Universal synchronous/asynchronous receiver transmitter (USART/UART) RM0453
1176/1461 RM0453 Rev 1
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 wakeup time parameter (refer to the device datasheet) and on the USART
receiver tolerance (see Section 35.5.8: Tolerance of the USART 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 241: Tolerance of the USART receiver when BRR
[3:0] = 0000, the USART receiver tolerance equals 3.41%.
DTRA + DQUANT + DREC + DTCL + DWU < USART receiver tolerance
D
WUmax
= t
WUUSART
/ (11 x T
bit Min
)
T
bit Min
= t
WUUSART
/ (11 x D
WUmax
)
where t
WUUSART
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
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
= 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.
35.6 USART in low-power modes
Table 244. Effect of low-power modes on the USART
Mode Description
Sleep No effect. USART interrupts cause the device to exit Sleep mode.
Stop
(1)
1. Refer to Section 35.4: USART 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 USART registers is kept
(2)
.
The USART is able to wake up the microcontroller from Stop mode when
the USART is clocked by an oscillator available in Stop mode.
2. The content of the USART registers is only kept in Stop 0 and Stop 1 modes. In Stop 2 mode the content of
the USART registers is lost and must be reinitialized after exiting Stop 2 mode.
Standby
The USART peripheral is powered down and must be reinitialized after
exiting Standby mode.
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