STMicroelectronics STM32F05 series Reference Manual page 330

General-purpose timers (TIM2 and TIM3)
Bit 2 URS: Update request source
Bit 1 UDIS: Update disable
Bit 0 CEN: Counter enable
Note: External clock, gated mode and encoder mode can work only if the CEN bit has been
CEN is cleared automatically in one-pulse mode, when an update event occurs.
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This bit is set and cleared by software to select the UEV event sources.
0: Any of the following events generate an update interrupt or DMA request if enabled.
These events can be:
– Counter overflow/underflow
– Setting the UG bit
– Update generation through the slave mode controller
1: Only counter overflow/underflow generates an update interrupt or DMA request if enabled.
This bit is set and cleared by software to enable/disable UEV event generation.
0: UEV enabled. The Update (UEV) event is generated by one of the following events:
– Counter overflow/underflow
– Setting the UG bit
– Update generation through the slave mode controller
Buffered registers are then loaded with their preload values.
1: UEV disabled. The Update event is not generated, shadow registers keep their value
(ARR, PSC, CCRx). However the counter and the prescaler are reinitialized if the UG bit is
set or if a hardware reset is received from the slave mode controller.
0: Counter disabled
1: Counter enabled
previously set by software. However trigger mode can set the CEN bit automatically by
hardware.
Doc ID 018940 Rev 1
RM0091