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What pinouts are you using for connecting to I²C and SPI modules?

ajcc

Hi!

 

I want to put some pin-headers for modules on a board with a RPi Zero W for sensors and other peripherals on modules (or break out boards). Starting with the assumption that there's a de facto standard for the pinout on modules, I tried to find one with Excel by simply entering the pin-out of different manufacturer's modules into columns. I found that some manufactured seemed to just use whatever worked for a 2-layer board, while others were using a consistent pinout (if you accept that they're sometimes mirrored).

From one manufacturer's modules I found that pin 1. GND, 2. VDD, 3. SDA and 4. SCL was used somewhat consistently for I²C modules (or its mirrored variant). But my joy was short lived when I read the Pmod specification: pin 1. SCL, 3. SDA, 5. GND and 7. VCC (single row on a 2x4 connector). GND and VDD are flipped compared to the other (interestingly this was the most common pinout used by the manufacturer that didn't seem to have a very consistent pinout at first glance). Another standard is the mikroBUS which has a consistent pins for SPI, I²C, UART, Analog, PWM, Power, etc. Their I²C pins are ordered in a row: GND, +5V, SDA and SCL (+3V3 is on another pin).

 

With SPI it was much harder to find something consistent since most SPI boards I found were configurable as I²C or SPI. It's the same story as with I²C when comparing Pmod: CS, SDO, SDI, SCK, GND and VCC, to MikroBUS: CS, SCK, SDI, SDO, 3v3 and GND (using the OSHWA naming scheme).

 

When I was looking at the Pmod connector which has muxed pins and fixed GND and VCC, I started to wonder if it would make more sense to do that with a RPi that have alternative functions for its GPIO pins? I'm even starting to wonder if it makes sense at all to use a smaller pinheader for modules, instead of just wiring them up on a breadboard and not have to think too much about it. One aspect I wanted from using a fixed pinout is that I can put voltage-level converters between the module and RPi (for example to use 5V with I/O expanders or 1V8 with cheap SPI flash memory). The high-to-low or low-to-high might be an issue since some voltage-level converters I've looked at required that the second input is a strictly lower voltage (not equal and absolutely not higher).

 

Anyway... I'm interesting to hear what your thoughts are about this?

  • Don't go with a manufacturer's standard. They usually want to sell only their parts, so the pinout might be deliberately different to make it not compatible to the competition. image

    If you need I2C and SPI, I would go with the pmod standard.

    For I2C only connections, the mikrobus seems to be identical to Sparkfun's Quiic system. Could be helpful to have access to other boards, cables.

     

    Most of all, don't make a new standard, because 927.

    - W.

  • in reply to wolfgangfriedrich

    That's a really good point, Quiic would just need a cable to work with any incompatible I²C board.

     

    If the alternative functions work out with Pmod then that's a really neat solution, so that you're not limited by fixing their function. Adding to your comment about cables to adapt other boards to Quiic, the same could be done with inbetween boards for voltage-levels and an alternative header.

  • Took some time to write up all the alternative pin functions of the Pmod 1.3.0 connector in a spreadsheet. There are three additional modes (or types) for H-bridges that I didn't included here.

    image

    The amazing people over at the Embedded Linux Wiki have already put together a chart over all of the BCM2835 GPIO functions and sadly there's no GPIO/I²C/SPI pins. So getting fully compatible Pmod headers isn't going to work if you're just wires and pin configurations.