General-purpose timers (TIM2 to TIM5) RM0402
486/1163 RM0402 Rev 6
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 (or underflow when downcounting) 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 detail 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 actual counter enable signal CNT_EN is set 1 clock cycle after CEN.
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/32-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 126 and Figure 127 give some examples of the counter behavior when the prescaler
ratio is changed on the fly:
Figure 126. Counter timing diagram with prescaler division change from 1 to 2
MS35833V1
CK_PSC
00
CNT_EN
Timerclock = CK_CNT
Counter register
Update event (UEV)
0
Prescaler control register
1
0
Write a new value in TIMx_PSC
Prescaler buffer
1
0
Prescaler counter
01010101
01 02 03FA FBF7 F8 F9 FC
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