timer_set has no documented restriction on this being not null, other
implementations explicitly tolerate it (rpx0xx checks inside the ISR,
but doing it at init time keeps the ISR slim).
This is useful when using a timer just to read, without any action when
it triggers (the action is taken depending on read values, eg. in a
thread context).
- boot the I2C after init in low power mode
- otherwise I2C will consume more power until the first time it is
used, which is surprising
- STM32 F1 only: reconfigure SCL and SDA as GPIOs while the I2C
peripheral is powered down
- When the I2C peripheral is not clocked, it drives SCL and SDA
down. This will dissipate power across the pull up resistor.
- 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
The functions `uart_poweron()`, `uart_poweroff()` and `uart_mode()`
can share code between the UART (UART without EasyDMA) and UARTE
(UART with EasyDMA) implementations, so let's do that.
- nRF51: Use `uart_conf_t` for consistency with nRF52
- nRF52832: Use UARTE (UART with EasyDMA) over UART (without DMA), as
done for all other nRF52 family members
- use `UARTE_PRESENT` to detect whether an UARTE can be used, rather
than family names
The `i2c_read_bytes()` and `i2c_write_bytes()` function return the
number of bytes written / read, instead of `0` as the API contract
says. This fixes the issue.
With only 8 possible prescalers, we can just loop over the values
and shift the clock. In addition to being much easier to read, using
shifts over divisions can be a lot faster on CPUs without hardware
division.
In addition an `assert()` is added that checks if the API contract
regarding the SPI frequency is honored. If the requested clock is too
low to be generated, we should rather have a blown assertion than
hard to trace communication errors.
Finally, the term prescaler is used instead of divider, as divider may
imply that the frequency is divided by the given value n, but
in fact is divided by 2^(n+1).
Previously, the /CS signal was performed by enabling / disabling the
SPI peripheral. This had the disadvantage that clock polarity settings
where not applied starting with `spi_acquire()`, as assumed by e.g.
the SPI SD card driver, but only just before transmitting data.
Now the SPI peripheral is enabled on `spi_acquire()` and only disabled
when calling `spi_release()`, and the `SPI_CR2_SSOE` bit in the `CR2`
register is used for hardware /CS handling (as supposed to).
