Posted on May 17, 2011 at 12:48Hi mdeneen:
Here is how I design a UART driver:
There are at least two ISRs for RS-232 operation, and one more additional ISR for RS-485 transmit complete (for RTS line control).
First ISR: Receive Interrupt. Read the character from UART hardware, store in circular buffer. Reading clears the interrupt flag. RXNEIE does not change and always remains enabled.
Second ISR: Transmit interrupt: Get a character from circular buffer, write to UART hardware. Writing clears the interrupt. If buffer is empty, turn of TXEIE flag. If RS-485, turn on TCIE flag. Otherwise, leave both flags alone.
Third ISR: Transmit Complete: Turn off TCIE flag, and turn off RTS control line.
Other ISRs: Provide for error conditions, and explicitly clear appropriate flags.
Furthermore, there are at least two driver functions which are not ISRs, but provide the mechanism to get characters into and out of the circular buffers, and therefore work in conjunction with the ISRs.
First driver function: Get Character: Checks the receive circular buffer and returns true with the character when the buffer is not empty.
Second driver function: Put Character: Puts the sending character into a circular buffer then turns on the TXEIE flag. If RS-485 design, first turn off TCIE, then enable RTS control line, then put character in to circular buffer, then enable TXEIE flag. The function returns true if the character was successfully put into the transmit buffer.
Now to answer your question. You want the UART hardware to generate and interrupt as soon as TXEIE is set. That is the mechanism to start transmitting.
Fortunately, the STM32 UART is relatively simple to program and appears to work well. Unfortunately, there is no double buffering on the Receiver. The ISR has to get the character out of the USART_DR (data register) before the next character arrives. Attempting to use the DMA to transfer the DR into RAM has its own problems because the DMA is usually programmed to transfer a specified number of bytes, but it is not known how many bytes are incoming with a serial data stream, so there is always a remainder problem to be handled. Too bad ST cheaped-out trying to save a few pennies, when it would have been much better to provide a least one extra RX buffer DR.
By the way, I recommend using the ST Firmware Library. It saves a lot of coding, headaches, and other un-related bugs and problems.
Best wishes
Garry Anderson.
[ This message was edited by: design6 on 23-10-2008 09:17 ]
