Philips Semiconductors

Product data sheet

LM75A

Digital temperature sensor and thermal Watchdog

™

2004 Oct 05

12

Data communication

The communication between the host and the LM75A must strictly

protocols for LM75A register read/write operations are illustrated by

the Figures as follows with these definitions:

means that the SCL and SDA lines must be both released by all

devices on the bus, and they become HIGH by the bus pull-up

resistors.

2. The host must provide SCL clock pulses necessary for the

communication. Data is transferred in sequence of 9 SCL clock

pulses for every 8-bit data byte followed by 1-bit status of the

acknowledgement.

3. During data transfer, except the Start and Stop signals, the SDA

signal must be stable while the SCL signal is HIGH. It means

that SDA signal can be changed only during the LOW duration

of the SCL line.

4. S: Start signal, initiated by the host to start a communication,

the SDA goes from HIGH-to-LOW while the SCL is HIGH.

5. RS: Re-start signal, same as the Start signal, to start a read

command that follows a write command.

6. P: Stop signal, generated by the host to stop a communication,

the SDA goes from LOW-to-HIGH while the SCL is HIGH. The

bus becomes free thereafter.

7. W: Write bit, when the Write/Read bit = LOW in a write

command.

8. R: Read bit, when the Write/Read bit = HIGH in a read

command.

9. A: Device Acknowledge bit, returned by the LM75A. It is LOW if

the device works properly and HIGH if not. The host must

release the SDA line during this period in order to give the

device the control on the SDA line.

10. A

′: Master Acknowledge bit, not returned by the device, but set

by the master or host in reading 2-byte data. During this clock

period, the host must set the SDA line to LOW in order to notice

the device that the first byte has been read for the device to

provide the second byte onto the bus.

11. NA: Not-Acknowledge bit. During this clock period, both the

device and host release the SDA line at the end of a data

transfer, the host is then enabled to generate the Stop signal.

12. In a write protocol, data is sent from the host to the device and

the host controls the SDA line, except during the clock period

when the device sends to the bus the device acknowledgement

signal.

13. In a read protocol, data is sent to the bus by the device and the

host must release the SDA line during the time that the device is

providing data onto the bus and controlling the SDA line, except

during the clock period when the master sends to the bus the

master acknowledgement signal.