Actions
Forum Thread Details
  • State Not Answered
  • Replies 6 replies
  • Subscribers 6 subscribers
  • Views 2092 views
  • Users 0 members are here
Related

i2c bus ACK problems

Former Member

Hallo all,

I’m designing an i2c bus at the moment with an ADuC842 master and a PCF 8574 i2c expander slave. I have a 74HC573 latch in between the master and slave, which I believe to be standard practice.. The problem that I am facing is that I cannot get the acknowledge signal to show up on the master side of the latch. When an ACK is generated by my PCF the pulldown brings the SDAT line from 5 volts to about 2. It shows up quite obviously on the slave side but not on the master side. I am pretty sure the issue is that the latch is not backwards transparent is blocking the pulldown from propagating through to the master side OR the pulldown is not sufficient and the ACK is being detected as logic 1.

Solutions tried:

Shorting out latch -> didn’t work

Swapping PCF chip -> didn’t work

Any ideas???

It would be the job if someone could shed a bit of light on this for me or if you have experienced it yourself..?

 

Regards,

Ronan

image

Parents
  • 0

    Why do you have the latch? This is certainly not normal for I2C. I2C devices MUST be open-drain so that both master & slaves can pull the SCL and SDA lines down. Remove the latch completely. You can see from your waveforms that the slave is trying to ACK but can't pull down the output of the latch. Anyway how do you expect the master to see the ACK when you have a latch in the way?

Reply
  • 0

    Why do you have the latch? This is certainly not normal for I2C. I2C devices MUST be open-drain so that both master & slaves can pull the SCL and SDA lines down. Remove the latch completely. You can see from your waveforms that the slave is trying to ACK but can't pull down the output of the latch. Anyway how do you expect the master to see the ACK when you have a latch in the way?

Children