The difference between AVR MCU communication transmission identifier UDRE and TXC

Single chip microcomputer STM32L151CCU6

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The difference between the AVR MCU communication transmission identifier UDRE and TXC:

The instructions of the AVR say:

"The TXC flag can be used to check if the transmission of a data frame has been completed. The RXC flag can be used to check if there is still data in the receive buffer. Before each data transmission (before writing the data register UDR) The TXC flag must be cleared."

"The data register empty UDRE flag indicates whether the transmit buffer can accept a new data. This bit is set to "1" when the transmit buffer is empty; it is cleared when the transmit buffer contains data to be transmitted."

"When the entire data frame moves out of the transmit shift register and there is no new data in the transmit buffer, the transmit end flag TXC is set. TXC is automatically cleared when the transfer end interrupt is executed, or "1" can be written in this bit. To clear."

After reading the above instructions, I have been wondering if I want to do two things at the same time when sending data:

1. Determine UDRE to be 1.

2. Clear the TXC logo.

But in the utility that I saw on the Internet, there is no second operation above, and it seems to work. With this doubt, I found a better answer on the Internet:

"About the AVR serial port, explain as follows:

For transmission, there is a UDR buffer register and a shift register. When you write a UDR once, the microcontroller will immediately transfer this data to the shift register, so you can write the second data immediately. In the future, whenever the UDR buffer register is empty, a UDRE interrupt will be generated. To generate a TXC interrupt, it must be generated after the data of the shift register has been transmitted.

For reception, there are two UDR buffer registers and one shift register. The two receive buffers are equivalent to a FIFO structure. When there is data reception, if a complete data is received into the shift register, it will be transferred to the buffer register. This will result in an RXC interrupt.

AVR is different from 51, and such a structure will be better. For example, when your program is busy in another interrupt, there is a serial port to receive data. Two buffers will win you time without losing data. The same is true for sending data. And 51 is not like this. "

"If two buffer data are written consecutively, because one data of the write buffer is shifted to the shift register, one data can be written immediately. That is, when the UDRE is set, the microcontroller has one byte shifted. The register is being sent."

The conclusion is:

The common cyclic transmission procedure (that is, the transmission procedure that only judges the UDRE without determining the TXC) can work. The root of the problem lies in the continuity of the transmitted data: that is, TXC=0 at the beginning, and the transmission condition is satisfied; and when the data is continuously transmitted, Since UDRE is set and data is still being transmitted in the shift register, TXC is not set and the transmission condition is satisfied. TXC is asserted until all data is transmitted and there is no data in both the shift register and the transmit buffer.

have to be aware of is:

1. If you have sent a round of data before, when you send it again, you must clear the TXC logo, that is, write "1" to this bit. Preferably, the TXC flag is detected after a round of data transmission is completed.

2. If the transmission completion interrupt is used, it is not necessary to manually clear it, because the hardware can automatically clear the TXC flag after entering the transmission interruption program.

3. If cyclic transmission is used without interrupt transmission, if the transmission program is suspended for more than a certain period of time due to other interrupts during transmission, the TXC flag will be set after the data transmission in the shift register is completed. Subsequent transmissions will not be possible.

4. If 485 is used for communication, the status of 485 is changed only when TXC is detected. Because only TXC is set, it represents the completion of the transmission process.

I think the better sending program is as follows:

Void uart_putchar(unsigned char c)

{

While(!(UCSR0A&(1<

If(UCSR0A&(1<

UCSR0A|=(1<

UDR0 = c;

}