- Move common code for USART (shared SPI / UART peripheral) to its
own file and allow sharing the USART peripheral to provide both
UART and SPI in round-robin fashion.
- Configure both UART and SPI bus via a `struct` in the board's
`periph_conf.h`
- this allows allocating the two UARTs as needed by the use case
- since both USARTs signals have a fixed connection to a single
GPIO, most configuration is moved to the CPU
- the board now only needs to decide which bus is provided by
which USART
Note: Sharing an USART used as UART requires cooperation from the app:
- If the UART is used in TX-only mode (no RX callback), the driver
will release the USART while not sending
- If the UART is used to also receive, the application needs to power
the UART down while not expecting something to send. An
`spi_acquire()` will be blocked while the UART is powered up.
RIOT supports two distinct families of the MSP430: The [MSP430 x1xx]
MCU family and the [MSP430 F2xx/G2xx] MCU family. For both incompatible
MCU families the code was located in the msp430fxyz folder, resulting
in case of the UART driver in particularly bizarre code looking roughly
like this:
#ifndef UART_USE_USCI
/* implementation of x1xx peripheral ... */
#else
/* implementation of F2xx/G2xx peripheral ... */
#endif
/* zero shared code between both variants */
This splits the peripheral drivers for USCI and USART serial IP blocks
into separate files and relocates everything in cpu/msp430, similar to
how cpu/stm32 is organized.
[MSP430 x1xx]: https://www.ti.com/lit/ug/slau049f/slau049f.pdf
[MSP430 F2xx/G2xx]: https://www.ti.com/lit/ug/slau144k/slau144k.pdf
