SMBus, SBS, I2CWrite, I2CRead

[HKMathews]

This is kind of a new direction, but people on this thread may be curious - I am trying to program a smart battery (smbus master), to recognize whether or not a level 2 smart charger (smbus slave) is attached to it. I figure if the charger is not there, I could look for an acknowledge bit to be false when I send the charger address. The problem is, the ACKSTAT bit never sets and is always low (asserted) with no charger present. I thought it would be easy because the master is just talking to an open bus (with pullups attached), but it won't work for me. Here is my code:

SSP1CON1 = %00101000 'MSSP1 is the smbus master
SSP1CON2 = %00000000 'default
SSP1ADD = $27 'baud rate for 100KHz smbus (16 MHz osc, PIC18F67K90)

SSP1IF = 0 'reset interrupt flag
WCOL = 0 'clear the collision flag
SEN = 1 '(SSP1CON2.0) initiate smbus START
DO WHILE SEN = 1 'wait for start delay to complete
LOOP
SSP1BUF = $12 'send address 18 (level 2 smart charger)
SSP1IF = 0 'reset interrupt flag
DO WHILE BF = 1 'wait for buffer to empty
LOOP
IF ACKSTAT = 1 THEN.....(skip smart charger module)

The hardware is functioning correct - the /ACK bit is high at the falling edge of the ninth clock with no charger. I tried to use I2CWRITE but it hangs up for some reason and my 1 second watchdog resets the pic.

[HKMathews]

Solved my own problem! I have to wait at least 1 clock pulse (15 usec) after the BF flag resets (buffer is empty) before looking at ACKSTAT, to allow the ninth clock pulse to complete. My fix:

SEN = 1 '(SSP1CON2.0) initiate smbus START
DO WHILE SEN = 1 'wait for start delay to complete
LOOP
SSP1BUF = $12 'send address 18 (level 2 smart charger)
SSP1IF = 0 'reset interrupt flag
DO WHILE BF = 1 'wait for buffer to empty
LOOP
PAUSEUS 15
IF ACKSTAT = 1 THEN.....(NACK - skip smart charger module)

[tumbleweed]

After you write to the SSPBUF if you wait for SSPIF instead of BF then ACKSTAT will be valid.

[dsicon]

great stuff H & T thanks for the info
i need to learn more about handling the MSSP hardware directly as you are showing

[HKMathews]

T- I tried waiting for the SSP1IF interrupt, but it never happened! The pic spec indicates that you clear it after the start condition is complete (SEN = 1) and you have written to SSP1BUF (PIC18F87K90 Family figure 21-23), but maybe I have to clear it before writing to the buffer.

D - I prefer to talk directly to the MSSP module because I2CWRITE wouldn't let me send just an address and look for a NACK.

[tumbleweed]

I tried waiting for the SSP1IF interrupt, but it never happened!

Hmm. After you initiate the START (SEN=1) and then wait for SEN to go low (SEN=0) you should be able to clear SSPIF before you write to the SSPBUF.

If you look at that figure once you write to the SSPBUF you'll see BF being asserted on the 8th clock, but SSPIF isn't asserted until the 9th after the ACK.