Description of the methods used to manage the DE signal

AN3070

6/12

Doc ID 16312 Rev 1

2

Description of the methods used to manage the DE

signal

The purpose of this section is to provide two methods to control the DE signal and switch

between the USART send and receive modes.

The first method uses two interrupts: the transmit complete interrupt of the DMA and the

transmit complete interrupt of the USART.

The second method uses two USART interrupts: the transmit complete interrupt and the

transmit buffer empty interrupt.

2.1

Method using the DMA interrupt

In this method, the DMA manages the data buffer transmission entirely. It continuously

sends the data buffer to the USART data register until the DMA counter reaches 0. When

the DMA transmit complete interrupt occurs, the USART transmit complete interrupt is

enabled. In this interrupt, the DE pin is driven low.

The DE signal goes high just before the DMA transfer is enabled.

Figure 2 shows an example of management of the DE signal by using the DMA transmit

complete interrupt.

Figure 2.

DMA interrupt method to control the DE signal

2.2

Method using the USART interrupt

The transmit complete interrupt of the USART is used to drive low the DE pin, and so inform

the slave to send its response. The transmit data register empty interrupt is also used to

drive this pin high to inform that the master is to send data to the slave. Figure 3 shows an

example of the DE signal management using the USART interrupts.

The DE pin is pulled high before the first byte is sent. It is driven low after the last bit of the

last byte is sent. This is done by enabling the TC interrupt during the last byte transmission.

The next occurrence of the transmit complete interrupt drives the DE pin low.