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):

• TIM3_CH1 is by default connected to USB_SOF, and not to PA6/PC6/PB4/PE3
• pin PB5 is by default connected to TIM22_CH2 rather than TIM3_CH2
• pin PC9 is by default connected to USB_NOE rather than TIM3_CH4

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