ST STM32L4+ Series Reference Manual page 1480

Low-power timer (LPTIM) applied to STM32L4Rxxx and STM32L4Sxxx only
41.4.4 LPTIM reset and clocks
The LPTIM can be clocked using several clock sources. It can be clocked using an internal
clock signal which can be chosen among APB, LSI, LSE or HSI16 sources through the
Reset and Clock controller (RCC). Also, the LPTIM can be clocked using an external clock
signal injected on its external Input1. When clocked with an external clock source, the
LPTIM may run in one of these two possible configurations:
•
The first configuration is when the LPTIM is clocked by an external signal but in the
same time an internal clock signal is provided to the LPTIM either from APB or any
other embedded oscillator including LSE, LSI and HSI16.
•
The second configuration is when the LPTIM is solely clocked by an external clock
source through its external Input1. This configuration is the one used to realize Timeout
function or Pulse counter function when all the embedded oscillators are turned off
after entering a low-power mode.
Programming the CKSEL and COUNTMODE bits allows controlling whether the LPTIM will
use an external clock source or an internal one.
When configured to use an external clock source, the CKPOL bits are used to select the
external clock signal active edge. If both edges are configured to be active ones, an internal
clock signal should also be provided (first configuration). In this case, the internal clock
signal frequency should be at least four times higher than the external clock signal
frequency.
41.4.5 Glitch filter
The LPTIM inputs, either external (mapped to GPIOs) or internal (mapped on the chip-level
to other embedded peripherals), are protected with digital filters that prevent any glitches
and noise perturbations to propagate inside the LPTIM. This is in order to prevent spurious
counts or triggers.
Before activating the digital filters, an internal clock source should first be provided to the
LPTIM. This is necessary to guarantee the proper operation of the filters.
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Table 294. LPTIM2 external trigger connection
TRIGSEL
lptim_ext_trig0
lptim_ext_trig1
lptim_ext_trig2
lptim_ext_trig3
lptim_ext_trig4
lptim_ext_trig5
lptim_ext_trig6
lptim_ext_trig7
RM0432 Rev 6
External trigger
GPIO
RTC alarm A
RTC alarm B
RTC_TAMP1 input detection
RTC_TAMP2 input detection
RTC_TAMP3 input detection
COMP1_OUT
COMP2_OUT
RM0432