TMP75B-Q1

SBOS721 – OCTOBER 2014

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7.3.3 Serial Interface

The TMP75B-Q1 operates as a slave device only on the two-wire bus and SMBus. Connections to the bus are

made using the open-drain I/O lines, SDA and SCL. The SDA and SCL pins feature integrated spike-suppression

filters and Schmitt triggers to minimize the effects of input spikes and bus noise. The TMP75B-Q1 supports the

transmission protocol for both fast (1 kHz to 400 kHz) and high-speed (1 kHz to 3 MHz) modes. All data bytes

are transmitted MSB first.

7.3.3.1 Bus Overview

The device that initiates the transfer is called a master, and the devices controlled by the master are slaves. The

bus must be controlled by a master device that generates the serial clock (SCL), controls the bus access, and

generates the start and stop conditions.

To address a specific device, initiate a start condition by pulling the data line (SDA) from a high to a low logic

level while SCL is high. All slaves on the bus shift in the slave address byte; the last bit indicates whether a read

or write operation follows. During the ninth clock pulse, the slave being addressed responds to the master by

generating an acknowledge bit and pulling SDA low.

Data transfer is then initiated and sent over eight clock pulses followed by an acknowledge bit. During data

transfer, SDA must remain stable while SCL is high because any change in SDA while SCL is high is interpreted

as a start or stop signal.

After all data have been transferred, the master generates a stop condition indicated by pulling SDA from low to

high, while SCL is high.

7.3.3.2 Serial Bus Address

To communicate with the TMP75B-Q1, the master must first communicate with slave devices using a slave

address byte. The slave address byte consists of seven address bits, and a direction bit indicating the intent of

executing either a read or write operation. The TMP75B-Q1 features three address pins that allow up to eight

devices to be addressed on a single bus. The TMP75B-Q1 latches the status of the address pins at the start of a

communication. Table 2 describes the pin logic levels and the corresponding address values.

Table 2. Address Pin Connections and Slave Addresses

DEVICE TWO-WIRE ADDRESS

A2

A1

A0

1001000

GND

GND

GND

1001001

GND

GND

1001010

GND

GND

1001011

GND

1001100

GND

GND

1001101

GND

1001110

GND

1001111

7.3.3.3 Writing and Reading Operation

Accessing a particular register on the TMP75B-Q1 is accomplished by writing the appropriate value to the pointer

register. The value for the pointer register is the first byte transferred after the slave address byte with the R/W

bit low. Every write operation to the TMP75B-Q1 requires a value for the pointer register (see Figure 9).

When reading from the TMP75B-Q1, the last value stored in the pointer register by a write operation is used to

determine which register is read by a read operation. To change the register pointer for a read operation, a new

value must be written to the pointer register. This action is accomplished by issuing a slave address byte with the

R/W bit low, followed by the pointer register byte. No additional data are required. The master can then generate

a start condition and send the slave address byte with the R/W bit high to initiate the read command. See

Figure 10 for details of this sequence. If repeated reads from the same register are desired, there is no need to

continually send the pointer register bytes because the TMP75B-Q1 stores the pointer register value until it is

changed by the next write operation.

Note that register bytes are sent with the most significant byte first, followed by the least significant byte.

10

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