RM0377 Datasheet, PDF (420/862 Pages) STMicroelectronics – This reference manual targets application developers

RM0377
General-purpose timers (TIM21/22)
17.3
17.3.1
TIM21/22 functional description
Time-base unit
The main block of the timer is a 16-bit counter with its related auto-reload register. The
counters counts up, down or both up and down but also down or both up and down. The
counter clock can be divided by a prescaler.
The counter, the auto-reload register and the prescaler register can be written or read by
software. This is true even when the counter is running.
The time-base unit includes:
• Counter register (TIMx_CNT)
• Prescaler register (TIMx_PSC)
• Auto-reload register (TIMx_ARR)
The auto-reload register is preloaded. Writing to or reading from the auto-reload register
accesses the preload register. The content of the preload register are transferred into the
shadow register permanently or at each update event (UEV), depending on the auto-reload
preload enable bit (ARPE) in TIMx_CR1 register. The update event is sent when the counter
reaches the overflow and if the UDIS bit equals 0 in the TIMx_CR1 register. It can also be
generated by software. The generation of the update event is described in detailed for each
configuration.
The counter is clocked by the prescaler output CK_CNT, which is enabled only when the
counter enable bit (CEN) in TIMx_CR1 register is set (refer also to the slave mode controller
description to get more details on counter enabling).
Note that the counter starts counting 1 clock cycle after setting the CEN bit in the TIMx_CR1
register.
Prescaler description
The prescaler can divide the counter clock frequency by any factor between 1 and 65536. It
is based on a 16-bit counter controlled through a 16-bit register (in the TIMx_PSC register).
It can be changed on the fly as this control register is buffered. The new prescaler ratio is
taken into account at the next update event.
Figure 118 and Figure 119 give some examples of the counter behavior when the prescaler
ratio is changed on the fly.
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