The RTC and RTT share the same peripheral, so they can also
share the same code.
This is needed to integrate the Tamper Detection into common
RTC/RTT code.
We don't need to read-modify-write the CTRLA register to disable
the UART.
The entire CTRLA register is re-written just a few lines below, so
we can just set it to 0 to disable the UART.
There is also no need to reset the UART since we re-write all config
registers in init.
The Atmel I2C peripheral supports arbitrary I2C frequencies.
Since the `i2c_speed_t` enum just encodes the raw frequency values,
we can just use them in the peripheral definition.
We just have to remove the switch-case block that will generate an error
for values outside of `i2c_speed_t`.
Currently only samd21 used the 32 kHz clock for EXTI which makes it miss fast events.
All newer members of the family use the MAIN clock.
While touching this, also make the clock source configurable to this can be overwritten,
e.g. in the board config if desired.
If a write to a full tsrb is attempted with disabled interrupts
or in a interrupt then a deadlock will occure. To avoid this make
space in the ringbuffer by synchronously writing to uart.
The UART TX and TX lines on SAMD5x and SAML1x can be inverted.
However, the flags don't do exactly what one would expect.
See errata 2.18.5: SERCOM-UART: TXINV and RXINV Bits Reference:
> The TXINV and RXINV bits in the CTRLA register have inverted functionality.
>
> Workaround:
> In software interpret the TXINV bit as a functionality of RXINV, and conversely,
> interpret the RXINV bit as a functionality of TXINV.
When the SPI peripheral is disabled, the output lines will become HIGH-Z.
If the clk pin is not pulled HIGH or LOW, connected SPI slaves will start drawing current expectedly.
Writing a 1 bit clears the interrupt flag, writing with |= is thus
uneccecary (and actually an error as this would clear *all* flags).
This cleanup was already done for rtt.c, but rtc.c missed out.
The sam0 MCUs all have a DAC peripheral.
The DAC has a resulution of 10 or 12 bits and can have one or two
output channels.
The output pins are always hard-wired to PA2 for DAC0 and PA5 for DAC1
if it exists.
On the same54-xpro I would only get a max value of ~1V when using the
internal reference, so I configured it to use an external voltage reference.
The external reference pin is hard-wired to PA3, so you'll have to connect
that to 3.3V to get results.
