SWDIO and SWCLK pads on XIAO SAMD21

Products & Technology XIAO
1

Is it possible to use the SWDIO and SWCLK pads on the back of the XIAO SAMD21 as general purpose GPIO pins ?

2

According to the circuit diagram,
SWCLK = PA30 and SWDIO = PA31.
Please try it.

2 Likes
3

Thanks for the reply.

I did try to setup as input and output on pin 30,31 but it does not work, locks up the SAMD21, even if you start setup() with a short delay.

pins_Arduino.h apparently has them defined as;

#define D29 (30u)
#define D30 (31u)

(BTW I am fully aware that the same physical pads on the ESP32S3 XIAO can be used as GPIOs, bit its not relevant to the SAMD21 or nRF52840 variants)

4

This post on the Microchip forum;

https://forum.microchip.com/s/topic/a5C3l000000UhMYEA0/t179797

Suggests you can use the CLK and DIO as GPIO pins ………………

1 Like
5

You might need to set the port muxes first.
I delay by a few seconds before doing this to have time to erase or flash via SWD if necessary.

  // Allow time for SWD access...
  delay(5000);

  // Disable peripheral multiplexing to ensure full GPIO control
  PORT->Group[PORTA].PINCFG[30].bit.PMUXEN = 0;  // PA30 (SWCLK)
  PORT->Group[PORTA].PINCFG[31].bit.PMUXEN = 0;  // PA31 (SWDIO)

  // Set pins as outputs
  PORT->Group[PORTA].DIRSET.reg = (1 << 30) | (1 << 31);  // Set PA30 and PA31 as outputs


//*** In loop ***
  // Toggle pins
  PORT->Group[PORTA].OUTTGL.reg = (1 << 30);
  PORT->Group[PORTA].OUTTGL.reg = (1 << 31);

See Datasheet page 29.

SAMD21_PMux
SAMD21_PMux1681×390 62.9 KB

1 Like
6

Excellant, that works !.

After setting up the IO Multiplexing, as per your post, I was able to blink an LED on the SW pins like so;

PORT->Group[PORTA].OUTSET.reg = 1 << 30 ; //set pin 30 high
delay(100);
PORT->Group[PORTA].OUTCLR.reg = 1 << 30 ; //set pin 30 low
delay(899);

And presumably its also possible to set the pins as inputs etc.

Whilst that does allow direct drive of the SW pins would there be a way of passing the pins as a reference for a control pin to an Arduino library, say if you wanted to use one of the SW pins as the select pin on a TFT display or SD card ?

7

You would probably need to modify the library to support those pins. I could check in the morning, 10pm here now. :zzz: