M38041FFHKP datasheet(241/387 Pages) RENESAS

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APPLICATION

2.5 Multi-master I2C-BUS interface

Rev.1.00 Jan 14, 2004

REJ09B0212-0100Z

3804 Group (Spec.H)

2.5.8 Notes on programming for SMBUS interface

(1)

Time out process

For a smart battery system, the time out process with a program is required so that the communication

can be completed even when communication is interrupted. It is because there is possibility of

extracting a battery from a PC.

The specifications are defined so that communication has been able to be completed within 25 ms

from START condition to STOP condition and within 10 ms from the ACK pulse to the ACK pulse of

each byte. Accordingly, the following two should be considered as count start conditions.

➀ SDA falling edge caused by SCL/SDA interrupt

This is the countermeasure for a communication interrupt in the middle of from START condition

to a slave address. However, the detection condition must be considered because a interrupt is

also generated by communication from other masters to other slaves.

➁ SMBUS interrupt after receiving slave address

This is the countermeasure for when communication is interrupted from receiving a slave address

until receiving a command.

(2)

Low hold of communication line

The I2C-BUS interface conforms to the I2C-BUS Standard Specifications. However, because the use

condition of SMBUS differs from the I2C-BUS’s, there is possibility of occurrence of the following

problem.

➀ Low hold of SDA line caused by ACK pulse at voltage drop of communication line

When the SMBUS voltage slowly drops, that is caused by extracting a battery from equipment or

turning off a PC’s power or etc., it might be incorrectly treated as the SCL pulse near the threshold

level voltage.

When the SDA is judged “L” in that condition, it becomes the general call and the ACK is transmitted.

However, when the SCL remains “L” at the ACK pulse, the SDA continuously remains “L” until

input of the next SCL pulse.

Countermeasure:

As explained before, start the time out count at the falling of SDA line of START condition and

reset ES0 bit of the S1D register when the time out is satisfied (Note).

Note: Do not use the read-modify-write instruction at this time. Furthermore, when the ES0 bit is

set to “0”, it becomes a general-purpose port ; so that the port must be set to input mode

or “H”.

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