General-purpose timers (TIM2 and TIM3) RM0091
300/742 Doc ID 018940 Rev 1
Figure 103. Counter timing diagram, internal clock divided by N
Figure 104. Counter timing diagram, Update event when repetition counter is not
used
Center-aligned mode (up/down counting)
In center-aligned mode, the counter counts from 0 to the auto-reload value (content of the
TIMx_ARR register) – 1, generates a counter overflow event, then counts from the auto-
reload value down to 1 and generates a counter underflow event. Then it restarts counting
from 0.
Center-aligned mode is active when the CMS bits in TIMx_CR1 register are not equal to
'00'. The Output compare interrupt flag of channels configured in output is set when: the
counter counts down (Center aligned mode 1, CMS = "01"), the counter counts up (Center
aligned mode 2, CMS = "10") the counter counts up and down (Center aligned mode 3,
CMS = "11").
In this mode, the direction bit (DIR from TIMx_CR1 register) cannot be written. It is updated
by hardware and gives the current direction of the counter.
The update event can be generated at each counter overflow and at each counter underflow
or by setting the UG bit in the TIMx_EGR register (by software or by using the slave mode
controller) also generates an update event. In this case, the counter restarts counting from
0, as well as the counter of the prescaler.
Timer clock = CK_CNT
Counter register
36
20
1F
Update interrupt flag (UIF)
Counter underflow
Update event (UEV)
CK_INT
00
36
CNT_EN
Timer clock = CK_CNT
Counter register
Update interrupt flag (UIF)
Counter underflow
Update event (UEV)
35 34 33 32 31 30 2F04 03 02 01 0005
Auto-reload register
FF 36
Write a new value in TIMx_ARR
CK_INT
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