STM32 gotchas
154. In 'L07x, some TIM3_CHx pins don't work - unless remapped in TIM3_OR

In some STM32, some of the timer input channels can be switched from the usual GPIO connection (through the AF matrix) into an alternative signal source, usually an internal clock or other singal. This enables to conveniently measure mtual relationship between clocks, or for example to count number of occurences of some event. Such feature is usually controlled by so called remap bits in TIMx_OR (Option Register), their default being the usual GPIO connection.

In STM32L07x, there are some surprising default mappings of some of pins related to TIM3 channels (in the 'L0xx family, only 'L07x has TIM3):

To be able to use TIM3_CHx on these pins, the respective remap bits in TIM3_OR have to be set.

Issue spotted by user JZhan.33.