This implements `pm_set_lowest()` for the MSP430. Unlike most other
platforms, it intentionally does not use pm_layered. It is pretty
similar to `pm_layered` in that is does use reference counters, but it
uses them for two independent clock sources.
The main difference is that the low frequency clock domain can be
disabled even when the high frequency clock is still active. With the
layers, disabling layer n-1 while layer n is still blocked would not
work.
For super low symbol rates the auxiliary clock (ACLK) is used to
conserve power. But with only 32,678 Hz clock just prescaling will
result in poor bit timing, hence correct modulation control settings
to compensate are needed. Since computing this is too expensive, a
look-up table (as switch statement) for the four most common symbol
rates was used.
The datasheet gave the prescaler values ordered by ascending symbol
rate, the switch statement was ordered descending.
This changes the order to match the datasheets order and matches the
correct prescaler setting to the corresponding symbol rate.
Fixes https://github.com/RIOT-OS/RIOT/issues/20620
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).
