ST STM32F301 6 Series Reference Manual page 753

RM0366
Note: If the USART kernel clock is kept on during Stop mode, there is no constraint on the
maximum baud rate that allows waking up from Stop mode. It is the same as in Run mode.
•
USART source clock is LSE
Same principle as described in case of USART source clock is HSI with the difference
that the LSE is on in Stop mode, but the LSE clock is not propagated to USART if the
USART is not requesting it. The LSE clock is not OFF but there is a clock gating to
avoid useless consumption.
The MCU wake-up from Stop mode can be done using the standard RXNE interrupt. In this
case, the RXNEIE bit must be set before entering Stop mode.
Alternatively, a specific interrupt may be selected through the WUS bit fields.
In order to be able to wake up the MCU from Stop mode, the UESM bit in the USART_CR1
control register must be set prior to entering Stop mode.
When the wake-up event is detected, the WUF flag is set by hardware and a wake-up
interrupt is generated if the WUFIE bit is set.
Note: Before entering Stop mode, the user must ensure that the USART is not performing a
transfer. BUSY flag cannot ensure that Stop mode is never entered during a running
reception.
The WUF flag is set when a wake-up event is detected, independently of whether the MCU
is in Stop or in an active mode.
When entering Stop mode just after having initialized and enabled the receiver, the REACK
bit must be checked to ensure the USART is actually enabled.
When DMA is used for reception, it must be disabled before entering Stop mode and re-
enabled upon exit from Stop mode.
The wake-up from Stop mode feature is not available for all modes. For example it doesn't
work in SPI mode because the SPI operates in master mode only.
Using mute mode with Stop mode
If the USART is put into mute mode before entering Stop mode:
• Wake-up from mute mode on idle detection must not be used, because idle detection
cannot work in Stop mode.
•
If the wake-up from mute mode on address match is used, then the source of wake-up
from Stop mode must also be the address match. If the RXNE flag is set when entering
the Stop mode, the interface will remain in mute mode upon address match and wake
up from Stop.
•
If the USART is configured to wake up the MCU from Stop mode on START bit
detection, the WUF flag is set, but the RXNE flag is not set.
Determining the maximum USART baud rate allowing to wake up correctly
from Stop mode when the USART clock source is the HSI clock
The maximum baud rate allowing to wake up correctly from Stop mode depends on:
•
the parameter t
•
the USART receiver tolerance provided in the
receiver to clock
Let us take this example: OVER8 = 0, M bits = 10, ONEBIT = 1, BRR [3:0] = 0000.
Universal synchronous/asynchronous receiver transmitter (USART/UART)
provided in the device datasheet
WUUSART
deviation.
RM0366 Rev 5
Section 26.5.5: Tolerance of the USART
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