I2C pin choices

sylvie369

I'm trying to use an LIS302DL accelerometer with an I2C interface on a Propeller Protoboard. I set it up with SCL and SDA as pins 0 and 1, but I get no response from the device. To see where the problem was, I took it off and set it up on a Propstick, using pins 28 and 29 there: that worked fine. Then I moved the wires over to pins 0 and 1 (and changed the code), and again, no response. I did dig through the various objects called by the program (lis302dlobject and Basic_i2c_Driver) to see if pin 28 was hard-coded anywhere: that didn't seem to be the problem. I did some other trouble-shooting, such as calling readlocation in the Basic_i2c_Driver directly from my own program: same results. It works if I use pins 28 and 29, and does not work if I use pins 0 and 1. This matters, by the way, because pins 28 and 29 are not available on the Protoboard.

So just now I remembered that on the BS2pe Mobo there are only a couple of different pin options for i2c, and I thought maybe that's the case for the Propeller as well. I moved it over to pins 8 and 9 on the Propstick, and sure enough, it worked. That should work for me on the Protoboard (those pins are available and in a good position), but I'm a bit mystified about why. I'd thought that you could use any consecutive pair of pins for i2c. Are only certain pins available for SCL and SDA i2c connections?

Rayman

Sounds like a pull-up problem to me... You're supposed to have pull-up resistors on SCL and SDA. Parallax puts always puts one on Pin29 (SDA), but leaves it off 28, so maybe you don't need one on SCL.

sylvie369

Ah, I hadn't thought of that. I'll take a look. Basically just run the appropriate resistors from the pins I'm using to one of the +3.3V rails, right?

Thanks.

Rayman

Yes, that's it... I think 10k is a good value.

sylvie369

Yup, a pair of 10K pullup resistors did the trick. Thanks.