Queue, Semaphore and timing

gerhardkreuzer wrote on Tuesday, October 04, 2011:

my problem:
I want to transmit some messages, which takes time (USART …). I post one message after the other using a queue and after the first post I tell the hardware to start transmitting. After the hardware has sent the first message, the next message is taken and so on, in the meantime more messages will be created and put in the queue.
After creating all messages, I know the number, I have to wait for the end of transmission of the last message. Not the end of any message or the beginning of the transmission of the last message or whatever.
So, waiting until the queue runs dry isn’t the correct way.
Has anybody some idea to manage this situation in some elegant way with FreeRTOS?


With best regards


rtel wrote on Tuesday, October 04, 2011:

You would have to implement something in the UART interrupt handler.  For example, presumably the UART will continue transmitting for as long as there is data to transmit.  When the queue is empty, and the last message received from the queue has been sent completely, the UART interrupt will know, and can set a semaphore to indicate such.  The task can then take the semaphore before it starts sending messages to the Tx queue, then block on the semaphore again once it has posted all the messages to the queue.  When the task unblocks, it knows the UART Tx interrupt has finished.