TickTimer Accuracy

[JayPRab]

Hi,

• We are using SD110 with SDK 7.2.0 in our application
• Our application needs to be quite accurate in timing
• We set out to determine what the accuracy of the TickTimer was, which is provided by the SD (110 in our case)
• Our hardwaresetup is shown in the attached file, "nRF51TimerTickAccuracyTestSetup.pdf"
  • What you see is a 1MHz oscillator (accurate to a few PPM)
    
  • it is connected to a 12-stage counter (hardware, so NO time lost due to freq. division)
    
  • I take the 12th stage output (which is 1MHz / 2^12 => 244 Hz) and connect it to P0.0 on the nRF
• The code (shown in file "nRF51TimerTickAccuracyTest.pdf") does the following:
  
  • Set up GPIOTE for my input pin (P0.0)
    
  • Init Timer2 to be a counter in 16 bit mode
  • Configure the PPI channel 0 to get Events in from P0.0 (NRF_PPI->CH[0].EEP)
  • Configure the PPI channel 0 to count up on T2 (NRF_PPI->CH[0].TEP)
• All this works very nicely, thanks to your previous help!!
• I run the nRF without ANY BLE activity at all
  • This should mean that the SD is quite undisturbed by asynchronous events like BLE traffic, etc.
• I expect to see CC[0] get a value close to 14,604:
  • every sec is 1,000,000 / 2^12 = 244 counts / second
  • 244 * 60 seconds = 14,604 counts / minute
• HOWEVER, what I actually see in the CC[0] register (using the Keil debugger) is 14,751 counts
  • This is the equivalent of 420,000 counts difference between expected and actual
  • this is 0.42 seconds gained every minute!
• Adding BLE doesn't change this number at all
• My question is:
  • What is causing this 420mSec per minute increase?
  • BTW, this is constant across many boards...
  • Is there a way to fine-tune the timer such that it is more accurate than this?

Files attached

Thanks for your help!

nRF51TimerTickAccuracyTestSetup.pdf

nRFTimerTickAccuracyTest.pdf

[Hein]

You are using an app timer to run a 1ms counter to get the 60 seconds ? What is the app timer prescaler ? It seems likely that the inaccuracy is from the app timer, not from your hardware clocking system ?

[JayPRab]

Hi,

Thanks for the FAST response!

Yes. 1mSec rep rate. That is the basis for ALL my timing in my application.

In the ISR, I up a counter ever time the ISR is hit (for this test)

So every count represents 1 mSec

I start the counter.

In 60,000 visits to the ISR (1 minute), I should have 14,604 counts in CC[0] because my Counter input is 244Hz

APP_TIMER_PRESCALER is 0

The inaccuracy is DEFINITELY from the App Timer...

Hence, my question... How can I get rid of this inaccuracy?

Thanks!

[Hein]

So, the app timer runs from the 32.768kHz clock, I assume you have a crystal for this? Either way, you cannot get a 1ms interrupt because 32.768kHz does not divide cleanly into 1ms.  You will probably get much more reasonable results if you set a repeating app timer to expire after 1 second and then check your counter value. But as stated by Ambystomalabs, it still takes time to service events, so there will always be some jitter in your measurements. 

[JayPRab]

I did not know that. I thought it used the HF clock...

Yes, I do have a crystal.

Thanks for all the help!