MAX6955AAX ,2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key ScanFeatures2The MAX6955 is a compact display driver that interfaces 400kbps 2-Wire Interface Compatib ..
MAX6955AAX+ ,2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key ScanFeaturesThe MAX6955 is a compact display driver that interfaces• Simplifies Driving 5 x 7 Matrix LE ..
MAX6955AAX+ ,2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key ScanApplicationsFunctional DiagramSet-Top Boxes Bar Graph DisplaysPanel Meters Audio/Video EquipmentGPI ..
MAX6955AAX+T ,2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key ScanElectrical Characteristics(Typical Operating Circuit, V+ = 2.7V to 5.5V, T = T to T , unless otherw ..
MAX6956AAI+ ,2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or 28-Port LED Display Driver and I/O ExpanderApplicationsTOP VIEWSet-Top Boxes Bar Graph DisplaysISET 1 28 V+Panel Meters Industrial Controllers ..
MAX6956AAX/V+T ,2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or 28-Port LED Display Driver and I/O ExpanderELECTRICAL CHARACTERISTICS(Typical Operating Circuit, V+ = 2.5V to 5.5V, T = T to T , unless otherw ..
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MB40166 ,AD/DA CONVERTERFUJITSU SEMICONDUCTORDS04-28500-5EDATA SHEETASSPAD/DA CONVERTERMB40166/MB401761-CHANNEL 6-BIT AD/DA ..
MB40176 ,AD/DA CONVERTERFUJITSU SEMICONDUCTORDS04-28500-5EDATA SHEETASSPAD/DA CONVERTERMB40166/MB401761-CHANNEL 6-BIT AD/DA ..
MB40176 ,AD/DA CONVERTERFUJITSU SEMICONDUCTORDS04-28500-5EDATA SHEETASSPAD/DA CONVERTERMB40166/MB401761-CHANNEL 6-BIT AD/DA ..
MAX6955AAX
2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan
General DescriptionThe MAX6955 is a compact display driver that interfaces
microprocessors to a mix of 7-segment, 14-segment,
and 16-segment LED displays through an I2C™-compati-
ble 2-wire serial interface. The MAX6955 drives up to 16
digits 7-segment, 8 digits 14-segment, 8 digits 16-seg-
ment, or 128 discrete LEDs, while functioning from a
supply voltage as low as 2.7V. The driver includes five
I/O expander or general-purpose I/O (GPIO) lines, some
or all of which can be configured as a key-switch reader.
The key-switch reader automatically scans and
debounces a matrix of up to 32 switches.
Included on chip are full 14- and 16-segment ASCII
104-character fonts, a hexadecimal font for 7-segment
displays, multiplex scan circuitry, anode and cathode
drivers, and static RAM that stores each digit. The max-
imum segment current for the display digits is set using
a single external resistor. Digit intensity can be inde-
pendently adjusted using the 16-step internal digital
brightness control. The MAX6955 includes a low-power
shutdown mode, a scan-limit register that allows the
user to display from 1 to 16 digits, segment blinking
(synchronized across multiple drivers, if desired), and a
test mode, which forces all LEDs on. The LED drivers
are slew-rate limited to reduce EMI.
For an SPI™-compatible version, refer to the MAX6954
data sheet. An evaluation kit* (EV kit) for the MAX6955
is available.
*Future product—contact factory for availability.
ApplicationsSet-Top BoxesAutomotive
Panel MetersBar Graph Displays
White GoodsAudio/Video Equipment
Features400kbps 2-Wire Interface Compatible with I2C2.7V to 5.5V OperationDrives Up to 16 Digits 7-Segment, 8 Digits
14-Segment, 8 Digits 16-Segment, 128 Discrete
LEDs, or a Combination of Digit TypesDrives Common-Cathode Monocolor and Bicolor
LED DisplaysBuilt-In ASCII 104-Character Font for 14-Segment
and 16-Segment Digits and Hexadecimal Font for
7-Segment DigitsAutomatic Blinking Control for Each Segment10µA (typ) Low-Power Shutdown (Data Retained)16-Step Digit-by-Digit Digital Brightness Control Display Blanked on Power-UpSlew-Rate-Limited Segment Drivers for Lower EMIFive GPIO Port Pins Can Be Configured as Key-
Switch Reader to Scan and Debounce Up to 32
Switches with n-Key RolloverIRQ Output when a Key Input is Debounced36-Pin SSOP and 40-Pin DIP PackagesAutomotive Temperature Range Standard
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Ordering Information19-2548; Rev 0; 7/02
I2C is a trademark of Philips Corp.
SPI is a trademark of Motorola, Inc.
Pin Configurations and Typical Operating Circuits appear
at end of data sheet.
Functional Diagram
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
ABSOLUTE MAXIMUM RATINGSStresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Voltage (with Respect to GND)
V+.........................................................................-0.3V to +6V
SCL, SDA, AD0, AD1 ...........................................-0.3V to +6V
All Other Pins............................................-0.3V to (V+ + 0.3V)
Current
O0–O7 Sink Current......................................................935mA
O0–O18 Source Current.................................................55mA
SCL, SDA, AD0, AD1, BLINK, OSC, OSC_OUT, ISET....20mA
P0, P1, P2, P3, P4...........................................................40mA
GND.....................................................................................1A
Continuous Power Dissipation (TA= +70°C)
36-Pin SSOP (derate at 11.8mW/°C above +70°C).....941mW
40-Pin PDIP (derate at 16.7mW/°C above +70°C).....1333mW
Operating Temperature Range
(TMINto TMAX)...............................................-40°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
DC ELECTRICAL CHARACTERISTICS
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
DC ELECTRICAL CHARACTERISTICS (continued)
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
TIMING CHARACTERISTICS
Note 1:All parameters tested at TA= +25°C. Specifications over temperature are guaranteed by design.
Note 2:Guaranteed by design.
Note 3:A master device must provide a hold time of at least 300ns for the SDA signal (referred to VIL- of the SCL signal) in order to
bridge the undefined region of SCL’s falling edge.
Note 4:CB= total capacitance of one bus line in pF. tRand tFmeasured between 0.3V+ and 0.7V+.
Note 5:ISINK≤6mA. CB= total capacitance of one bus line in pF. tRand tFmeasured between 0.3V+ and 0.7V+.
Note 6:Input filters on the SDA and SCL inputs suppress noise spikes less than 50ns.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Typical Operating Characteristics(V+ = 3.3V, LED forward voltage = 2.4V, Typical Application Circuit, TA= +25°C, unless otherwise noted.)
MAX6955
Detailed DescriptionThe MAX6955 is a serially interfaced display driver that
can drive up to 16 digits 7-segment, 8 digits 14-seg-
ment, 8 digits 16-segment, 128 discrete LEDs, or a
combination of these display types. Table 1 shows the
drive capability of the MAX6955 for monocolor and
bicolor displays.
The MAX6955 includes 104-character ASCII font maps
for 14-segment and 16-segment displays, as well as
the hexadecimal font map for 7-segment displays. The
characters follow the standard ASCII font, with the addi-
tion of the following common symbols: £, , ¥, °, µ, ±,
↑, and ↓. Seven bits represent the 104-character font
map; an 8th bit is used to select whether the decimal
point (DP) is lit. Seven-segment LED digits can be con-
trolled directly or use the hexadecimal font. Direct seg-
ment control allows the MAX6955 to be used to drive
bar graphs and discrete LED indicators.
Tables 2, 3, and 4 list the connection schemes for 16-,
14-, and 7-segment digits, respectively. The letters in
Tables 2, 3, and 4 correspond to the segment labels
shown in Figure 1. (For applications that require mixed
display types, see Tables 38–41.)
Serial Interface
Serial AddressingThe MAX6955 operates as a slave that sends and
receives data through an I2C-compatible 2-wire inter-
face. The interface uses a serial data line (SDA) and a
serial clock line (SCL) to achieve bidirectional commu-
nication between master(s) and slave(s). A master (typ-
ically a microcontroller) initiates all data transfers to and
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scanfrom the MAX6955, and generates the SCL clock that
synchronizes the data transfer (Figure 2).
The MAX6955 SDA line operates as both an input and
an open-drain output. A pullup resistor, typically 4.7kΩ,
is required on the SDA. The MAX6955 SCL line oper-
ates only as an input. A pullup resistor, typically 4.7kΩ,
is required on SCL if there are multiple masters on the
2-wire interface, or if the master in a single-master sys-
tem has an open-drain SCL output.
Each transmission consists of a START condition
(Figure 3) sent by a master, followed by the MAX6955
7-bit slave address plus R/Wbit (Figure 4), a register
address byte, 1 or more data bytes, and finally a STOP
condition (Figure 3).
Start and Stop ConditionsBoth SCL and SDA remain high when the interface is
not busy. A master signals the beginning of a transmis-
sion with a START (S) condition by transitioning SDA
from high to low while SCL is high. When the master
has finished communicating with the slave, it issues a
STOP (P) condition by transitioning the SDA from low to
high while SCL is high. The bus is then free for another
transmission (Figure 3).
Bit TransferOne data bit is transferred during each clock pulse.
The data on the SDA line must remain stable while SCL
is high (Figure 5).
AcknowledgeThe acknowledge bit is a clocked 9th bit that the recipi-
ent uses to handshake receipt of each byte of data
(Figure 6). Thus, each byte transferred effectively
requires 9 bits. The master generates the 9th clock
pulse, and the recipient pulls down SDA during the
acknowledge clock pulse, such that the SDA line is sta-
ble low during the high period of the clock pulse. When
the master is transmitting to the MAX6955, the
MAX6955 generates the acknowledge bit because the
MAX6955 is the recipient. When the MAX6955 is trans-
mitting to the master, the master generates the
acknowledge bit because the master is the recipient.
Slave AddressThe MAX6955 has a 7-bit-long slave address (Figure
4). The eighth bit following the 7-bit slave address is the
R/Wbit. It is low for a write command, high for a read
command.
The first 3 bits (MSBs) of the MAX6955 slave address
are always 110. Slave address bits A3, A2, A1, and A0
are selected by the address input pins AD1 and AD0.
These two input pins can be connected to GND, V+,
SDA, or SCL. The MAX6955 has 16 possible slave
addresses (Table 5) and therefore a maximum of 16
MAX6955 devices can share the same interface.
Table 1. MAX6955 Drive CapabilityFigure 1. Segment Labeling for 7-Segment Display, 14-Segment Display, and 16-Segment Display
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Message Format for WritingA write to the MAX6955 comprises the transmission of
the MAX6955’s slave address with the R/Wbit set to
zero, followed by at least 1 byte of information. The first
byte of information is the command byte, which deter-
mines which register of the MAX6955 is to be written by
the next byte, if received. If a STOP condition is detect-
ed after the command byte is received, then the
MAX6955 takes no further action (Figure 7) beyond
storing the command byte.
Any bytes received after the command byte are data
bytes. The first data byte goes into the internal register of
the MAX6955 selected by the command byte (Figure 8).
If multiple data bytes are transmitted before a STOP
condition is detected, these bytes are generally stored
in subsequent MAX6955 internal registers because the
command byte address generally autoincrements
(Table 6) (Figure 9).
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key ScanFigure 4. Slave Address
Figure 2. 2-Wire Serial Interface Timing Details
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Message Format for ReadingThe MAX6955 is read using the MAX6955’s internally
stored command byte as address pointer, the same
way the stored command byte is used as address
pointer for a write. The pointer generally autoincre-
ments after each data byte is read using the same rules
as for a write (Table 6). Thus, a read is initiated by first
configuring the MAX6955’s command byte by perform-
ing a write (Figure 7). The master can now read n con-
secutive bytes from the MAX6955, with the first data
byte being read from the register addressed by the ini-
tialized command byte (Figure 9). When performing
read-after-write verification, reset the command byte’s
address because the stored byte address generally is
autoincremented after the write (Table 6).
Operation with Multiple MastersIf the MAX6955 is operated on a 2-wire interface with
multiple masters, a master reading the MAX6955
should use a repeated start between the write, which
sets the MAX6955’s address pointer, and the read(s)
that takes the data from the location(s). This is because
it is possible for master 2 to take over the bus after
master 1 has set up the MAX6955’s address pointer but
before master 1 has read the data. If master 2 subse-
quently changes the MAX6955’s address pointer, then
master 1’s delayed read may be from an unexpected
location.
Command Address AutoincrementingAddress autoincrementing allows the MAX6955 to be
configured with the shortest number of transmissions
by minimizing the number of times the command byte
needs to be sent. The command address or the font
pointer address stored in the MAX6955 generally incre-
ments after each data byte is written or read (Table 6).
Digit Type RegistersThe MAX6955 uses 32 digit registers to store the char-
acters that the user wishes to display. These digit regis-
ters are implemented with two planes, P0 and P1. Each
digit is represented by 2 bytes of memory, 1 byte in
plane P0 and the other in plane P1. The digit registers
are mapped so that a digit’s data can be updated in
plane P0, plane P1, or both planes at the same time
(Table 7).
If the blink function is disabled through the Blink Enable
Bit E (Table 20) in the configuration register, then the
digit register data in plane P0 is used to multiplex the
display. The digit register data in P1 is not used. If the
blink function is enabled, then the digit register data in
both plane P0 and plane P1 are alternately used to mul-
tiplex the display. Blinking is achieved by multiplexing
the LED display using data plane P0 and plane P1 on
alternate phases of the blink clock (Table 21).
The data in the digit registers does not control the digit
segments directly for 14- and 16-segment displays.
Instead, the register data is used to address a charac-
ter generator that stores the data for the 14- and 16-
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scansegment fonts (Tables 8 and 9). The lower 7 bits of the
digit data (D6 to D0) select the character from the font.
The most significant bit of the register data (D7) con-
trols the DP segment of the digits; it is set to 1 to light
DP, and to zero to leave DP unlit (Table 10).
For 7-segment displays, the digit plane data register
can be used to address a character generator, which
contains the data of a 16-character font containing the
hexadecimal font. The decode mode register can be
used to disable the character generator and allow the
segments to be controlled directly. Table 11 shows the
one-to-one pairing of each data bit to the appropriate
segment line in the digit plane data registers. The hexa-
decimal font is decoded according to Table 12.
The digit-type register configures the display driver for
various combinations of 14-segment digits, 16-segment
digits, and/or pairs, or 7-segment digits. The function of
this register is to select the appropriate font for each
digit and route the output of the font to the appropriate
MAX6955 driver output pins. The MAX6955 has four
digit drive slots. A slot can be filled with various combi-
nations of monocolor and bicolor 16-segment displays,
14-segment displays, or two 7-segment displays. Each
pair of bits in the register corresponds to one of the four
digit drive slots, as shown in Table 13. Each bit also cor-
responds to one of the eight common-cathode digit
drive outputs, CC0 to CC7. When using bicolor digits,
the anode connections for the two digits within a slot are
always the same. This means that a slot correctly drives
two monocolor or one bicolor 14- or 16-segment digit.
The digit type register can be written, but cannot be
read. Examples of configuration settings required for
some display digit combinations are shown in Table 14.
7-Segment Decode-Mode RegisterIn 7-segment mode, the hexadecimal font can be dis-
abled (Table 15). The decode-mode register selects
between hexadecimal code or direct control for each of
eight possible pairs of 7-segment digits. Each bit in the
register corresponds to one pair of digits. The digit
pairs are {digit 0, digit 0a} through {digit 7, digit 7a}.
Disabling decode mode allows direct control of the 16
LEDs of a dual 7-segment display. Direct control mode
can also be used to drive a matrix of 128 discrete LEDs.
A logic high selects hexadecimal decoding, while a
logic low bypasses the decoder. When direct control is
selected, the data bits D7 to D0 correspond to the seg-
ment lines of the MAX6955.
Display Blink ModeThe display blinking facility, when enabled, makes the
driver flip automatically between displaying the digit
register data in planes P0 and P1. If the digit register
data for any digit is different in the two planes, then that
digit appears to flip between two characters. To make a
character appear to blink on or off, write the character
to one plane, and use the blank character (0x20) for the
other plane. Once blinking has been configured, it con-
tinues automatically without further intervention.
Blink SpeedThe blink speed is determined by the frequency of the
multiplex clock, OSC, and by the setting of the Blink
Rate Selection Bit B (Table 19) in the configuration reg-
ister. The Blink Rate Selection Bit B sets either fast or
slow blink speed for the whole display.
Initial Power-UpOn initial power-up, all control registers are reset, the
display is blanked, intensities are set to minimum, and
shutdown is enabled (Table 16).
Configuration RegisterThe configuration register is used to enter and exit shut-
down, select the blink rate, globally enable and disable
the blink function, globally clear the digit data, select
between global or digit-by-digit control of intensity, and
reset the blink timing (Tables 17–20 and 22–25).
The configuration register contains 7 bits: S bit selects shutdown or normal operation
(read/write).B bit selects the blink rate (read/write).E bit globally enables or disables the blink function
(read/write).T bit resets the blink timing (data is not stored—tran-
sient bit).R bit globally clears the digit data for both planes P0
and P1 for ALL digits (data is not stored—transient bit).I bit selects between global or digit-by-digit control
of intensity (read/write).P bit returns the current phase of the blink timing
(read only—a write to this bit is ignored).
Character Generator Font MappingThe font is composed of 104 characters in ROM. The
lower 7 bits of the 8-bit digit register represent the char-
acter selection. The most significant bit, shown as x in
the ROM map of Tables 8 and 9, is 1 to light the DP
segment and zero to leave the DP segment unlit.
The character map follows the standard ASCII font for
96 characters in the x0101000 through x1111111
range. The first 16 characters of the 16-segment ROM
map cover 7-segment displays. These 16 characters
are numeric 0 to 9 and characters A to F (i.e., the hexa-
decimal set).
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Multiplex Clock and Blink TimingThe OSC pin can be fitted with capacitor CSETto GND
to use the internal RC multiplex oscillator, or driven by
an external clock to set the multiplex clock frequency
and blink rate. The multiplex clock frequency deter-
mines the frequency that the complete display is updat-
ed. With OSC at 4MHz, each display digit is enabled
for 200µs.
The internal RC oscillator uses an external resistor,
RSET, and an external capacitor, CSET, to set the oscil-
lator frequency. The suggested values of RSET(56kΩ)
and CSET(22pF) set the oscillator at 4MHz, which
makes the blink frequency 0.5Hz or 1Hz.
The external clock is not required to have a 50:50 duty
cycle, but the minimum time between transitions must
be 50ns or greater and the maximum time between
transitions must be 750ns.
The on-chip oscillator may be accurate enough for
applications using a single device. If an exact blink rate
is required, use an external clock ranging between
1MHz and 8MHz to drive OSC. The OSC inputs of multi-
ple MAX6955s can be connected to a common external
clock to make the devices blink at the same rate. The
relative blink phasing of multiple MAX6955s can be syn-
chronized by setting the T bit in the control register for
all the devices in quick succession. If the serial inter-
faces of multiple MAX6955s are daisy-chained by con-
necting the DOUT of one device to the DIN of the next,
then synchronization is achieved automatically by
updating the configuration register for all devices simul-
taneously. Figure 10 is the multiplex timing diagram.
OSC_OUT OutputThe OSC_OUT output is a buffered copy of either the
internal oscillator clock or the clock driven into the OSC
pin if the external clock has been selected. The feature
is useful if the internal oscillator is used, and the user
wishes to synchronize other MAX6955s to the same
blink frequency.
Scan-Limit RegisterThe scan-limit register sets how many 14-segment dig-
its or 16-segment digits or pairs of 7-segment digits are
displayed, from 1 to 8. A bicolor digit is connected as
two monocolor digits. The scan register also limits the
number of keys that can be scanned.
Since the number of scanned digits affects the display
brightness, the scan-limit register should not be used to
blank portions of the display (such as leading-zero sup-
pression). Table 26 shows the scan-limit register format.
Intensity RegistersDigital control of display brightness is provided and
can be managed in one of two ways: globally or indi-
vidually. Global control adjusts all digits together.
Individual control adjusts the digits separately.
The default method is global brightness control, which
is selected by clearing the global intensity bit (I data bit
D6) in the configuration register. This brightness setting
applies to all display digits. The pulse-width modulator
is then set by the lower nibble of the global intensity
register, address 0x02. The modulator scales the aver-
age segment current in 16 steps from a maximum of
15/16 down to 1/16 of the peak current. The minimum
interdigit blanking time is set to 1/16 of a cycle. When
using bicolor digits, 256 color/brightness combinations
are available.
Individual brightness control is selected by setting the
global intensity bit (I data bit D6) in the configuration
register. The pulse-width modulator is now no longer
set by the lower nibble of the global intensity register,
address 0x02, and the data is ignored. Individual digi-
tal control of display brightness is now provided by a
separate pulse-width modulator setting for each digit.
Each digit is controlled by a nibble of one of the four
intensity registers: intensity10, intensity32, intensity54,
and intensity76 for all display types, plus intensity10a,
intensity32a, intensity54a, and intensity76a for the extra
eight digits possible when 7-segment displays are
used. The data from the relevant register is used for
each digit as it is multiplexed. The modulator scales the
average segment current in 16 steps in exactly the
same way as global intensity adjustment.
Table 27 shows the global intensity register format.
Table 28 shows individual segment intensity registers.
Table 29 shows the even individual segment intensity
format. Table 30 shows the odd individual segment
intensity format.
GPIO and Key ScanningThe MAX6955 features five general-purpose input/out-
put (GPIO) ports: P0 to P4. These ports can be individ-
ually enabled as logic inputs or open-drain logic
outputs. The GPIO ports are not debounced when con-
figured as inputs. The ports can be read and the out-
puts set using the 2-wire interface.
Some or all of the five ports can be configured to per-
form key scanning of up to 32 keys. Ports P0 to P4 are
renamed Key_A, Key_B, Key_C, Key_D, and IRQ,
respectively, when used for key scanning. The full key-
scanning configuration is shown in Figure 11. Table 31
is the GPIO data register.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan*Do NOT write to register.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
One diode is required per key switch. These diodes
can be common-anode dual diodes in SOT23 pack-
ages, such as the BAW56. Sixteen diodes would be
required for the maximum 32-key configuration.
The MAX6955 can only scan the maximum 32 keys if
the scan-limit register is set to scan the maximum eight
digits. If the MAX6955 is driving fewer digits, then a
maximum of (4 x n) switches can be scanned, where n
is the number of digits set in the scan-limit register. For
example, if the MAX6955 is driving four 14-segment
digits, cathode drivers O0 to O3 are used. Only 16 keys
can be scanned in this configuration; the switches
shown connected to O4 through O7 are not read.
If the user wishes to scan fewer than 32 keys, then
fewer scan lines can be configured for key scanning.
The unused Key_x ports are released back to their orig-
inal GPIO functionality. If key scanning is enabled,
regardless of the number of keys being scanned, P4 is
always configured as IRQ (Table 32).
The key-scanning circuit utilizes the LEDs’ common-
cathode driver outputs as the key-scan drivers. O0 to
O7 go low for nominally 200µs (with OSC = 4MHz) in
turn as the displays are multiplexed. By varying the
oscillator frequency, the debounce time changes,
though key scanning still functions. Key_x inputs have
internal pullup resistors that allow the key condition to
be tested. The Key_x input is low during the appropri-
ate digit multiplex period when the key is pressed. The
timing diagram of Figure 12 shows the normal situation
where all eight LED cathode drivers are used.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Note:Unused register bits read as zero.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key ScanThe timing in Figure 12 loops over time, with 32 keys
experiencing a full key-scanning debounce over typi-
cally 25.6ms. Four keys are sampled every 1.6ms, or
every multiplex cycle. If at least one key that was not
previously pressed is found to have been pressed dur-
ing both sampling periods, then that key press is
debounced, and an interrupt is issued. The key-scan
circuit detects any combination of keys being pressed
during each debounce cycle (n-key rollover).
Port Configuration RegisterThe port configuration register selects how the five port
pins are used. The port configuration register format is
described in Table 33.
Key Mask RegistersThe Key_A Mask, Key_B Mask, Key_C Mask, and
Key_D Mask write-only registers (Table 34) configure
the key-scanning circuit to cause an interrupt only when
selected (masked) keys have been debounced. Each
bit in the register corresponds to one key switch. The bit
is clear to disable interrupt for the switch, and set to
enable interrupt. Keys are always scanned (if enabled
through the port configuration register), regardless of
the setting of these interrupt bits, and the key status is
stored in the appropriate Key_x pressed register.
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key ScanFigure 11. Key-Scanning Configuration
Figure 12. Key-Scan Timing Diagram
MAX6955
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
Key Debounced RegistersThe Key_A debounced, Key_B debounced, Key_C
debounced, and Key_D debounced read-only registers
(Table 35) show which keys have been detected as
debounced by the key-scanning circuit.
Each bit in the register corresponds to one key switch.
The bit is set if the switch has been correctly
debounced since the register was read last. Reading a
debounced register clears that register (after the data
has been read) so that future keys pressed can be
identified. If the debounced registers are not read, the
key-scan data accumulates. However, as there is no
FIFO in the MAX6955, the user is not able to determine
key order, or whether a key has been pressed more
than once, unless the debounced key status registers
are read after each interrupt, and before the next key-
scan cycle.
Reading any of the four debounced registers clears the
IRQ output. If a key is pressed and held down, the key is
reported as debounced (and IRQ issued) only once. The
key must be detected as released by the key-scanning
circuit, before it debounces again. If the debounced reg-
isters are being read in response to the IRQ being
asserted, then the user should generally read all four
registers to ensure that all the keys that were detected by
the key-scanning circuit are discovered.
Key Pressed RegistersThe Key_A pressed, Key_B pressed, Key_C pressed,
and Key_D pressed read-only registers (Table 36)
show which keys have been detected as pressed by
the key-scanning circuit during the last test.
Each bit in the register corresponds to one key switch.
The bit is set if the switch has been detected as
pressed by the key-scanning circuit during the last test.
The bit is cleared if the switch has not been detected
as pressed by the key-scanning circuit during the last
test. Reading a pressed register does not clear that
register or clear the IRQ output.
Display Test RegisterThe display test register (Table 37) operates in two
modes: normal and display test. Display test mode
turns all LEDs on (including DPs) by overriding, but not
altering, all controls and digit registers (including the
shutdown register), except for the digit-type register
and the GPIO configuration register. The duty cycle,
while in display test mode, is 7/16 (see the Choosing
Supply Voltage to Minimize Power Dissipationsection).
Selecting External Components RSETand
CSETto Set Oscillator Frequency and
Peak Segment CurrentThe RC oscillator uses an external resistor, RSET, and
an external capacitor, CSET, to set the frequency, fOSC.
The allowed range of fOSCis 1MHz to 8MHz. RSETalso
sets the peak segment current. The recommended val-
ues of RSETand CSETset the oscillator to 4MHz, which
makes the blink frequencies selectable between 0.5Hz
and 1Hz. The recommended value of RSETalso sets the
peak current to 40mA, which makes the segment cur-
rent adjustable from 2.5mA to 37.5mA in 2.5mA steps.
ISEG= KL/ RSETmA
fOSC= KF/ (RSETx CSET) MHz
where: = 2240= 5376
RSET= external resistor in kΩ
CSET= external capacitor in pF
CSTRAY= stray capacitance from OSC pin to GND in
pF, typically 2pF
The recommended value of RSETis 56kΩand the rec-
ommended value of CSETis 22pF.
The recommended value of RSETis the minimum
allowed value, since it sets the display driver to the
maximum allowed peak segment current. RSETcan be
set to a higher value to set the segment current to a
lower peak value where desired. The user must also
ensure that the peak current specifications of the LEDs
connected to the driver are not exceeded.
The effective value of CSETincludes not only the actual
external capacitor used, but also the stray capacitance
from OSC to GND. This capacitance is usually in the
1pF to 5pF range, depending on the layout used.
Applications Information
Driving Bicolor LEDsBicolor digits group a red and a green die together for
each display element, so that the element can be lit red
or green (or orange), depending on which die (or both)
is lit. The MAX6955 allows each segment’s current to
be set individually from the 1/16th (minimum current
and LED intensity) to 15/16th (maximum current and
LED intensity), as well as off (zero current). Thus, a
bicolor (red-green) segment pair can be set to 256
color/intensity combinations.
MAX6955
Choosing Supply Voltage to Minimize
Power DissipationThe MAX6955 drives a peak current of 40mA into LEDs
with a 2.2V forward-voltage drop when operated from a
supply voltage of at least 3.0V. The minimum voltage
drop across the internal LED drivers is therefore (3.0V -
2.2V) = 0.8V. If a higher supply voltage is used, the dri-
ver absorbs a higher voltage, and the driver’s power
dissipation increases accordingly. However, if the LEDs
used have a higher forward-voltage drop than 2.2V, the
supply voltage must be raised accordingly to ensure
that the driver always has at least 0.6V of headroom.
The voltage drop across the drivers with a nominal 5V
supply (5.0V - 2.2V) = 2.8V is nearly 3 times the drop
across the drivers with a nominal 3.3V supply (3.3V -
2.2V) = 1.1V. In most systems, consumption is an
important design criterion, and the MAX6955 should be
operated from the system’s 3.3V nominal supply. In
other designs, the lowest supply voltage may be 5V.
The issue now is to ensure the dissipation limit for the
MAX6955 is not exceeded. This can be achieved by
inserting a series resistor in the supply to the MAX6955,
ensuring that the supply decoupling capacitors are still
on the MAX6955 side of the resistor. For example, con-
sider the requirement that the minimum supply voltage
to a MAX6955 must be 3.0V, and the input supply
range is 5V ±5%. Maximum supply current is 35mA +
(40mA x 17) = 715mA. Minimum input supply voltage is
4.75V. Maximum series resistor value is (4.75V -
3.0V)/0.715A = 2.44Ω. We choose 2.2Ω±5%. Worst-
case resistor dissipation is at maximum toleranced
resistance, i.e., (0.715A) 2 x (2.2Ωx 1.05) = 1.18W. The
maximum MAX6955 supply voltage is at maximum
input supply voltage and minimum toleranced resis-
tance, i.e., 5.25V - (0.715A x 2.2Ωx 0.95) = 3.76V.
Low-Voltage OperationThe MAX6955 works over the 2.7V to 5.5V supply
range. The minimum useful supply voltage is deter-
mined by the forward-voltage drop of the LEDs at the
peak current ISEG, plus the 0.8V headroom required by
the driver output stages. The MAX6955 correctly regu-
lates ISEGwith a supply voltage above this minimum
voltage. If the supply drops below this minimum volt-
age, the driver output stages can brown out, and be
unable to regulate the current correctly. As the supply
voltage drops further, the LED segment drive current
becomes effectively limited by the output driver's on-
resistance, and the LED drive current drops. The char-
acteristics of each individual LED in a display digit are
well matched, so the result is that the display intensity
dims uniformly as supply voltage drops out of regula-
tion and beyond.
Computing Power DissipationThe upper limit for power dissipation (PD) for the
MAX6955 is determined from the following equation:= (V+ x 35mA) + (V+ - VLED) (DUTY x ISEGx N)
where:
V+ = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 17)
VLED= LED forward voltage at ISEG
ISEG= segment current set by RSET= Power dissipation, in mW if currents are in mA
Dissipation example:
ISEG= 30mA, N = 17, DUTY = 15/16,
VLED= 2.4V at 30mA, V+ = 3.6V= 3.6V (35mA) + (3.6V - 2.4V)(15/16 x
30mA x 17) = 0.700W
Thus, for a 36-pin SSOP package (TJA= 1 / 0.0118 =
+85°C/W from Operating Ratings), the maximum
allowed ambient temperature TAis given by:
TJ(MAX)= TA+ (PDx TJA) = +150°C
= TA+ (0.700 x +85°C/W)
So TA= +90.5°C. Thus, the part can be operated safely
at a maximum package temperature of +85°C.
Power SuppliesThe MAX6955 operates from a single 2.7V to 5.5V
power supply. Bypass the power supply to GND with a
0.1µF capacitor as close to the device as possible. Add
a 47µF capacitor if the MAX6955 is not close to the
board’s input bulk decoupling capacitor.
2-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
