Table 252. Effect Of Low-Power Modes On The Lpuart - STMicroelectronics STM32WL5 Series Reference Manual

RM0453
Determining the maximum LPUART baud rate that enables to correctly wake
up the MCU from low-power mode
The maximum baud rate that enables to correctly wake up the MCU from low-power mode
depends on the wake-up time parameter (refer to the device datasheet) and on the LPUART
receiver tolerance (see
deviation).
Let us take the example of OVER8 = 0, M bits = '01', ONEBIT = 0 and BRR [3:0] = 0000.
In these conditions, according to
LPUART receiver tolerance equals 3.41%.
DTRA + DQUANT + DREC + DTCL + DWU < LPUART 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
lpuart_ker_ck inaccuracy.
For example, if HSI is used as lpuart_ker_ck, and the HSI inaccuracy is of 1%, then we
obtain:
t
WULPUART
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.
36.5 LPUART 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.
Low-power universal asynchronous receiver transmitter (LPUART)
Section 36.4.8: Tolerance of the LPUART receiver to clock
= t / (11 x T
WULPUART
= t / (11 x D
WULPUART
is the wake-up time from low-power mode.
WULPUART
= 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 252. Effect of low-power modes on the LPUART

Mode
Sleep No effect. LPUART interrupts cause the device to exit Sleep mode.
The content of the LPUART registers is kept.
(1)
Stop
The LPUART is able to wake up the microcontroller from Stop mode when
the LPUART is clocked by an oscillator available in Stop mode.
The LPUART peripheral is powered down and must be reinitialized after
exiting Standby mode.
Section 36.3: LPUART implementation
Table 250: Tolerance of the LPUART
)
bit Min
)
WUmax
Description
to know if the wake-up from Stop mode is supported for a
RM0453 Rev 5
receiver, the
1227/1450
1253