Pulse, timer and UART with GPIOTE and PPI

[RO]

Hello,

My problem statement is something like this:

• • Implement a Transmit Enable (TxEn) on a GPIO pin.
  • TxEn should be initialized low.
  • TxEn should be pulled high 30us before the first start bit transmission begins.
  • TxEn should remain high during serial transmission of multiple bytes.
  • TxEn should be pulled low 60us after the last stop is transmitted.
  • TxEn handling and serial transmission should not substantially block main program execution.

I figured I need to use the GPIOTE and PPI functionality. 

My question is that since I have 3 functions to take care of: GPIO transition, timer compare and UART start/stop, how best can I utilize GPIOTE and PPI to accomplish this.

Option 1: Event - GPIO low to high, Task- timer compare and then start UART

Option 2: Set GPIO high,  Event - Timer compare, Task- UART start

What approach should I follow? Is there another option,  is there a way to use all 3 in a GPIOTE-PPI combination?

Thanks

[Susheel Nuguru]

If you want this to happen completely without MCU involving after configuration then Option 1) seems wise choice. Since in option 2 you need to manually set GPIO high which might add possible delays based on MCU availability.

[RO]

Hmm. I don't have an external trigger for the GPIO in my setup. So I think I have to manually set/clear anyway to pull it high/low.

In this case, does it make any sense to use it as a GPIOTE to be coupled with a PPI task?

Or should I use PPI for the timer and UART event-task pair?

Or is there a way to use all 3 in a GPIOTE-PPI combination?

[Susheel Nuguru]

TimerCC_event --> PPI -->UART_START

UART_ENDTX _event--> PPI --> Timer_start

These two connections seem necessary.

But the main challenge is that if you have only one timer, then you need to reconfigure the CC from 30us to 60us. And it is difficult to do it exactly when last byte is transmitted (if UART_ENDTX event generates interrupt that is with highest priority, then you can do it). But if you have two timers to spare, then you can easily do it.

Good luck with it, let me know if you have some challenges, it seems interesting.

[nordev]

Hi Susheel Nuguru, sorry if this is a basic Q, but how do you start the UART, and what happens after starting the UART? 

The below is from the UART example code, and it seems to print "UART Start!", but none of the commands surrounding this log statement seems to have anything to do with UART. Please explain how the UART is actual started, thanks a lot!

[Susheel Nuguru]

nordev, please open a new ticket for that and keep this thread focussed on one thing.

[nordev]

Sorry, will do. Thanks!