I2C SDA not going to zero! 3.3V --> 1.14V !

[jimbostlawrence]

Hi,

I have a strange one... I have had some boards made up. Some work but one in particular fails to talk to some I2C sensors. The clock line is correctly jumping between 3.3V and 0V but the data (SDA) line is bouncing between 3.3V and 1.15V !

I changed the code to something simple that simply set data line high for a second and then low but it's just not having it. I can't find any shorts anywhere between this line and anything else! Could the PIC port be damaged?

Cheers

Mark

[Demon]

Are you sure you didn't forget a resistor? (I have pull-down in mind).

PICs are pretty resilient as some have tested here. If you have identical boards and one is acting up, I'd check for missing parts, polarity, etc.

Robert

Edit: I'm assuming the boards, PICs and program are all identical.

[jimbostlawrence]

Yup, all components accounted for. I still need to have another good look at it but I've followed the track from PIC to all sensors on that data line through each Via, looking for possible shorts and nothing. All boards and firmware identical yes.

I have a pull-up on the line for I2C; it just seems like the PIC can't pull the line down, even when I stick on a simple program to pull it down for a second or more.

If I find out what it is/was, I'll let you know. I'd prefer to not have to resort to air gunning off the PIC...

[Jumper]

Hi,

What PIC do you use? Are you sure that pin does not have any other functions that you might have forgotten to set (comparators, AD and similar).

What if you remove the I2C device? Will the data line go low?

[Charlie]

Check the tracks with a meter. I've seen vias with internal cracks cause invisible opens, especially when there is solder resist over them.

[rsocor01]

Start replacing components. Start with the pull-ups resistors, then the sensors and the PIC. If the problem persists then trash the board .

[jimbostlawrence]

Check the tracks with a meter. I've seen vias with internal cracks cause invisible opens, especially when there is solder resist over them.

Many thanks for your reply. I'll try some more continuity tests. Not given up on this one yet.

[jimbostlawrence]

Hi,

What PIC do you use? Are you sure that pin does not have any other functions that you might have forgotten to set (comparators, AD and similar).

What if you remove the I2C device? Will the data line go low?

Many thanks for your reply. It's an 18F26J53. It's set for output so should automatically be digital. The firmware works on other board (identical). I may have to start removing components but they're all QFN types, including the PIC.