This cleans up the USCI based UART and SPI implementations and allows
multiple instances of either interface to be configured by the
boards. In addition, it allows sharing the USCI peripherals to provide
multiple serial interfaces with the same hardware (round-robin).
- add support for multiple timers
- add support for selecting clock source in the board's `periph_conf.h`
- add support for the prescaler
- implement `periph_timer_query_freqs`
- add a second timer to all MSP430 boards
- the first timer is fast ticking, high-power
- the second is slow ticking, low-power
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
