Цитата
18.3.3 Clock selection
The counter clock can be provided by the following clock sources:
• Internal clock (CK_INT)
• External clock mode1 (for TIM9): external input pin (TIx)
• External clock mode2: external trigger input (ETR connected internally to LSE)
• Internal trigger inputs (ITRx) (for TIM9): connecting the trigger output from another
timer. Refer to Section : Using one timer as prescaler for another timer for more details.
А дальше вдумчиво читать описания slave mode и TIMx_SMCR.
Конкретикой не поделюсь - сам я этим режимом не пользовался.