scottbconsult
Posts: 7
Joined: Sat Nov 17, 2018 3:42 am

Which Compute Module 3 pins for I2C bus to R-Pi Display?

Sat Nov 17, 2018 4:25 am

Hello,
Please forgive what may be a duplicate question, but I've been searching the forums and yet am still unsure of the answer to the question of which pins on the SODIMM socket holding a Compute Module 3 should be used for the I2C interface to the official R-Pi Touch Display. We want to use whatever is the default so that we don't have to modify code to get it working. If there is a thread which makes that clear, please point me to it.
Thanks!
Scott

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Raspberry Pi Engineer & Forum Moderator
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Re: Which Compute Module 3 pins for I2C bus to R-Pi Display?

Sat Nov 17, 2018 5:21 pm

https://www.raspberrypi.org/documentati ... display.md
Connect these pins together with jumper wires:

GPIO0 - CD1_SDA
GPIO1 - CD1_SCL
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aBUGSworstnightmare
Posts: 1286
Joined: Tue Jun 30, 2015 1:35 pm

Re: Which Compute Module 3 pins for I2C bus to R-Pi Display?

Mon Nov 19, 2018 8:53 am

I2C0 is used as default.
You can use it from below GPIOs:
GPIO0+1
GPIO28+29
GPIO44+45

You can use the ' i2c0-bcm2708' overlay for re-mapping them.
Name: i2c0-bcm2708
Info: Change i2c0 pin usage. Not all pin combinations are usable on all
platforms - platforms other then Compute Modules can only use this
to disable transaction combining.
Load: dtoverlay=i2c0-bcm2708,<param>=<val>
Params: sda0_pin GPIO pin for SDA0 (deprecated - use pins_*)
scl0_pin GPIO pin for SCL0 (deprecated - use pins_*)
pins_0_1 Use pins 0 and 1 (default)
pins_28_29 Use pins 28 and 29
pins_44_45 Use pins 44 and 45
pins_46_47 Use pins 46 and 47
combine Allow transactions to be combined (default
"yes")


scottbconsult
Posts: 7
Joined: Sat Nov 17, 2018 3:42 am

Re: Which Compute Module 3 pins for I2C bus to R-Pi Display?

Wed Nov 21, 2018 5:45 pm

Sorry to bring this up again, but I want to confirm one detail: is it true that the CM3 has pull-ups on those I2C signals? Or, since the I2C signals can be mapped to different GPIOs, are we responsible for putting the pull-ups on our host board? Looking at the various schematics for the CM3, the CMIO and the CMCDA, I don't see pull-ups.
Thanks,
Scott

aBUGSworstnightmare
Posts: 1286
Joined: Tue Jun 30, 2015 1:35 pm

Re: Which Compute Module 3 pins for I2C bus to R-Pi Display?

Wed Nov 21, 2018 5:54 pm

scottbconsult wrote: Looking at the various schematics for the CM3, the CMIO and the CMCDA, I don't see pull-ups.
look for the pull-ups at the camera connector. They are on CMIO, not on the Compute Module

scottbconsult
Posts: 7
Joined: Sat Nov 17, 2018 3:42 am

Re: Which Compute Module 3 pins for I2C bus to R-Pi Display?

Wed Nov 21, 2018 8:03 pm

Ah, excellent! It wasn't clear from the schematic which I2C buses those were.

Thanks,
Scott

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