cpu/esp32/include/periph_cpu.h overrides the default definition of adc_res_t from periph/adc with a definition which contains only four resolution, two new resolutions and two resolutions defined by the default definition of adc_res_t. This gives compilation errors if an application uses other resolutions. According to the documentation, adc_sample should return -1 if the resolution is not supported. All other CPUs override adc_res_t either to add new resolutions or to mark resolutions as unsupported. But they all allow to use them at the interface. Therefore, esp32 overrides now the definition of adc_res_t with all resolutions that are defined by the default definition of adc_res_t and new platform specific resolutions. It returns -1 if a resolution is used in adc_sample that is not supported.
Some ESP32 boards (like my SparkFun ESP32 Thing) have a main clock
crystal that runs at 26MHz, not 40MHz. RIOT appears to assume 40MHz.
The mismatch causes the UART to not sync properly, resulting in
garbage written to the terminal instead of log output.
I’ve added:
* A new board configuration constant ESP32_XTAL_FREQ that defaults
to 40, but can be overridden by a board def or at build time to
force a specific value (i.e. 26).
* Some code spliced into system_clk_init() to check this constant and
call rtc_clk_init() to set the correct frequency.
* A copy of the rtf_clk_init() function from the ESP-IDF sources.
Fixes#10272
