DS1631/DS1631A/DS1731

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2-WIRE SERIAL DATA BUS

The DS1631, DS1631A, and DS1731 communicate over a bidirectional 2-wire serial data bus that

consists of a serial clock (SCL) signal and serial data (SDA) signal. The DS1631, DS1631A, and DS1731

interface to the bus through their SCL input pins and open-drain SDA I/O pins.

The following terminology is used to describe 2-wire communication:

Master Device: Microprocessor/microcontroller that controls the slave devices on the bus. The master

device generates the SCL signal and START and STOP conditions.

Slave: All devices on the bus other than the master. The DS1631, DS1631A, and DS1731 always

function as slaves.

Bus Idle or Not Busy: Both SDA and SCL remain high. SDA is held high by a pullup resistor when the

bus is idle, and SCL must either be forced high by the master (if the SCL output is push-pull) or pulled

high by a pullup resistor (if the SCL output is open-drain).

Transmitter: A device (master or slave) that is sending data on the bus.

Receiver: A device (master or slave) that is receiving data from the bus.

START Condition: Signal generated by the master to indicate the beginning of a data transfer on the

bus. The master generates a START condition by pulling SDA from high to low while SCL is high (see

Figure 8). A “repeated” START is sometimes used at the end of a data transfer (instead of a STOP) to

indicate that the master will perform another operation.

STOP Condition: Signal generated by the master to indicate the end of a data transfer on the bus. The

master generates a STOP condition by transitioning SDA from low to high while SCL is high (see Figure

8). After the STOP is issued, the master releases the bus to its idle state.

Acknowledge (ACK): When a device is acting as a receiver, it must generate an acknowledge (ACK) on

the SDA line after receiving every byte of data. The receiving device performs an ACK by pulling the

SDA line low for an entire SCL period (see Figure 8). During the ACK clock cycle, the transmitting

device must release SDA. A variation on the ACK signal is the “not acknowledge” (NACK). When the

master device is acting as a receiver, it uses a NACK instead of an ACK after the last data byte to indicate

that it is finished receiving data. The master indicates a NACK by leaving the SDA line high during the

ACK clock cycle.

Slave Address: Every slave device on the bus has a unique 7-bit address that allows the master to access

the corresponding input pins. The three address pins allow up to eight DS1631s, DS1631As, or DS1731s

to be multidropped on the same bus.

Control Byte: The control byte is transmitted by the master and consists of the 7-bit slave address plus a

Command Byte: The command byte can be any of the command protocols described in the COMMAND

SET section of this data sheet.

Figure 7. CONTROL BYTE

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

1

0

0

1

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