App timer misfire after 4000 mSec interval

Question

Hi, 
 
 Greetings ! 
 
 I want to take continuous 40 reading of adc by varying time interval between readings based on RPM value. I want timer interval resolution from 6 Sec to 15 mSec. For that I used app_timer for repeated mode. I start timer when reading count is zero and stop it after completion of 40 readings. Here I used, app_timer as like... 
 
 void start_normal_mode()
{
 ret_code_t err_code;
 SamplingRate = 60/(40 * RPM);
 SamplingRate_ms = SamplingRate * 1000;
 
 NORMAL_MODE_MEAS_INTERVAL = APP_TIMER_TICKS(SamplingRate_ms);

 if(normal_mode_reading_count == 0)
 {
 err_code = app_timer_start(m_normalmode_timer_id, NORMAL_MODE_MEAS_INTERVAL, NULL);
 APP_ERROR_CHECK(err_code);
 }
} 
 
 I got perfect reading when timer interval is less than 4000mSec but timer is misfires above 4000mSec. It instantly fires and didn’t get perfect time interval. 
 In same code, I used one more app_timer for fixed interval 1 Sec and I also check it for 6 Sec time interval. It works perfectly. So I didn’t recognized why this second one timer misbehaving like this 
 
 Will you please help me out in this matter? 
 
 Thanks, 
 Pooja Uchagaonkar

Jørgen Holmefjord · Accepted Answer

The APP_TIMER_TICKS macro is defined like this: 
 ((uint32_t)ROUNDED_DIV( \
 (MS) * (uint64_t)APP_TIMER_CLOCK_FREQ, \
 1000 * (APP_TIMER_CONFIG_RTC_FREQUENCY + 1))) 
 As you can see, it returns a 32-bit unsigned integer, and uses a 64-bit unsigned integer internally to make sure the accuracy is not lost due to variable overflow. 
 This is why I asked what data type NORMAL_MODE_MEAS_INTERVAL is in your application. If this is declared as uint16_t and this is the variable you check for the output from APP_TIMER_TICKS, you will get the behavior that you describe.

App timer misfire after 4000 mSec interval

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