Universal synchronous asynchronous receiver transmitter (USART) RM0351
1344/1830 DocID024597 Rev 5
DWU max = t
WUUSART
/ (11 x Tbit Min)
Tbit Min = t
WUUSART
/ (11 x DWU max)
If we consider an ideal case where the parameters DTRA, DQUANT, DREC and DTCL are
at 0%, the DWU max is 4.86 %. In reality, we need to consider at least the HSI inaccuracy.
Let us consider the HSI inaccuracy = 1 %, t
WUUSART
= 1.7 s (in case of wakeup from Stop
mode 0).
DWU max = 4.86 % - 1 % = 3.86 %
Tbit min = 1.7 µs / (9 3.86 %) = 4.89 µs
In these conditions, the maximum baud rate allowing to wakeup correctly from Stop mode is
1/4.89 µs = 204 Kbaud.
40.6 USART low-power modes
40.7 USART interrupts
Table 238. Effect of low-power modes on the USART
Mode Description
Sleep No effect. USART interrupt causes the device to exit Sleep mode.
Low-power run No effect.
Low-power sleep
No effect. USART interrupt causes the device to exit Low-power sleep
mode.
Stop 0 / Stop 1
The USART registers content is kept. The USART is able to wake up the
MCU from Stop 0 and Stop 1 modes when the UESM bit is set and the
USART clock is set to HSI16 or LSE.
The MCU wakeup from Stop 0 and Stop 1 modes can be done using either
a standard RXNE or a WUF interrupt.
Stop 2
The USART registers content is kept. The USART must either be disabled
or put in reset state.
Standby
The USART is powered down and must be reinitialized when the device
has exited from Standby or Shutdown mode.
Shutdown
Table 239. USART interrupt requests
Interrupt event Event flag
Enable Control
bit
Transmit data register empty TXE TXEIE
CTS interrupt CTSIF CTSIE
Transmission Complete TC TCIE
Receive data register not empty (data ready to be read) RXNE
RXNEIE
Overrun error detected ORE
Idle line detected IDLE IDLEIE
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