Table 244. Effect Of Low-Power Modes On The Usart - STMicroelectronics STM32WL5 Series Reference Manual

RM0453
Determining the maximum USART baud rate that enables to correctly wake
up the device from low-power mode
The maximum baud rate that enables to correctly wake up the device from low-power mode
depends on the wake-up time parameter (refer to the device datasheet) and on the USART
receiver tolerance (see
Let us take the example of OVER8 = 0, M bits = '01', ONEBIT = 0 and BRR [3:0] = 0000.
In these conditions, according to
[3:0] = 0000, the USART receiver tolerance equals 3.41%.
DTRA + DQUANT + DREC + DTCL + DWU < USART receiver tolerance
D
WUmax
T
bit Min
where t
If we consider the ideal case where DTRA, DQUANT, DREC and DTCL parameters are at
0%, the maximum value of DWU is 3.41%. In reality, we need to consider at least the
usart_ker_ck inaccuracy.
For example, if HSI is used as usart_ker_ck, and the HSI inaccuracy is of 1%, then we
obtain:
t
WUUSART
device datasheet).
D
WUmax
T
bit min
As a result, the maximum baud rate that enables to wake up correctly from low-power
mode is: 1/11.32 µs = 88.36 Kbaud.
35.6 USART in low-power modes
Standby
1. Refer to
given peripheral instance. If an instance is not functional in a given Stop mode, it must be disabled before
entering this Stop mode.
2. The content of the USART registers is kept only in Stop 0 and Stop 1 modes. In Stop 2 mode, the content
of the USART registers is lost and must the reinitialized after exiting from Stop 2 mode.
Universal synchronous/asynchronous receiver transmitter (USART/UART)
Section 35.5.8: Tolerance of the USART receiver to clock
= t / (11 x T
WUUSART
= t / (11 x D
WUUSART
is the wake-up time from low-power mode.
WUUSART
= 3 µs (values provided only as examples; for correct values, refer to the
= 3.41% - 1% = 2.41%
= 3 µs/ (11 x 2.41%) = 11.32 µs.

Table 244. Effect of low-power modes on the USART

Mode
Sleep No effect. USART interrupts cause the device to exit Sleep mode.
The content of the USART registers is kept
(1)
Stop
The USART is able to wake up the microcontroller from Stop mode when
the USART is clocked by an oscillator available in Stop mode.
The USART peripheral is powered down and must be reinitialized after
exiting Standby mode.
Section 35.4: USART implementation
Table 241: Tolerance of the USART receiver when BRR
)
bit Min
)
WUmax
Description
to know if the wake-up from Stop mode is supported for a
RM0453 Rev 5
deviation).
(2)
.
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