interrupt
(normal default condition)
logic1= enable the TXRDY (ISR level 3) interrupt IER[0] Receive Holding Register interrupt. In the non-FIFO mode, this interrupt will be
issued when the RHR has data, or is cleared when the RHR is empty. In the FIFO
mode, this interrupt will be issued when the FIFO has reached the programmed
trigger level or is cleared when the FIFO drops below the trigger level.
logic0= disable the receiver ready (ISR level 2, RXRDY) interrupt (normal
default condition)
logic1= enable the RXRDY (ISR level 2) interrupt
Table 9. Interrupt Enable Register bits description …continued
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.3 FIFO Control Register (FCR)This register is used to enable the FIFOs, clear the FIFOs, and set the receive FIFO
trigger levels.
7.3.1 FIFO mode[1] For 128-byte FIFO mode, refer to Section 7.16, Section 7.17, Section 7.18.
[2] For 128-byte FIFO mode, refer to Section 7.15, Section 7.17, Section 7.18.
[1] When RXINTLVL, TXINTLVL, FLWCNTL or FLWCNTH contains any value other than 0x00, receive and
transmit trigger levels are set by RXINTLVL, TXINTLVL registers (see Section 6.4 “FIFO operation”).
Table 10. FIFO Control Register bits description7:6 FCR[7:6] Receive trigger level in 32-byte FIFO mode[1]
These bits are used to set the trigger levels for receive FIFO interrupt and flow
control. The SC16C850 will issue a receive ready interrupt when the number of
characters in the receive FIFO reaches the selected trigger level. Refer to
Table11.
5:4 FCR[5:4] Transmit trigger level in 32-byte FIFO mode[2].
These bits are used to set the trigger level for the transmit FIFO interrupt and
flow control. The SC16C850 will issue a transmit empty interrupt when the
number of characters in FIFO drops below the selected trigger level. Refer to
Table 12. FCR[3] reserved FCR[2] XMIT FIFO reset.
logic0= no FIFO transmit reset (normal default condition)
logic1= clears the contents of the transmit FIFO and resets the FIFO
counter logic. This bit will return to a logic 0 after clearing the FIFO. FCR[1] RCVR FIFO reset.
logic0= no FIFO receive reset (normal default condition)
logic1= clears the contents of the receive FIFO and resets the FIFO counter
logic. This bit will return to a logic 0 after clearing the FIFO. FCR[0] FIFO enable.
logic0= disable the transmit and receive FIFO (normal default condition)
logic1= enable the transmit and receive FIFO
Table 11. RCVR trigger levels 8 16 24 28
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder[1] When RXINTLVL, TXINTLVL, FLWCNTL or FLWCNTH contains any value other than 0x00, receive and
transmit trigger levels are set by RXINTLVL, TXINTLVL registers (see Section 6.4 “FIFO operation”).
7.4 Interrupt Status Register (ISR)The SC16C850 provides six levels of prioritized interrupts to minimize external software
interaction. The Interrupt Status Register (ISR) provides the user with six interrupt status
bits. Performing a read cycle on the ISR will provide the user with the highest pending
interrupt level to be serviced. No other interrupts are acknowledged until the pending
interrupt is serviced. A lower level interrupt may be seen after servicing the higher level
interrupt and re-reading the interrupt status bits. Table 13 “Interrupt source” shows the
data values (bits 5:0) for the six prioritized interrupt levels and the interrupt sources
associated with each of these interrupt levels.
Table 12. TX FIFO trigger levels 16 8 24 30
Table 13. Interrupt source 0 00110LSR (Receiver Line Status
Register) 0 00100RXRDY (Received Data Ready) 0 01100RXRDY (Receive Data time-out) 0 00010TXRDY (Transmitter Holding
Register Empty) 0 00000MSR (Modem Status Register) 0 10000RXRDY (Received Xoff signal)/
Special character 1 00000CTS, RTS change of state
Table 14. Interrupt Status Register bits description7:6 ISR[7:6] FIFOs enabled. These bits are set to a logic 0 when the FIFOs are not being
used in the non-FIFO mode. They are set to a logic 1 when the FIFOs are
enabled in the SC16C850 mode.
logic 0 or cleared= default condition
5:4 ISR[5:4] INT priority bits 4:3. These bits are enabled when EFR[4] is set to a logic1.
ISR[4] indicates that matching Xoff character(s) have been detected. ISR[5]
indicates that CTS, RTS have been generated. Note that once set to a
logic 1, the ISR[4] bit will stay a logic 1 until Xon character(s) are received.
logic 0 or cleared= default condition
3:1 ISR[3:1] INT priority bits 2:0. These bits indicate the source for a pending interrupt at
interrupt priority levels 1, 2, and 3 (see Table 13).
logic 0 or cleared= default condition
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.5 Line Control Register (LCR)The Line Control Register is used to specify the asynchronous data communication
format. The word length, the number of stop bits, and the parity are selected by writing the
appropriate bits in this register. ISR[0] INT status.
logic0= an interrupt is pending and the ISR contents may be used as a
pointer to the appropriate interrupt service routine
logic1= no interrupt pending (normal default condition)
Table 14. Interrupt Status Register bits description …continued
Table 15. Line Control Register bits description LCR[7] Divisor latch enable. The internal baud rate counter latch and Enhanced
Feature mode enable.
logic0 = divisor latch disabled (normal default condition)
logic1= divisor latch enabled LCR[6] Set break. When enabled, the Break control bit causes a break condition to
be transmitted (the TX output is forced to a logic 0 state). This condition
exists until disabled by setting LCR[6] to a logic0.
logic0= no TX break condition (normal default condition)
logic1= forces the transmitter output (TX) to a logic 0 for alerting the
remote receiver to a line break condition
5:3 LCR[5:3] Programs the parity conditions (see Table 16). LCR[2] Stop bits. The length of stop bit is specified by this bit in conjunction with the
programmed word length (see Table 17).
logic 0 or cleared= default condition
1:0 LCR[1:0] Word length bits 1, 0. These two bits specify the word length to be
transmitted or received (see Table 18).
logic 0 or cleared= default condition
Table 16. LCR[5:3] parity selection 0 no parity 01odd parity 11even parity 0 1 forced parity ‘1’ 1 1 forced parity ‘0’
Table 17. LCR[2] stop bit length 5, 6, 7, 8 1 11⁄2
16, 7, 8 2
NXP Semiconductors SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.6 Modem Control Register (MCR)This register controls the interface with the modem or a peripheral device.
Table 18. LCR[1:0] word length 5 6 7 8
Table 19. Modem Control Register bits description MCR[7] Clock select
logic0 = divide-by-1 clock input
logic1= divide-by-4 clock input MCR[6] IR enable (see Figure 22).
logic 0 = enable the standard modem receive and transmit input/output
interface (normal default condition)
logic 1 = enable infrared IrDA receive and transmit inputs/outputs. While
in this mode, the TX/RX output/inputs are routed to the infrared
encoder/decoder. The data input and output levels will conform to the
IrDA infrared interface requirement. As such, while in this mode, the
infrared TX output will be a logic 0 during idle data conditions. MCR[5] Interrupt type (Intel mode only). In Intel mode (16/68= 1), this pin determines the interrupt output pin configuration.
logic0 = CMOS output
logic1= open-source. A 300 Ω to 500 Ω pull-down resistor is required.
In Motorola mode (16/68= 0), the output is always open-drain. MCR[4] Loopback. Enable the local loopback mode (diagnostics). In this mode the
transmitter output (TX) and the receiver input (RX), CTS, DSR, CD, and RI
are disconnected from the SC16C850 I/O pins. Internally the modem data and control pins are connected into a loopback data configuration (see
Figure 9). In this mode, the receiver and transmitter interrupts remain fully
operational. The Modem Control Interrupts are also operational, but the
interrupts’ sources are switched to the lower four bits of the Modem Control. Interrupts continue to be controlled by the IER register.
logic0= disable Loopback mode (normal default condition)
logic1= enable local Loopback mode (diagnostics)
3MCR[3] OP2. This bit is used for internal Loopback mode only. In Loopback mode, this bit is used to write the state of the modem CD interface signal.
2MCR[2] OP1. This bit is used for internal Loopback mode only. In Loopback mode,
this bit is used to write the state of the modem RI interface signal.
1MCR[1] RTS
logic0 = force RTS output to a logic 1 (normal default condition)
logic1= force RTS output to a logic0
0MCR[0] DTR
logic0 = force DTR output to a logic 1 (normal default condition)
logic1= force DTR output to a logic0
