Wakeup From Stop Mode - STMicroelectronics STM32F05 series Reference Manual

Universal synchronous asynchronous receiver transmitter (USART)
RS485 Driver Enable
The driver enable feature is enabled by setting bit DEM in the USART_CR3 control register.
This allows the user to activate the external transceiver control, through the DE (Driver
Enable) signal. The assertion time is the time between the activation of the DE signal and
the beginning of the START bit. It is programmed using the DEAT [4:0] bit fields in the
USART_CR1 control register. The de-assertion time is the time between the end of the last
stop bit, in a transmitted message, and the de-activation of the DE signal. It is programmed
using the DEDT [4:0] bit fields in the USART_CR1 control register. The polarity of the DE
signal can be configured using the DEP bit in the USART_CR3 control register.
25.5.17

Wakeup from Stop mode

This section is relevant only when the Wakeup from Stop feature is supported. Please refer
to Table 83: STM32F05xxx USART features on page
The USART is able to wake up the MCU from Stop mode when the UESM bit is set and the
USART clock is set to HSI or LSE (refer to the Reset and clock control (RCC) section.
The MCU wakeup from Stop mode can be done using the standard RXNE interrupt. In this
case, the RXNEIE bit must be set before entering Stop mode.
Alternatively, a specific interrupt may be selected through the WUS bit fields.
In order to be able to wake up the MCU from Stop mode, the UESM bit in the USART_CR1
control register must be set prior to entering Stop mode.
When the wakeup event is detected, the WUF flag is set by hardware and a wakeup
interrupt is generated if the WUFIE bit is set.
Note: Before entering Stop mode, software must check that the USART is not performing a
transfer, by checking the BUSY flag in the USART_SR register.
The WUF flag is set when a wakeup event is detected, independently of whether the MCU is
in Stop or in an active mode.
When entering Stop mode just after having initialized and enabled the receiver, the REACK
bit must be checked to ensure the USART is actually enabled.
When DMA is used for reception, it must be disabled before entering Stop mode and re-
enabled upon exit from Stop mode.
The minimum bit duration to wake up properly from Stop mode is
t
WUSTOP
equals 0.
Caution: The wakeup from Stop mode feature is not available for all modes. For example it doesn't
work in SPI mode because the SPI operates in master mode only.
Using Mute mode with Stop mode
If the USART is put into Mute mode before entering Stop mode:
● Wakeup from Mute mode on idle detection must not be used, because idle detection
cannot work in Stop mode.
●
If the wakeup from Mute mode on address match is used, then the source of wake-up
from Stop mode must also be the address match.
●
If the USART is configured to wake up the MCU from Stop mode on START bit
detection, the WUF flag is set, but the RXNE flag is not set.
608/742
/(DWU (max)*11) when M equals 1 and t
Doc ID 018940 Rev 1
573.
/ (DWU (max) * 10) when M
WUSTOP
RM0091