UARTE ENDTX event latency

RE: UARTE ENDTX event latency

Edvin — Fri, 16 Feb 2018 10:01:58 GMT

Hello,

If you need to add a delay after the ENDTX event, you must use a timer. I have added this in the code below:

#define ENDTX_GPIOTE_CH 0
#define ENDTX_GPIOTE_CH_A 0
#define TIMER_CAPTURE_CH 1
#define TIMER_COMPARE_CC_NUM 0
#define TIMER_CAPTURE_DELAY_US 2

#define TIMER_RELOAD 1024
#define TIMER_RELOAD_CC_NUM 5

void timer_init(void)
{
    NRF_TIMER3->BITMODE                 = TIMER_BITMODE_BITMODE_24Bit << TIMER_BITMODE_BITMODE_Pos;
    NRF_TIMER3->PRESCALER               = 4;
    NRF_TIMER3->SHORTS                  = TIMER_SHORTS_COMPARE0_CLEAR_Msk << TIMER_RELOAD_CC_NUM;
    NRF_TIMER3->MODE                    = TIMER_MODE_MODE_Timer << TIMER_MODE_MODE_Pos;
    NRF_TIMER3->CC[TIMER_RELOAD_CC_NUM] = TIMER_RELOAD;
    NRF_TIMER3->CC[TIMER_COMPARE_CC_NUM]= TIMER_CAPTURE_DELAY_US;
}

void timer_start(void)
{
    NRF_TIMER3->TASKS_START = 1;
}

void attach_ENDTX_to_pin(uint32_t pinselect)
{
    NRF_GPIOTE->CONFIG[ENDTX_GPIOTE_CH]     = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                                              GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                                              pinselect << GPIOTE_CONFIG_PSEL_Pos |
                                              GPIOTE_CONFIG_OUTINIT_High << GPIOTE_CONFIG_OUTINIT_Pos;
    
    NRF_PPI->CH[ENDTX_GPIOTE_CH_A].EEP      = (uint32_t)&NRF_UARTE0->EVENTS_ENDTX;
    NRF_PPI->CH[ENDTX_GPIOTE_CH_A].TEP      = (uint32_t)&NRF_TIMER3->TASKS_CLEAR;
    NRF_PPI->FORK[ENDTX_GPIOTE_CH_A].TEP    = (uint32_t)&NRF_GPIOTE->TASKS_SET;
    NRF_PPI->CH[TIMER_CAPTURE_CH].EEP       = (uint32_t)&NRF_TIMER3->EVENTS_COMPARE[TIMER_COMPARE_CC_NUM];
    NRF_PPI->CH[TIMER_CAPTURE_CH].TEP       = (uint32_t)&NRF_GPIOTE->TASKS_CLR[ENDTX_GPIOTE_CH];
    
    
    NRF_PPI->CHENSET = (1 << ENDTX_GPIOTE_CH_A) | (1 << TIMER_CAPTURE_CH);
}

int main(void)
{
    ...init_uart...
    timer_init();
    timer_start();
    attach_ENDTX_to_pin(22);
}

This will toggle the pin after TIMER_CAPTURE_DELAY_US microseconds.

You can use the EGU (event generation unit) to do something useful instead of toggling the pin, but as you can see, the pin will go low (in this case) 2µs after the last byte is sent.

RE: UARTE ENDTX event latency

KV — Thu, 15 Feb 2018 15:54:50 GMT

Thanks Edvin.

For our use case, the transmit section and receive section of hardware cannot be enabled at the same time. If I have to wait until all the bytes are completely transmitted, this method may not work because the transmit section would be turned off before the stop bit of the last byte is transmitted. Ther could be framing errors on other side.

Some MCUs have TX_COMPLETE interrupt which is asserted after stop bit is sent out.

How do I address this scenario?

RE: UARTE ENDTX event latency

Edvin — Thu, 15 Feb 2018 12:53:54 GMT

If you want to test yourself, here is the piece of code that I used to test:

void attach_ENDTX_to_pin(uint32_t pinselect)
{
    NRF_GPIOTE->CONFIG[ENDTX_GPIOTE_CH] =   GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                                            pinselect << GPIOTE_CONFIG_PSEL_Pos |
                                            GPIOTE_CONFIG_OUTINIT_High << GPIOTE_CONFIG_OUTINIT_Pos;
    
    NRF_PPI->CH[ENDTX_GPIOTE_CH_A].EEP  =   (uint32_t)&NRF_UARTE0->EVENTS_ENDTX;
    NRF_PPI->CH[ENDTX_GPIOTE_CH_A].TEP  =   (uint32_t)&NRF_GPIOTE->TASKS_CLR[ENDTX_GPIOTE_CH];
    
    NRF_PPI->CHENSET = (1 << ENDTX_GPIOTE_CH_A);
}

int main(void)
{
    ...init_uart...
    
    attach_ENDTX_to_pin(22);    //use e.g. pin 20 to use LED4 (nRF52832)
    
    printf("Start: \r\n");

}

RE: UARTE ENDTX event latency

Edvin — Thu, 15 Feb 2018 12:50:19 GMT

Hello,

I tested with 115200 baud rate, and a PPI set up to change a pin at the ENDTX event. As you can see, the event is set at the very end of the last payload bit, and right before the stop bit. Screenshot below shows the TX line on Channel 0 and the pin that switched on ENDTX on Channel 1.

I also tested it with the 1M baudrate. As you can see there is a tiny delay, but it is less than 0.3µs.

The same delay is on the 115200 baudrate, but of course it is neglectable with that baudrate.

Best regards,

Edvin