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DS2490
USB to 1-Wire Bridge Chip
19-4993; 10/09
DS2490
USB to 1-Wire Bridge Chip
FEATURES PIN ASSIGNMENT Communicates at regular and overdrive 1-
Wire® speeds
Supports stiff 5V pullup for EEPROM,
sensors, and crypto iButton® Slew rate controlled 1-Wire timing and active
pullup to accommodate long 1-Wire network
lines and reduce radiation Programmable 1-Wire timing and driver
characteristics accommodate a wide range of
1-Wire network configurations Low- to high-level command types, including
macros, for generating 1-Wire communication Crystal oscillator timebase provides precision
timed 1-Wire waveforms High-speed 12Mbps Universal Serial Bus
(USB) interface Integrated USB-compliant transceiver Supports USB remote wake-up from a 1-Wire
device event to resume a suspended host
system 0oC to +70oC operating temperature range
VD
VD2
1-WIRE
PMOD
GND13
14
15
16
17
18
19
20
21
22
23
24
SUSO
24-Pin SO
Top View
(300-mil)
ORDERING INFORMATION
PART NUMBER TEMP
RANGE PIN-PACKAGE DS2490S+ 0oC to +70oC 24 SO (300 mil)
DS2490S+T&R 0oC to +70oC 24 SO (300mil)
+ Denotes a lead(Pb)-free/RoHS-compliant package.
T&R = Tape and reel.
DESCRIPTION The DS2490 is a bridge chip that enables communication between a USB host system and a 1-Wire bus.
It provides regular, overdrive, and flexible 1-Wire communication speeds and a full-speed 12Mbps
connection to USB. USB vendor-specific commands defined in this specification are used to control the
DS2490 and communicate with attached 1-Wire devices. A functional block diagram of the DS2490 is
DS2490
SIGNAL SUMMARY Table 1
SIGNAL NAME TYPE FUNCTION VD PWR Power supply input for digital and 1-Wire functions. Range: 5.0 10%
VD2 PWR Second VD supply, must be tied to VD externally to the IC (the two pins
are not tied together inside the package).
VB PWR Power supply input for USB functions. Range: 3.3V 10% supply
regulated from USB supplied VBUS.
D+ I/O USB data—non-inverted of differential data pair.
D- I/O USB data—inverted signal of differential data pair.
1-Wire I/O 1-Wire input/output.
PMOD I Reserved for future use. Must be tied to GND.
SUSO O Suspend Output—buffered USB suspend-state output from USB device
controller. When HIGH the USB is in an active non-suspended state,
when LOW the USB has entered a suspended state. This is an open drain
output and requires an external pullup.
XI I Crystal input. Use a 12.0 MHz, fundamental-mode, parallel-resonant
crystal. A 12.0MHz CMOS clock source may also be used.
XO O Crystal output. Connect to other side of crystal 1 if used.
GND PWR Ground reference and ground return for 1-Wire bus.
NC No connect. For factory use or reserved, do not connect to these pins.
RELATED DOCUMENTS This specification uses terms from and references or complies with the Universal Serial Bus Specification
v1.1, which may be obtained from the USB Implementers Forum website: www.usb.org. The USB
specification is considered to be part of the DS2490 specification.
DOCUMENT ORGANIZATION The remainder of this document is organized into the following major sections:
SECTION SUMMARY OVERVIEW Device functional summary and application examples
1-WIRE I/F CONTROLLER Edge control and timing diagrams of 1-Wire signals
USB COMMUNICATION Configuration model, core and vendor-specific command
summaries
MODE COMMANDS Commands used to configure 1-Wire interface operational
characteristics
CONTROL COMMANDS Commands used to control 1-Wire communication
command processing
COMMUNICATION COMMANDS Commands used to communicate with an attached 1-Wire
device
DEVICE FEEDBACK Technique to obtain device status information
USB TRANSCEIVER Transceiver connection requirements
DS2490
SECTION SUMMARY ELECTRICAL CHARACTERISTICS DC and AC specifications
APPLICATION INFORMATION HW application example
APPENDIX 1 CONTROL COMMANDS—USB setup packet encoding
APPENDIX 2 COMMUNICATION COMMANDS—USB setup packet
encoding
APPENDIX 3 MODE COMMANDS—USB setup packet encoding
APPENDIX 4 USB command and command type constant codes
OVERVIEW The DS2490 directly interfaces a USB port to a 1-Wire bus. As shown in Figure 1, the DS2490
incorporates a USB physical interface, a USB device controller coupled with a 1-Wire-specific USB
function core, and a 1-Wire bus interface controller. The 1-Wire interface controller shapes the slopes of
the 1-Wire waveforms, applies strong pullup to 5V, and reads the 1-Wire bus using a non-TTL threshold
to maximize the noise margin for best performance on large 1-Wire Networks. 1-Wire waveform timing
is accurately controlled with a crystal-based oscillator.
The DS2490 also supports USB remote wake-up which enables the DS2490-based USB peripheral to
send resume signaling to a suspended host system. If the remote wake-up function is enabled and the host
system is in a suspended state, a 1-Wire device attachment will cause the DS2490 to perform a host
system wake-up and allow the 1-Wire device to be serviced.
DS2490 FUNCTIONAL BLOCK DIAGRAM Figure 1 USB
XCVR
USB
DEVICE
CONTROLLER
PWR
CNTL
USB
1-WIRE
FUNCTION
CORE
FIFOS
OSCCLK
GEN
USB
DESC
ROM
1-WIRE
I/F
CONTROLLER
SUSO
1-WIREXOGNDVPPVD2
Typical application examples of the DS2490 are shown in Figure 2 (a to c). As shown in all the examples,
all host control and communication with the device is accomplished over a USB communication link. A
USB vendor-specific command set, as defined in this document, is used to select operational modes
(MODE COMMANDS), control command processing (CONTROL COMMANDS), and communicate
over the 1-Wire interface (COMMUNICATION COMMANDS). Shown in Figure 2, example (a) is a
DS2490-based USB peripheral application. The peripheral function is a USB to 1-Wire adapter and
provides both USB and 1-Wire I/O connections. In this example, the peripheral is attached to the USB
DS2490 the various 1-Wire bus topologies simple multi-drop to complex 1-Wire Network. Example (b) and (c) in
Figure 2 are variations of example (a) in which the DS2490 is embedded in the host computer or a USB
HUB.
DS2490 APPLICATION EXAMPLES Figure 2
1-WIRE INTERFACE CONTROLLER 1-Wire communication commands sent to the DS2490 are ultimately processed by the 1-Wire interface
controller. One of the tasks of the interface controller is to actively shape the edges of the 1-Wire
communication waveforms. This speeds up the recharging of the 1-Wire bus (rising edges) and reduces
ringing of long lines (falling edges). The circuitry for shaping rising edges is always active. The slew rate
of falling edges is actively controlled only at flexible speed and requires the parameter for slew rate
control being different from its power-on default value. See the MODE COMMANDS section for
parameter control and power-on defaults.
All Rising Edges The active pullup of the rising edges reduces the rise time on the 1-Wire bus significantly compared to a
simple resistive pullup. Figure 3 shows how the DS2490 is involved in shaping a rising edge.
DS2490
ACTIVE PULLUP Figure 3 1-Wire bus
is discharged
VIAPO
VIAPTOAPUOTt2t3
The circuit operates as follows: At t1, the pulldown (induced by the DS2490 or a device on the bus) ends.
From this point on the 1-Wire bus is pulled high by the weak pullup current IWEAKPU provided by the
DS2490. The slope is determined by the load on the bus and the value of the pullup current. At t2, the
voltage crosses the threshold voltage VIAPO. Now, the DS2490 switches over from the weak pullup
current IWEAKPU to the higher current IACTPU. As a consequence, the voltage on the bus now rises faster.
As the voltage on the bus crosses the threshold VIAPTO at t3, a timer is started. As long as this timer is on
(tAPUOT), the IACTPU current will continue to flow. After the timer is expired, the DS2490 will switch back
to the weak pullup current.
Falling Edges (DS2490-initiated) Whenever the DS2490 begins pulling the 1-Wire bus low to initiate a time slot, for example, it first turns
off the weak pullup current IWEAKPU. Then, at regular and overdrive speed it will generate a falling edge at
a slew rate of typically 15 V/µs. This value is acceptable for short 1-Wire busses and adequate for
communication at overdrive speed. For 1-Wire networks of more than roughly 30m length, flexible speed
should always be used. One of the parameters that is adjustable at flexible speed is the slew rate of
DS2490-initiated falling edges. The effect of the slew rate control is shown in Figure 4.
SLEW RATE CONTROL Figure 4 low slew rate
high slew rate
0.8 V
pull-down begins
weak pull-up ends,1-Wire bus
is pulled up
Target for
long lines:
4 ± 0.5 µs
Extensive tests have shown that 1-Wire networks with lengths of up to 300m will perform best if the fall
time tF is in the range of 4 ± 0.5µs. This translates into a slew rate of approximately 1V/µs. This slew rate
