RE: Timeslot / Radio signal callback hander sometimes called with 10us delay

m.wagner — Wed, 11 Mar 2020 14:23:41 GMT

Okay, perfect, thanks.

RE: Timeslot / Radio signal callback hander sometimes called with 10us delay

Hung Bui — Wed, 11 Mar 2020 13:43:36 GMT

If you use timeslot, it for sure will cause more latency as the interrupt will get to the softdevice first, after that be forwarded to the timeslot event.

1us when using bare metal is more reasonable. It can be varied a bit chip to chip and by the sample rate + accuracy of logic analyzer. In my case it's about 450ns.

RE: Timeslot / Radio signal callback hander sometimes called with 10us delay

m.wagner — Wed, 11 Mar 2020 07:54:07 GMT

Hi Hung Bui,

Thanks for your response. The wakeup from system ON IDLE indeed seems to be the cause for this!

[quote userid="2121" url="~/f/nordic-q-a/58879/timeslot-radio-signal-callback-hander-sometimes-called-with-10us-delay/239155"]But 3.5 us is a little bit long from my experience. [/quote]

The above measurements were made in a timeslot based application and the delay is between the peripheral event and the invocation of the signal handler by the softdevice. I assume this brings some additional latency and could explain this discrepanc, couldn't it?

With a "bare metal" application, I measure about 1us from the peripheral event to the pin being set in the interrupt handler. Is that reasonable?

RE: Timeslot / Radio signal callback hander sometimes called with 10us delay

Hung Bui — Tue, 10 Mar 2020 14:12:52 GMT

Hi Michael,

The 10us delay extra is most likely the time it take for the CPU to wake up from system ON IDLE. Did you put the CPU to sleep (IDLE) ?

When the debugger is connected the CPU is kept running all the time (including the HFCLK). This explain why you have shorter latency.

But 3.5 us is a little bit long from my experience.

I was using the following code and can see 0.5us latency if I don't put the CPU to sleep about 11.8us if I put the CPU to sleep (with the __WFE and __SEV).

#include <nrf.h>
#include "nrf_delay.h"
#define PPI_CHANNEL (0)
#define PIN_GPIO    (17)
#define PIN_GPIO2    (18)
void RADIO_IRQHandler(void)
{
    if (NRF_RADIO->EVENTS_DISABLED)
    {
        NRF_GPIO->OUTCLR = (1UL << PIN_GPIO2);  
        NRF_RADIO->EVENTS_DISABLED=0;
    }
  /*  if (NRF_RADIO->EVENTS_READY )
    {
        NRF_GPIO->OUTSET = (1UL << PIN_GPIO2);  
        NRF_RADIO->EVENTS_READY=0;
     }
    */

 }
int main(void)
{
  // Packet to send
  uint8_t packet[16] = "demopacket";
  
  // Start HFCLK from crystal oscillator. The radio needs crystal to function correctly.
  NRF_CLOCK->TASKS_HFCLKSTART = 1;
  while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
  
  // Configure radio with 2Mbit Nordic proprietary mode
  NRF_RADIO->MODE = RADIO_MODE_MODE_Nrf_2Mbit << RADIO_MODE_MODE_Pos;
  
  // Configure packet with no S0,S1 or Length fields and 8-bit preamble.
  NRF_RADIO->PCNF0 = (0 << RADIO_PCNF0_LFLEN_Pos) |
                     (0 << RADIO_PCNF0_S0LEN_Pos) |
                     (0 << RADIO_PCNF0_S1LEN_Pos) | 
                     (RADIO_PCNF0_S1INCL_Automatic << RADIO_PCNF0_S1INCL_Pos) |
                     (RADIO_PCNF0_PLEN_8bit << RADIO_PCNF0_PLEN_Pos);
  
  // Configure static payload length of 16 bytes. 3 bytes address, little endian with whitening enabled.
  NRF_RADIO->PCNF1 =  (16 << RADIO_PCNF1_MAXLEN_Pos) |
                      (16 << RADIO_PCNF1_STATLEN_Pos) |
                      (2  << RADIO_PCNF1_BALEN_Pos) | 
                      (RADIO_PCNF1_ENDIAN_Little << RADIO_PCNF1_ENDIAN_Pos) |
                      (RADIO_PCNF1_WHITEEN_Enabled << RADIO_PCNF1_WHITEEN_Pos);
  
  // initialize whitening value
  NRF_RADIO->DATAWHITEIV = 0x55;
  
  // Configure address Prefix0 + Base0
  NRF_RADIO->BASE0   = 0x0000BABE;
  NRF_RADIO->PREFIX0 = 0x41 << RADIO_PREFIX0_AP0_Pos;
  
  // Use logical address 0 (BASE0 + PREFIX0 byte 0)
  NRF_RADIO->TXADDRESS = 0 << RADIO_TXADDRESS_TXADDRESS_Pos;
  
  // Initialize CRC (two bytes)
  NRF_RADIO->CRCCNF = (RADIO_CRCCNF_LEN_Two << RADIO_CRCCNF_LEN_Pos) |
                      (RADIO_CRCCNF_SKIPADDR_Skip << RADIO_CRCCNF_SKIPADDR_Pos);
  NRF_RADIO->CRCPOLY = 0x0000AAAA;
  NRF_RADIO->CRCINIT = 0x12345678;
  
  // Enable fast rampup, new in nRF52
  NRF_RADIO->MODECNF0 = (RADIO_MODECNF0_DTX_B0 << RADIO_MODECNF0_DTX_Pos) |
                        (RADIO_MODECNF0_RU_Fast << RADIO_MODECNF0_RU_Pos);
                        
  // 0dBm output power, sending packets at 2400MHz
  NRF_RADIO->TXPOWER = RADIO_TXPOWER_TXPOWER_0dBm << RADIO_TXPOWER_TXPOWER_Pos;
  NRF_RADIO->FREQUENCY = 0 << RADIO_FREQUENCY_FREQUENCY_Pos;
  
  // Configure address of the packet and logic address to use
  NRF_RADIO->PACKETPTR = (uint32_t)&packet[0];
  
  // Configure shortcuts to start as soon as READY event is received, and disable radio as soon as packet is sent.
  NRF_RADIO->SHORTS = (RADIO_SHORTS_READY_START_Enabled << RADIO_SHORTS_READY_START_Pos) |
                      (RADIO_SHORTS_END_DISABLE_Enabled << RADIO_SHORTS_END_DISABLE_Pos);
                      
                      
  NRF_GPIO->DIRSET = (1UL << PIN_GPIO);
  
  // Configure GPIOTE->TASKS_OUT[0] to toggle PIN_GPIO
  NRF_GPIOTE->CONFIG[0] = (GPIOTE_CONFIG_MODE_Task       << GPIOTE_CONFIG_MODE_Pos) |
                          (GPIOTE_CONFIG_OUTINIT_Low     << GPIOTE_CONFIG_OUTINIT_Pos) |
                          (GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos) |
                          (PIN_GPIO                      << GPIOTE_CONFIG_PSEL_Pos);   

  NRF_GPIO->DIRSET = NRF_GPIO->DIRSET|1UL << PIN_GPIO2;
  NRF_GPIO->PIN_CNF[PIN_GPIO2] = (GPIO_PIN_CNF_DIR_Output << GPIO_PIN_CNF_DIR_Pos) |
                                (GPIO_PIN_CNF_DRIVE_S0S1 << GPIO_PIN_CNF_DRIVE_Pos) |
                                (GPIO_PIN_CNF_INPUT_Connect << GPIO_PIN_CNF_INPUT_Pos) |
                                (GPIO_PIN_CNF_PULL_Disabled << GPIO_PIN_CNF_PULL_Pos) |
                                (GPIO_PIN_CNF_SENSE_Disabled << GPIO_PIN_CNF_SENSE_Pos);
  // Configure GPIOTE->TASKS_OUT[0] to toggle PIN_GPIO
  
  NRF_PPI->CH[PPI_CHANNEL].EEP = (uint32_t)&NRF_RADIO->EVENTS_DISABLED ;
  NRF_PPI->CH[PPI_CHANNEL].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[0];
  
  // Enable PPI channel
  NRF_PPI->CHENSET = (1UL << PPI_CHANNEL);
  
  NVIC_ClearPendingIRQ(RADIO_IRQn);
  NVIC_EnableIRQ(RADIO_IRQn);  
  NRF_RADIO->INTENSET      = RADIO_INTENSET_DISABLED_Msk;    

  // Continuously send the same packet
  while (1)
  { 
    NRF_RADIO->TASKS_TXEN = 1;
    NRF_GPIO->OUTSET = (1UL << PIN_GPIO2);  
    /*__SEV();
    __WFE();
    __WFE();*/
    
    nrf_delay_ms(500);
  }
}