xiao_usarts

Extra hardware serial ports on the SAM D21 XIAO

Three PlatformIO projects that show how to enable up to 3 extra hardware serial ports for a total of four on the SAM D21 XIAO by SeeedStudio.

March 25, 2022

Table of Content

• 1. xiao_usarts.cpp
• 2. 3usarts.cpp
• 3. 4usarts.cpp
• 4. Arduino IDE
• 5. References
• 6. License

1. xiao_usarts.cpp

(in ./xiao_usarts/src/)

This is a one file project that will create 2 additional instances of the Uart class.

Serial2: Uses the ALT-SERCOM0 module which, normally, is set up as a SPI interface.

• TX pin: A10 (= ALT-SERCOM0 Pad 2)
• RX pin: A9 (= ALT-SERCOM0 Pad 1)

Serial3: Uses the ALT-SERCOM2 module which, normally, is set up as an I²C interface.

• TX pin: A4 (= ALT-SERCOM2 Pad 0)
• RX pin: A5 (= ALT-SERCOM2 Pad 1)

Alternatively, defining the USE_ALT_SERIAL macro will set the Serial3 pin assignment as follows:

• TX pin: A2 (= ALT-SERCOM2 Pad 2)
• RX pin: A3 (= ALT-SERCOM2 Pad 3)

Another macro, ORDER_MATTERS sets the order in which the serial port is initialized:

If ORDER_MATTERS is defined, then the sequence is as follows

  Serial3.begin(USART_BAUD);
  pinPeripheral(A2, PIO_SERCOM_ALT);
  pinPeripheral(A3, PIO_SERCOM_ALT);

which is the correct order. If the macro is not defined, the sequence will be

  pinPeripheral(A2, PIO_SERCOM_ALT);
  pinPeripheral(A3, PIO_SERCOM_ALT);
  Serial3.begin(USART_BAUD);

and the A2 and A3 peripheral function will be reset by Serial3.begin() and fail.

2. 3usarts.cpp

(in ./xiao_usarts/3usarts/src/)

Basically the same project as xio_usarts except for two changes.

• The definitions of the Uart instances have been moved to libraries Serial2, Serial3 and Serial3Alt in the lib library (.xio_usarts/3usarts/lib/XIAO_extra_serail.
• The ORDER_MATTERS macro is no longer use and the correct sequence when initializing a serial port is always used.

3. 4usarts.cpp

(in ./xiao_usarts/3usarts/src/)

Added yet another extra serial port whose TX and RX signals are brought out on the SWD pads on the back of the XIAO.

This requires two additional entries in the pin description table in variants.cpp:

 const PinDescription g_APinDescription[]=
 
  ...
  { PORTA, 24, PIO_COM, PIN_ATTR_NONE, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, // USB/DM
  { PORTA, 25, PIO_COM, PIN_ATTR_NONE, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE } // USB/DP
 
  // 17,18 - SWD Interface - added for 4th hardware USART
  ,
  { PORTA, 30, PIO_SERCOM_ALT, PIN_ATTR_NONE, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, //SWCLK, TX: SERCOM1/PAD[2]
  { PORTA, 31, PIO_SERCOM_ALT, PIN_ATTR_NONE, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE } // SWDIO, RX: SERCOM1/PAD[3]
  } ;

4. Arduino IDE

If the Arduino IDE is the preferred development environment, then for each of the three <proj>usarts (where <proj> = xiao_, 3 and 4) :

1. Create a directory name <proj>usarts
2. Copy the file <proj>usarts/src/<proj>usarts.cpp to the newly created directory.
3. Rename the copied file so that its execption is .ino instead of .cpp.
4. For the 3usarts and 4usarts project, copy the content of the <proj>usarts/src/XIAO_extra_serial/ directory into the newly created directory.
5. For the 3usarts project,
   • rename the Serial3Alt library
     • Serial3Alt.h --> Serial3Alt.h.hide
     • Serial3Alt.cpp --> Serial3Alt.cpp.hide
   • make sure that USE_ALT_SERIAL3 is NOT defined in 3usarts.ino.

Here is the directory tree after all these changes.

.
├── 3usarts
│   ├── 3usarts.ino
│   ├── Serial2.cpp
│   ├── Serial2.h
│   ├── Serial3Alt.cpp.hide
│   ├── Serial3Alt.h.hide
│   ├── Serial3.cpp
│   └── Serial3.h
├── 4usarts
│   ├── 4usarts.ino
│   ├── Serial2.cpp
│   ├── Serial2.h
│   ├── Serial3.cpp
│   ├── Serial3.h
│   ├── Serial4.cpp
│   └── Serial4.h
└── xiao_usarts
    └── xiao_usarts.ino

3 directories, 15 files

When the Serial3 alternate pin assignement is to be used, "hide" the Serial3 library and unhide the Serial3Alt library

.
├── 3usarts
│   ├── 3usarts.ino
│   ├── Serial2.cpp
│   ├── Serial2.h
│   ├── Serial3Alt.cpp
│   ├── Serial3Alt.h
│   ├── Serial3.cpp.hide
│   └── Serial3.h.hide

and define USE_ALT_SERIAL3 in 3usarts.ino. It may be necessary to close and restart the IDE if there's a complaint about a twice defined SERCOM2_Handler; a lot of things are cached in that environment.

See Getting Started with Seeeduino XIAO on the SeeedStuoio Wiki for details about using the Arduino IDE and obtaining the correct board defintions.

5. References

• Three, Nay Four Hardware Serial Ports on a SAM D21 XIAO (2022/03/23) by Michel Deslierres

• XIAO Serial Extension 2 (2021/03/03) by kio denshi

• Seeeduino XIAO Serial Communication Interfaces (SERCOM) (2020/05/04-2022/03/23) by Michel Deslierres

6. License

Copyright 2022, Michel Deslierres, no rights reserved.

In those jurisdictions where releasing a work into the public domain may be a problem, the BSD Zero Clause License SPDX:0BSD applies.