Usart Low-Power Management - ST STM32WL5 Series Reference Manual

RM0453
35.5.21

USART low-power management

The USART has advanced low-power mode functions, that enables transferring properly
data even when the usart_pclk clock is disabled.
The USART is able to wake up the MCU from low-power mode when the UESM bit is set.
When the usart_pclk is gated, the USART provides a wakeup interrupt (usart_wkup) if a
specific action requiring the activation of the usart_pclk clock is needed:
•
If FIFO mode is disabled
usart_pclk clock has to be activated to empty the USART data register.
In this case, the usart_wkup interrupt source is RXNE set to '1'. The RXNEIE bit must
be set before entering low-power mode.
•
If FIFO mode is enabled
usart_pclk clock has to be activated to:
–
–
In this case, the usart_wkup interrupt source can be:
–
–
–
This enables sending/receiving the data in the TXFIFO/RXFIFO during low-power
mode.
To avoid overrun/underrun errors and transmit/receive data in low-power mode, the
usart_wkup interrupt source can be one of the following events:
–
–
For example, the application can set the threshold to the maximum RXFIFO size if the
wakeup time is less than the time required to receive a single byte across the line.
Using the RXFIFO full, TXFIFO empty, RXFIFO not empty and RXFIFO/TXFIFO
threshold interrupts to wakeup the MCU from low-power mode enables doing as many
USART transfers as possible during low-power mode with the benefit of optimizing
consumption.
Alternatively, a specific usart_wkup interrupt can be selected through the WUS bitfields.
When the wakeup event is detected, the WUF flag is set by hardware and a usart_wkup
interrupt is generated if the WUFIE bit is set. In this case the usart_wkup interrupt is not
mandatory and setting the WUF being is sufficient to wake up the MCU from low-power
mode.
Universal synchronous/asynchronous receiver transmitter (USART/UART)
to fill the TXFIFO
or to empty the RXFIFO
RXFIFO not empty. In this case, the RXFNEIE bit must be set before entering low-
power mode.
RXFIFO full. In this case, the RXFFIE bit must be set before entering low-power
mode, the number of received data corresponds to the RXFIFO size, and the
RXFF flag is not set.
TXFIFO empty. In this case, the TXFEIE bit must be set before entering low-power
mode.
TXFIFO threshold reached. In this case, the TXFTIE bit must be set before
entering low-power mode.
RXFIFO threshold reached. In this case, the RXFTIE bit must be set before
entering low-power mode.
RM0453 Rev 1 1173/1461
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