ESP32-C6-DevKitM-1 Pins seem completely wrong

[konomikitten]

I've been tinkering with an ESP32-C6 from Waveshare specifically the ESP32-C6-Pico which as far as I can tell follows the pinout of the ESP32-C6-DevKitM-1. Now if I start up MicroPython and check the defined I2S and SPI hardware buses I get this:

>> import machine
>>> machine.I2C(0)
I2C(0, scl=18, sda=19, freq=400000, timeout=50000)
>>> machine.I2C(1)
I2C(1, scl=9, sda=8, freq=400000, timeout=50000)
>>> machine.SPI(1)
SPI(id=1, baudrate=500000, polarity=0, phase=0, bits=8, firstbit=0, sck=6, mosi=7, miso=2)

This seems to be in direct conflict with the pin layout as supplied by Expressif:

I'd expect at least one of the two defined I2C buses to use scl=7, sda=6 but instead they're on the SPI bus!?

Does anyone know why?

Firmware file used: https://micropython.org/download/ESP32_GENERIC_C6/

[have-you-tried-restarting]

Those pins are MicroPython's generic defaults for the chip, not a map of the DevKitM-1 labels. I checked the source.

• I2C: ports/esp32/machine_i2c.h sets the defaults per chip. I2C(0) gets SCL 9 / SDA 8 on the C3 and S3 and SCL 18 / SDA 19 on every other chip, which includes the C6. I2C(1) gets 25 / 26 on the classic ESP32 and SCL 9 / SDA 8 on everything else. That is exactly your output: I2C(0, scl=18, sda=19) and I2C(1, scl=9, sda=8).
• SPI: ports/esp32/machine_hw_spi.c defaults SPI(1) to the chip's IO_MUX pins (SPI2_IOMUX_PIN_NUM_CLK/MOSI/MISO), with a comment that a board can override them. Your sck=6, mosi=7, miso=2 is that default.
• Why they ignore the board: ports/esp32/boards/ESP32_GENERIC_C6 doesn't override any of these. It is a generic "ESP32C6 module" build, with no pin map for the DevKitM-1 or the Waveshare Pico. Boards that do have a map of their own override the defaults in their mpconfigboard.h: for example SEEED_XIAO_ESP32C6 sets I2C0 to SCL 23 / SDA 22, M5STACK_NANOC6 to 1 / 2 and UM_TINYC6 to 7 / 6.

So nothing is wrong. The defaults are only what you get when you pass no pins. The ESP32 docs say any available output-capable pins can be used, so pass the ones you want:

from machine import Pin, I2C

i2c = I2C(0, scl=Pin(7), sda=Pin(6), freq=400000)

machine.SoftI2C(scl=Pin(7), sda=Pin(6)) works the same way if you'd rather not use the hardware controller.

For SPI, the ESP32 quickref says hardware SPI on pins other than the defaults goes through the extra GPIO-multiplexing layer. That limits it to 40 MHz and can affect reliability at high speeds. At lower speeds you can pick any suitable pins.

I checked the pin defaults in the source tree and docs. I haven't run it on a C6 or looked up the Waveshare Pico's pinout, so check your two pins against its pin diagram before wiring.

[konomikitten]

I did look through the source code and it seems to me that the default pins are set per platform not per chip. At least in the case of ESP32 and ignoring the custom board defines. Looking at the Arduino library that Expressif publish themselves the base defines seem to be in this file https://github.com/espressif/arduino-esp32/blob/master/variants/esp32/pins_arduino.h which don't seem to line up with what MicroPython is choosing. And then what I'd expect to be the pins for ESP32_GENERIC_C6 seem to be defined here https://github.com/espressif/arduino-esp32/blob/master/variants/esp32c6/pins_arduino.h and they don't line up with what MicroPython defaults to either.

I just don't understand why MicroPython isn't taking the pin outs Expressif who make the chips in the first place?

[have-you-tried-restarting]

You're right that I overstated it. It isn't really per chip. machine_i2c.h special-cases only the C3 and S3, and every other target, the C6 included, falls into one shared default (SCL 18 / SDA 19 for I2C0, SCL 9 / SDA 8 for I2C1, with the classic ESP32 getting 25 / 26 on I2C1). So it's closer to one default for the whole port with a couple of exceptions.

On why MicroPython doesn't copy Espressif's tables: the file you linked is the Arduino core's variant map. For the generic C6 variant it sets SDA 23 / SCL 22 and SS 18 / MOSI 19 / MISO 20 / SCK 21, which is Arduino's choice of "what Wire and SPI mean on this board". MicroPython is built on ESP-IDF, not on the Arduino core, and as far as I can tell the IDF I2C driver has no default pins at all, you always have to supply sda_io_num and scl_io_num. So there's no official per-chip default for MicroPython to inherit, and it ended up with its own constants. A board port can still override them, which is why the XIAO C6 and a few others differ.

I read this from the sources only and haven't run it on a C6. For your board the practical fix is still passing the pins you want, as in the earlier snippet.

[scruss]

I'd expect at least one of the two defined I2C buses to use scl=7, sda=6 but instead they're on the SPI bus!?

Those are the low power I2C pins: note that GPIO 6 & 7 have the LP_I2C prefix. GPIO 6, 7 & 2 are on the Fast SPI bus of the C6.

ESP32s of all flavours are pretty flexible about pin functions. Best to set and specify the pins you want to use rather than relying on defaults.

[peterhinch]

Waveshare have produced a custom PCB intended to have similar pinout to the Pico. There is a pinout diagram in their docs. I would use this and follow the advice of @scruss re specifying pins in code.