RM0033 Rev 9 665/1381
RM0033 Universal synchronous asynchronous receiver transmitter (USART)
681
Figure 242. Transmission using DMA
Reception using DMA
DMA mode can be enabled for reception by setting the DMAR bit in USART_CR3 register.
Data is loaded from the USART_DR register to a SRAM area configured using the DMA
peripheral (refer to the DMA specification) whenever a data byte is received. To map a DMA
channel for USART reception, use the following procedure:
1. Write the USART_DR register address in the DMA control register to configure it as the
source of the transfer. The data will be moved from this address to the memory after
each RXNE event.
2. Write the memory address in the DMA control register to configure it as the destination
of the transfer. The data will be loaded from USART_DR to this memory area after each
RXNE event.
3. Configure the total number of bytes to be transferred in the DMA control register.
4. Configure the channel priority in the DMA control register
5. Configure interrupt generation after half/ full transfer as required by the application.
6. Activate the channel in the DMA control register.
When the number of data transfers programmed in the DMA Controller is reached, the DMA
controller generates an interrupt on the DMA channel interrupt vector. The DMAR bit should
be cleared by software in the USART_CR3 register during the interrupt subroutine.
TX line
USART_DR
Frame 1
TXE flag
F2
TC flag
F3
Frame 2
software waits until TC=1
Frame 3
set by hardware
cleared by DMA read
set by hardware
cleared by DMA read
set by hardware
set
Idle preamble
by hardware
F1
software configures
the DMA to send 3
data and enables the
USART
DMA request
ignored by the DMA
DMA writes
flag DMA TCIF
set by hardware
clear
by software
USART_DR
because DMA transfer is complete
DMA writes F1
into
USART_DR
DMA writes F2
into
USART_DR
DMA writes F3
into
USART_DR.
The DMA transfer
is complete
(TCIF=1 in
DMA_ISR)
(Transfer complete)
ai17192b
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