19-5047; 4/15
DS2480B
Serial to 1-Wire Line Driver
BENEFITS AND FEATURES
•
Simplifies the Design of a Low-Cost, Universal
RS-232 COM Port to 1-Wire® Interface
True-Ground
o
Interface to an RS-232 COM Port for Reading
and Writing 1-Wire Devices
o
Works with Bipolar as well as Unipolar Logic
Signals
o
Slew Rate Controlled 1-Wire Pulldown and
Active Pullup to Accommodate Long Lines
and Reduce Radiation
o
Communicates at Data Rates of 9.6kbps
(default), 19.2kbps, 57.6kbps, and 115.2kbps
o
Self-Calibrating Time Base with ±5%
Tolerance for Serial and 1-Wire
Communication
o
User-Selectable RXD/TXD Polarity
Minimizes Component Count When
Interfacing to 5V Based RS232 Systems or
Directly to UARTs
o
Smart Protocol Combines Data and Control
Information Without Requiring Extra Pins
o
Compatible with Optical, IR, and RF to
RS232 Converters
•
Single Product Supports Various iButton® or
1-Wire Device Types for Easy System Integration
o
Supports Reading and Writing at Standard
and Overdrive Speeds
o
Provides Strong Pullup to 5V for Temperature
and EEPROM 1-Wire Devices
o
Programs 1-Wire EPROM Devices with
External 12V Power Supply
o
Programmable 1-Wire Timing and Driver
Characteristics Accommodate a Wide Range
of Slave Device Configurations at Standard
Speed
o
Operates Over 4.5V to 5.5V from -40°C to
+85°C
PIN ASSIGNMENT
GND
1-W
NC
VDD
1
2
3
4
8
7
6
5
RXD
TXD
POL
VPP
8-Pin SO (150 mil)
PIN DESCRIPTION
GND
1-W
NC
V
DD
V
PP
POL
TXD
RXD
- Ground
- 1-Wire Input/Output
- No Connection
- 4.5V to 5.5V
- Optional EPROM
Programming Voltage
- RXD/TXD Polarity Select
- Serial Data from UART
- Serial Data to UART
ORDERING INFORMATION
PART
DS2480B+
DS2480B+T&R
TEMP
RANGE
-40°C to +85°C
-40°C to +85°C
PIN-
PACKAGE
8 SO
8 SO
+ Denotes a lead(Pb)-free/RoHS-compliant package.
T&R = Tape and reel.
1-Wire and iButton are registered trademarks of
Maxim Integrated Products, Inc.
DESCRIPTION
The DS2480B is a serial port to 1-Wire interface chip that supports standard and overdrive speeds. It con-
nects directly to UARTs and 5V RS232 systems. Interfacing to RS232C (±12V levels) requires a passive
clamping circuit and one 5V to ±12V level translator. Internal timers relieve the host of the burden of
generating the time-critical 1-Wire communication waveforms. In contrast to the DS9097(E) where a full
character must be sent by the host for each 1-Wire time slot, the DS2480B can translate each character
Maxim Integrated
1
DS2480B
into eight 1-Wire time slots, thereby increasing the data throughput significantly. In addition, the
DS2480B can be set to communicate at four different data rates, including 115.2kbps, 57.6kbps, and
19.2kbps, with 9.6kbps being the power-on default. Command codes received from the host’s crystal
controlled UART serve as a reference to continuously calibrate the on-chip timing generator. The
DS2480B uses a unique protocol that merges data and control information without requiring control pins.
This approach maintains compatibility to off-the-shelf serial to wireless converters, allowing easy
realization of 1-Wire media jumpers. The various control functions of the DS2480B are optimized for 1-
Wire networks and support the special needs of all current 1-Wire devices including EPROM-based add-
only memories, EEPROM devices, and 1-Wire thermometers. See
Application Note 192: Using the
DS2480B Serial 1-Wire Driver
for detailed software examples.
DETAILED PIN DESCRIPTION
PIN
1
2
3
4
SYMBOL
GND
1-W
N.C.
V
DD
DESCRIPTION
Ground Pin.
Common ground reference and ground return for 1-Wire bus
1-Wire Input/Output Pin.
1-Wire bus with slew-rate-controlled pulldown,
active pullup, ability to switch in V
PP
to program EPROM, and ability to switch
in V
DD
through a low-impedance path to program EEPROM, or perform a
temperature conversion.
No Connection
Power Input Pin.
Power supply for the chip and 1-Wire pullup voltage, 5V
±10%, must
always
be lower than or equal to V
PP
. V
DD
should be derived from
V
PP
by a separate voltage regulator whenever possible.
EPROM Programming Voltage.
12V supply input for EPROM programming.
If EPROM programming is not required, connect this pin directly to the system’s
5V supply.
RXD/TXD Polarity Select.
RXD/TXD polarity select; tied to GND for RS232
(12V or 5V) connection; tied to V
DD
for direct connection to UART chip.
Serial Data from UART.
Data input from host (inverted or true); maximum
voltage swing -0.3V to V
DD
+ 0.3V; for logic thresholds see DC specifications.
Serial Data to UART.
Signal output to host; push-pull driver with CMOS
compatible levels; for true
±12V
RS232 systems an external level translator must
be provided.
5
6
7
8
V
PP
POL
TXD
RXD
OVERVIEW
The DS2480B directly interfaces a 5V serial communication port with its lines TXD (transmit) and RXD
(receive) to a 1-Wire bus. In addition the device performs a speed conversion allowing the data rate at the
communication port to be different from the 1-Wire data rate. Several parameters relating to the 1-Wire
port and its timing as well as the communication speed at both the port and the 1-Wire bus are
configurable. The circuit to achieve these functions is outlined in the block diagram (see Figure 1).
The device gets its input data from the serial communication port of the host computer through pin TXD.
For compatibility with active-high as well as active-low systems, the incoming signal can be inverted by
means of the polarity input POL. The polarity chosen by hard-wiring the logic level of this pin is also
valid for the output pin RXD. If for minimizing the interface hardware an asymmetry between RXD and
TXD is desired, this can be achieved by setting the most significant bit of the speed control parameter to a
1 (see
Configuration Parameter Value Codes).
With the MS bit of the speed control set to 1, the polarity
at TXD is still selected by the logic level at POL, but the polarity at RXD will be the opposite of what the
logic level at POL specifies.
Maxim Integrated
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DS2480B
As data enters the core of the DS2480B’s logic circuitry, it is analyzed to separate data and command
bytes and to calibrate the device’s timing generator. The timing generator controls all speed relations of
the communication interface and the 1-Wire bus as well as the waveforms on the 1-Wire bus.
Command bytes either affect the configuration setting or generate certain waveforms on the 1-Wire bus.
Data bytes are simply translated by the protocol converter into the appropriate 1-Wire activities. Each
data byte generates a return byte from the 1-Wire bus that is communicated back to the host through the
RXD pin as soon as the activity on the 1-Wire bus is completed.
The 1-Wire driver shapes the slopes of the 1-Wire waveforms, applies programming pulses or 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.
Figure 1. DS2480B BLOCK DIAGRAM
DEVICE OPERATION
The DS2480B can be described as a complex state machine with two static and several dynamic states.
Two device-internal flags as well as functions assigned to certain bit positions in the command codes
determine the behavior of the chip, as shown in the state transition diagram (Figure 2). The DS2480B
requires and generates a communication protocol of 8 data bits per character, 1 stop bit and no parity. It is
permissible to use 2 stop bits on the TXD line. However, the DS2480B only asserts a single stop bit on
RXD.
When powering up, the DS2480B performs a master reset cycle and enters the
Command Mode,
which is one of the two
static states. The device now expects to receive one 1-Wire reset command on the TXD line sent by the host at a data rate of
9600bps (see
Communication Commands
section for details). This command byte is required solely for calibration of the baud
rate timing generator the DS2480B and is not translated into any activity on the 1-Wire bus. After this first command byte the
device is ready to receive and execute any command as described later in this document. NOTE: Baud rate calibration is valid
only for the V
DD
operating voltage at which calibration is performed. Post-calibration changes in V
DD
by more than 5% may
cause calibration error to exceed
±5%.
The DS2480B requires a 1-Wire reset command sent by the host at a data rate of
9600bps for calibration. Data rates of 115200bps or higher during calibration may put the DS2480B in an undefined state,
requiring a power-down reset to restore normal operation.
Maxim Integrated
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DS2480B
Figure 2.
STATE TRANSITION DIAGRAM
LEGEND:
V
SS
P
T
Q
H
ZZZ
VVV
X
BINARY VALUE (TYPE OF WRITE TIME SLOT)
1-WIRE SPEED SELECTION CODE
IF LOGIC 1, GENERATES STRONG PULLUP TO 5V IMMEDIATELY FOLLOWING THE
TIME SLOT
TYPE OF PULSE; 0 = STRONG PULLUP (5V), 1 = PROGRAMMING PULSE (12V)
1 = ARM STRONG PULLUP AFTER EVERY BYTE; 0 = DISARM
SEARCH ACCELERATOR CONTROL; 1 = ACCELERATOR ON, 0 = ACCELERATOR OFF
CONFIGURATION PARAMETER CODE (WRITE), 000 = READ CONFIGURATION
PARAMETER
CONFIGURATION PARAMETER VALUE CODE (WRITE), CONFIGURATION PARAMETER
CODE (READ)
DON’T CARE
Maxim Integrated
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DS2480B
A master reset cycle can also be generated by means of software. This may be necessary if the host for
any reason has lost synchronization with the device. The DS2480B will perform a master reset cycle
equivalent to the power-on reset if it detects start polarity in place of the stop bit. The host has several
options to generate this condition. These include making the UART generate a break signal, sending a
NULL character at a data rate of 4800bps and sending any character with parity enabled and selecting
space polarity for the parity bit. As with the power-on reset, the DS2480B requires a 1-Wire reset
command sent by the host at a data rate of 9600bps for calibration.
After the DS2480B has reached the Command Mode, the host can send commands such as 1-Wire Reset,
Pulse, Configuration, Search Accelerator, and Single Bit functions or switch over to the second static
state called
Data Mode.
In Data Mode the DS2480B simply converts bytes it receives at the TXD pin
into their equivalent 1-Wire waveforms and reports the results back to the host through the RXD pin. If
the Search Accelerator is on, each byte seen at TXD will generate a 12-bit sequence on the 1-Wire bus
(see
Search Accelerator
section for details). If the strong pullup to 5V is enabled (see
Pulse Command),
each byte on the 1-Wire bus will be followed by a pause of predefined duration where the bus is pulled to
5V via a low-impedance transistor in the 1-Wire driver circuit.
While being in the Data Mode the DS2480B checks each byte received from the host for the reserved
code that is used to switch back to Command Mode. To be able to write any possible code (including the
reserved one) to the 1-Wire bus, the transition to the Command Mode is as follows: After having received
the code for switching to Command Mode, the device temporarily enters the
Check Mode
where it waits
for the next byte. If both bytes are the same, the byte is sent
once
to the 1-Wire bus and the device returns
to the Data Mode. If the second byte is different from the reserved code, it will be executed as command
and the device finally enters the Command Mode. As a consequence, if the reserved code that normally
switches to Command Mode is to be written to the 1-Wire bus, this code byte must be sent twice
(duplicated). This detail must be considered carefully when developing software drivers for the
DS2480B.
After having completed a memory function with a device on the 1-Wire bus it is recommended to issue a
Reset Pulse. This means that the DS2480B has to be switched to Command Mode. The host then sends
the appropriate command code and continues performing other tasks. If during this time a device arrives
at the 1-Wire bus it will generate a presence pulse. The DS2480B will recognize this unsolicited presence
pulse and notify the host by sending a byte such as XXXXXX01b. The Xs represent undefined bit values.
The fact that the host receives the byte unsolicited together with the pattern 01b in the least significant 2
bits marks the bus arrival. If the DS2480B is left in Data Mode after completing a memory function
command it will not report any bus arrival to the host.
COMMAND CODE OVERVIEW
The DS2480B is controlled by a variety of commands. All command codes are 8 bits long. The most
significant bit of each command code distinguishes between communication and configuration
commands. Configuration commands access the configuration registers. They can write or read any of the
configurable parameters. Communication commands use data of the configuration register in order to
generate activity on the 1-Wire bus and/or (dis)arm the strong pullup after every byte or (de)activate the
Search Accelerator without generating activity on the 1-Wire bus. Details on the command codes are
included in the State Transition diagram (Figure 2). A full explanation is given in the subsequent
Communication Commands
and
Configuration Commands
sections.
Maxim Integrated
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