RTC prescaler overflow on nRF(52)

[ChristianHirsch]

Describe the bug
when setting prescaler of a (nRF52840) RTC to a value greater than 256, the value overflows. therefore, it is not possible to use all the 12bit of the prescaler as mentioned in the nRF52840 datasheet.

To Reproduce
Steps to reproduce the behavior:

1. set prescaler to a value >256 (e.g., RTC2 to 4096)
2. execute on nRF52840:

struct device *counter_dev = device_get_binding(DT_RTC_2_NAME);
counter_start(counter_dev);
k_sleep(1000);
counter_stop(counter_dev);
u32_t ticks = counter_read(counter_dev);

Expected behavior
the value of tick should be 8, however it is 128. the counter is 16 times faster because of the overflow: 256 * 16 = 4096).

Screenshots or console output
warning indicating the overflow:

zephyr/drivers/counter/counter_nrfx_rtc.c: In function 'counter_2_init':
zephyr/include/generated/generated_dts_board_unfixed.h:661:53: warning: unsigned conversion from 'int' to 'u8_t' {aka 'unsigned char'} changes value from '4095' to '255' [-Woverflow]
 #define DT_NORDIC_NRF_RTC_40024000_PRESCALER        4096
zephyr/include/generated/generated_dts_board_unfixed.h:663:53: note: in expansion of macro 'DT_NORDIC_NRF_RTC_40024000_PRESCALER'
 #define DT_NORDIC_NRF_RTC_RTC_2_PRESCALER           DT_NORDIC_NRF_RTC_40024000_PRESCALER
                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/chirsch/workspace/zephyrproject/zephyr/drivers/counter/counter_nrfx_rtc.c:662:5: note: in expansion of macro 'DT_NORDIC_NRF_RTC_RTC_2_PRESCALER'
     DT_NORDIC_NRF_RTC_RTC_##idx##_PRESCALER - 1);  \
     ^~~~~~~~~~~~~~~~~~~~~~
/home/chirsch/workspace/zephyrproject/zephyr/drivers/counter/counter_nrfx_rtc.c:702:1: note: in expansion of macro 'COUNTER_NRF_RTC_DEVICE'
 COUNTER_NRF_RTC_DEVICE(2);

Environment (please complete the following information):

• nRF52840 (particle_xenon board)
• OS: Linux
• Zephyr SDK 0.10.3
• 33929cb

Additional context
the effected function is static int init_rtc(struct device *dev, u8_t prescaler) in zephyr/drivers/counter/counter_nrfx_rtc.c. I already tried to change u8_t prescaler to u16_t prescaler wich eliminates the warning, but zephyr crashes..

[ChristianHirsch]

I just realized that I had some settings in the prj.conf file which caused the zephyr crashes. changing u8_t prescaler to u16_t prescaler actually fixed the problem.