Hello! First time posting here, hoping I have this in the right place.
I have 5 boards that someone else built for me (who is sadly no longer available to consult with), each with two SX1509 breakouts on them. I’m trying to control them all from a Pi Zero. I have each board’s I2C pins hooked up to different pins on the Pi, with custom I2C busses configured for each.
Four of the 5 boards work fine in this configuration; i2cdetect finds both SX1509s configured at each address no problem. But board #5 is having problems; the i2cdetect command just times out trying to query each address, and it doesn’t find either chip.
Testing with the voltmeter I found that the 4 working boards correctly have SDA and SCL pulled up to 3.3V. However, for the non-working board, SDA floats around 2.5V and SCL at 50mV.
I did a bunch of reading online about pull-up resistors and the like, and from the sounds of it, the SX1509 is supposed to have internal pull-up resistors for SDA and SCL, but it’s possible the guy that built these boards for me cut (intentionally or otherwise) the pull-up resistors on this one board? At least, that’s how I interpret the resulting voltage.
So I figured: no problem, just need an external pair of pull-ups, right? So I got some 4k7 resistors and put them on.
Now, SDA reads 3.3V and SCL…still at 50mV. ?!
I’ve quadruple-checked my work, figuring I must’ve screwed up the pull-up for SCL somehow, and I’ve tried both larger and smaller resistors just in case, but nothing changes, it just sits at 50mV the entire time.
Now, I’m not a hardware guy, I’m a software guy; the ins and outs of actual electrical circuits baffle me (I’m trying!) So I don’t know even where to go from here to troubleshoot, so I’m hoping someone on these forums can help? I don’t think I can remove either SX1509 from their breakout board (they’re both soldered on as far as I can tell), and I don’t know based on what I’m seeing if one or both of those chips are just bad or what? (I’ve checked voltage on the exposed tops of the pins on the 1509s themselves; 3.3V reads correctly, SDA and SCL give me the same values as the connectors on the board).
Any help, tips, or even just general steerage in the right direction would be greatly appreciated!