Low-power universal asynchronous receiver transmitter (LPUART) RM0453
1240/1461 RM0453 Rev 1
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 wakeup time parameter (refer to the device datasheet) and on the LPUART
receiver tolerance (see Section 36.4.8: Tolerance of the LPUART receiver to clock
deviation).
Let us take the example of OVER8 = 0, M bits = ‘01’, ONEBIT = 0 and BRR [3:0] = 0000.
In these conditions, according to Table 250: Tolerance of the LPUART receiver, the
LPUART receiver tolerance equals 3.41%.
DTRA + DQUANT + DREC + DTCL + DWU < LPUART receiver tolerance
D
WUmax
= t
WULPUART
/ (11 x T
bit Min
)
T
bit Min
= t
WULPUART
/ (11 x D
WUmax
)
where t
WULPUART
is the wakeup time from low-power mode.
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
= 3 µs (values provided only as examples; for correct values, refer to the
device datasheet).
D
WUmax
= 3.41% - 1% = 2.41%
T
bit min
= 3 µs/ (11 x 2.41%) = 11.32 µs.
As a result, the maximum baud rate that enables to wakeup correctly from low-power
mode is: 1/11.32 µs = 88.36 Kbaud.
36.5 LPUART in low-power modes
Table 252. Effect of low-power modes on the LPUART
Mode Description
Sleep No effect. LPUART interrupts cause the device to exit Sleep mode.
Stop
(1)
1. Refer to Section 36.3: LPUART implementation to know if the wakeup from Stop mode is supported for a
given peripheral instance. If an instance is not functional in a given Stop mode, it must be disabled before
entering this Stop mode.
The content of the LPUART registers is kept.
The LPUART is able to wake up the microcontroller from Stop mode when
the LPUART is clocked by an oscillator available in Stop mode.
Standby
The LPUART peripheral is powered down and must be reinitialized after
exiting Standby mode.
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