Table 238. Effect Of Low-Power Modes On The Usart; Table 239. Usart Interrupt Requests - ST STM32L4 5 Series Reference Manual

Universal synchronous asynchronous receiver transmitter (USART)
DWU max = t
Tbit Min = t
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
mode 0).
DWU max = 4.86 % - 1 % = 3.86 %
Tbit min = 1.7 µs / (9 ₓ 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
Sleep
Low-power run
Low-power sleep
Stop 0 / Stop 1
Stop 2
Standby
Shutdown
40.7 USART interrupts
Transmit data register empty
CTS interrupt
Transmission Complete
Receive data register not empty (data ready to be read)
Overrun error detected
Idle line detected
1344/1830
/ (11 x Tbit Min)
WUUSART
/ (11 x DWU max)
WUUSART

Table 238. Effect of low-power modes on the USART

Mode
No effect. USART interrupt causes the device to exit Sleep mode.
No effect.
No effect. USART interrupt causes the device to exit Low-power sleep
mode.
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.
The USART registers content is kept. The USART must either be disabled
or put in reset state.
The USART is powered down and must be reinitialized when the device
has exited from Standby or Shutdown mode.

Table 239. USART interrupt requests

Interrupt event
DocID024597 Rev 5
= 1.7 μs (in case of wakeup from Stop
WUUSART
Description
Event flag
TXE
CTSIF
RXNE
ORE
IDLE
RM0351
Enable Control
bit
TXEIE
CTSIE
TC TCIE
RXNEIE
IDLEIE