General-purpose timers (TIM2/TIM3) RM0367
508/1043 RM0367 Rev 7
The OPM waveform is defined by writing the compare registers (taking into account the
clock frequency and the counter prescaler).
• The t
DELAY
is defined by the value written in the TIMx_CCR1 register.
• The t
PULSE
is defined by the difference between the auto-reload value and the compare
value (TIMx_ARR - TIMx_CCR1+1).
• Let’s say one want to build a waveform with a transition from ‘0 to ‘1 when a compare
match occurs and a transition from ‘1 to ‘0 when the counter reaches the auto-reload
value. To do this PWM mode 2 must be enabled by writing OC1M=111 in the
TIMx_CCMR1 register. Optionally the preload registers can be enabled by writing
OC1PE=1 in the TIMx_CCMR1 register and ARPE in the TIMx_CR1 register. In this
case one has to write the compare value in the TIMx_CCR1 register, the auto-reload
value in the TIMx_ARR register, generate an update by setting the UG bit and wait for
external trigger event on TI2. CC1P is written to ‘0 in this example.
In our example, the DIR and CMS bits in the TIMx_CR1 register should be low.
Since only 1 pulse (Single mode) is needed, a 1 must be written in the OPM bit in the
TIMx_CR1 register to stop the counter at the next update event (when the counter rolls over
from the auto-reload value back to 0). When OPM bit in the TIMx_CR1 register is set to '0',
so the Repetitive Mode is selected.
Particular case: OCx fast enable:
In One-pulse mode, the edge detection on TIx input set the CEN bit which enables the
counter. Then the comparison between the counter and the compare value makes the
output toggle. But several clock cycles are needed for these operations and it limits the
minimum delay t
DELAY
min we can get.
If one wants to output a waveform with the minimum delay, the OCxFE bit can be set in the
TIMx_CCMRx register. Then OCxRef (and OCx) is forced in response to the stimulus,
without taking in account the comparison. Its new level is the same as if a compare match
had occurred. OCxFE acts only if the channel is configured in PWM1 or PWM2 mode.
For code example, refer to A.11.16: One-Pulse mode code example.
21.3.11 Clearing the OCxREF signal on an external event
1. The external trigger prescaler should be kept off: bits ETPS[1:0] in the TIMx_SMCR
register are cleared to 00.
2. The external clock mode 2 must be disabled: bit ECE in the TIMx_SMCR register is
cleared to 0.
3. The external trigger polarity (ETP) and the external trigger filter (ETF) can be
configured according to the application’s needs.
For code example, refer to A.11.10: ETR configuration to clear OCxREF code example.
Figure 139 shows the behavior of the OCxREF signal when the ETRF input becomes high,
for both values of the OCxCE enable bit. In this example, the timer TIMx is programmed in
PWM mode.
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