Please add Arduino support for Serial2, Serial3, etc, on the M4 Express boards

[Andy2No]

This have been discussed several times in the Adafruit forum, and appears to still be unresolved. There is apparently no definition for Serial2, Serial3 etc, for these board variants, and no documentation on how to make them work, despite there being configurable SERCOMs that should support them.

For example, search the forum for the words;

M4 Serial2

Here's a search link to do that:

https://forums.adafruit.com/search.php?keywords=serial2+m4&sort=BestMatch

[dhalbert]

Have you seen this: https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports ? It was written before SAMD51 boards came out but it should work the same.

[Andy2No]

I hadn't. Thanks. I'm still not clear how to apply it to each particular M4 board Adafruit makes though, and I'm not clear why Serial was redefined, on the second page, instead of defining Serial2. Is there a reason for that?;

https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports/muxing-it-up

Apart from the individual board and chip details, the recommendation seems to be for us to produce new variants, with the added feature. Perhaps I'm misunderstanding that part.

I would hope that us mere mortals would have the opportunity to just define a new Serial object with a pair of valid pins, e.g. just using this line from that page, but with a more sensible name;

Uart Serial2( &sercom5, PIN_SERIAL_RX, PIN_SERIAL_TX, PAD_SERIAL_RX, PAD_SERIAL_TX ) ;

Is that possible? If so, perhaps all that's needed is a new page of documentation that relates to the current M4 boards, with a table of details for each one.

It would be nice to have a table clearly stating which pins can be used for each board.

[dhalbert]

Uart Serial2( &sercom5, PIN_SERIAL_RX, PIN_SERIAL_TX, PAD_SERIAL_RX, PAD_SERIAL_TX ) ;

That kind of example is given in https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports/creating-a-new-serial. There is also discussion on that page of the SAMD51.

The problem is that the correspondence between pins, pads, and functionality on the SAMD boards is not very regular at all. Also, since SERCOMs can also do I2C, SPI, etc. you may end up blocking out some pins, but only because you've chosen other pins to do other functions. It's very interlocking and not predictable without looking at pinmux tables.

You can do that by hand. You can also use the CircuitPython program given here https://learn.adafruit.com/circuitpython-essentials/circuitpython-uart-serial#wheres-my-uart-2985036 to give you an idea of which pins to use, but you'll still need to specify the pads.

CircuitPython allocates the SERCOMs dynamically, but it can also run into interlocking assignment difficulties because of the order in which it chooses SERCOMs: https://learn.adafruit.com/circuitpython-essentials/circuitpython-uart-serial#trinket-m0-create-uart-before-i2c-2986086

Other chips like the nRF52840 and the ESP line have a full or almost full crossbar, so you can use almost any pin for anything.