Electronics

DIT Adafruit FeatherM0 RS232 Library

The source code can be found on GitHub.

2017-06-05 09:42

This library provides:

• The RS232_configure() function, that configures SERCOM0 of the MCU to use the pins that are connected to Feather's JP1.2 (labelled TX) and JP1.3 (labelled RX).
• The RS232_enable() function, that enables the USART and the on_received and on_transmitted callback functions (if assigned).

See the 02_RS232 tutorial for a demo on how to use the library.

Adafruit_FeatherM0_RS232.h

Adafruit_FeatherM0_RS232.c

RS232_cofigure()

In lines 43..65 we find the function RS232_configure() that configures SERCOM0 of an Atmel ATSAMD21G18A MCU for RS232 communication.

• Line 47 creates a variable of the usart_config type
• This variable is initialized with the default USART configuration in line 49
• Line 51 sets the desired baud rate
• Line 52 defines that the RX line of the USART should be routed to pad 3 and the TX line to pad 2
• The USART pads 0 and 1 are not used (lines 53 and 54)

All this pad and pinmux voodoo comes from the fact, that modern MCUs have more function-blocks than they have pins available to connect them all to the outside world.

Since usually not all the function-blocks are needed for every application the MCU designers work around that by connecting the pins to more than one MCU function-block using pin-multiplexing (pinumx). During the initialization of the MCU the application designer can connect the function-blocks in- and outputs (pads) to the pins they want them use to communicate to the outside world.

• Line 55 connects USART0 pad2 to the PA10 pin of the MCU
• Line 56 connects USART0 pad3 to the PA11 pin of the MCU
• Line 59 tries to initialize the USART until the initialization was successful

RS232_configure_callback()

In lines 70..81 we find the function RS232_configure_callback() that assigns a callback function to an USART callback event and enables the callback if a function is assigned.

Callback functions are called by the ASF USART module, for example:

• When the receive buffer is filled with characters received (USART_CALLBACK_BUFFER_RECEIVED)
• When the content of a buffer was transferred (USART_CALLBACK_BUFFER_TRANSMITTED)

There are a couple of other events you can attach a callback function to (see usart.h: enum usart_callback), depending on you application's needs.

• Line 75 checks if a callback_func
if
• Line 76 registers callback_func with the callback_type of the USART
• Line 77 enables the callback_type for the USART
otherwise
• Line 79 disables the callback_type for the USART

RS232_enable()

In lines 86..90 we find the function RS232_enable() that enables the USART.

There is nothing fancy here. We just call the usart library function. This function only exists to have a complete RS232 API.