Measuring Microsecond Time

Question

Hi, I am trying to expand on the ESB PRX example and want to recreate functions with the same functionality as Arduino's micros() and millis() functions. I'd like the following: 
 a) 1 μs resolution 
 b) Low latency time read 
 c) Safe to use from either a normal or interrupt context 
 My understanding is that the RTC module is not suitable as it has a low resolution. The Time API of the IEEE 802.15.4 stack appears to have the right functionality, especially as it returns the microsecond time as a 64-bit number. I don't want to overcomplicate the issue by trying to include the IEEE 802.15.4 stack when all I want from it is the time. 
 What is the best way to achieve this? 
 [Mac OS, Segger, SDK 17.0.2] 
 Kind regards, 
 Microderm

microderm · Answer

Thanks, that worked for me. Here is what I did: 
 1) Added the code below. 
 2) Copied the TIMER block from the sdk_config.h file of a TIMER example and set TIMER1_ENABLED 1. 
 static uint32_t curr_sys_micros_32 = 0;
static uint64_t curr_sys_micros_64 = 0;

/**
 * Initialises the timer.
 */
void micros64_init() {
 NRF_TIMER1->TASKS_STOP = 1; // Stop timer //
 NRF_TIMER1->MODE = 0; // Timer mode //
 NRF_TIMER1->BITMODE = 3; // 32-bit counter //
 NRF_TIMER1->PRESCALER = 4; // Prescalar 2^4 //
 NRF_TIMER1->TASKS_CLEAR = 1; // Clear counter //
 NRF_TIMER1->TASKS_START = 1; // Start timer //
}

/**
 * Returns the current 64-bit microsecond system time. Must be called within 71 minutes of previous call.
 * @return the current 64-bit microsecond system time.
 */
uint64_t micros64() {
 NRF_TIMER1->TASKS_CAPTURE[0] = 1; // Capture current value //
 uint32_t prev_value = curr_sys_micros_32;
 curr_sys_micros_32 = NRF_TIMER1->CC[0]; // Store value //
 curr_sys_micros_64 += curr_sys_micros_32 - prev_value;
 return curr_sys_micros_64;
} 
 The code accounts for the 32-bit overflow to give the correct 64-bit timestamp in microseconds, so long as micros64() is called periodically with period less than 71 minutes.