Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32l4/STM32L4x1/TIM15/SMCR#0x0
slave mode control register
Slave mode selection
Trigger selection
Master/slave mode
Slave mode selection - bit 3
https://github.com/modm-io/cmsis-svd-stm32