Low-power timer (LPTIM) RM0440
1474/2126 RM0440 Rev 4
32.4.13 Timer counter reset
In order to reset the content of LPTIM_CNT register to zero, two reset mechanisms are
implemented:
• The synchronous reset mechanism: the synchronous reset is controlled by the
COUNTRST bit in the LPTIM_CR register. After setting the COUNTRST bit-field to '1',
the reset signal is propagated in the LPTIM kernel clock domain. So it is important to
note that a few clock pulses of the LPTIM kernel logic will elapse before the reset is
taken into account. This will make the LPTIM counter count few extra pluses between
the time when the reset is trigger and it become effective. Since the COUNTRST bit is
located in the APB clock domain and the LPTIM counter is located in the LPTIM kernel
clock domain, a delay of 3 clock cycles of the kernel clock is needed to synchronize the
reset signal issued by the APB clock domain when writing '1' to the COUNTRST bit.
• The asynchronous reset mechanism: the asynchronous reset is controlled by the
RSTARE bit located in the LPTIM_CR register. When this bit is set to '1', any read
access to the LPTIM_CNT register will reset its content to zero. Asynchronous reset
should be triggered within a timeframe in which no LPTIM core clock is provided. For
example when LPTIM Input1 is used as external clock source, the asynchronous reset
should be applied only when there is enough insurance that no toggle will occur on the
LPTIM Input1.
It should be noted that to read reliably the content of the LPTIM_CNT register two
successive read accesses must be performed and compared. A read access can be
considered reliable when the value of the two read accesses is equal. Unfortunately
when asynchronous reset is enabled there is no possibility to read twice the
LPTIM_CNT register.
Warning: There is no mechanism inside the LPTIM that prevents the
two reset mechanisms from being used simultaneously. So
developer should make sure that these two mechanisms are
used exclusively.
32.4.14 Encoder mode
This mode allows handling signals from quadrature encoders used to detect angular
position of rotary elements. Encoder interface mode acts simply as an external clock with
direction selection. This means that the counter just counts continuously between 0 and the
auto-reload value programmed into the LPTIM_ARR register (0 up to ARR or ARR down to
0 depending on the direction). Therefore LPTIM_ARR must be configured before starting.
From the two external input signals, Input1 and Input2, a clock signal is generated to clock
the LPTIM counter. The phase between those two signals determines the counting direction.
The Encoder mode is only available when the LPTIM is clocked by an internal clock source.
The signals frequency on both Input1 and Input2 inputs must not exceed the LPTIM internal
clock frequency divided by 4. This is mandatory in order to guarantee a proper operation of
the LPTIM.
Direction change is signalized by the two Down and Up flags in the LPTIM_ISR register.
Also, an interrupt can be generated for both direction change events if enabled through the
DOWNIE bit.
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