Using NRF51822 for full RTC and low power

RE: Using NRF51822 for full RTC and low power

Roger Clark — Tue, 07 Feb 2017 19:58:17 GMT

The RTC in the nRF51 or nRF52 will not survive a reset.

Use and external RTC module.

RE: Using NRF51822 for full RTC and low power

tuyenpham — Tue, 07 Feb 2017 09:48:20 GMT

Thanks Roger,

I don't need very accurate time. I can sync time from mobile app. But I need timekeeping when NRF51 in low power mode or it was be reset.

RE: Using NRF51822 for full RTC and low power

Roger Clark — Tue, 07 Feb 2017 09:32:57 GMT

Sorry. I should have made it clear I was referring to using a 32khz crystal rather than the 32kHz RC oscillator

For very accurate time keeping even that would not be enough as I think you require temperature compensation.

It probably depends on how accurate and over what time period and under what conditions you need to keep accurate time.

Ultimately if you want to be very accurate, you should probably use a external GPS for your timekeeping.

RE: Using NRF51822 for full RTC and low power

tuyenpham — Tue, 07 Feb 2017 09:24:50 GMT

Thanks Bjørn Spockeli,

RE: Using NRF51822 for full RTC and low power

bjorn-spockeli — Tue, 07 Feb 2017 09:04:13 GMT

@tuyenpham: Yes you are correct, the RTC will be turned off in System OFF, I meant to write System ON . I have correct my comment.

RE: Using NRF51822 for full RTC and low power

tuyenpham — Tue, 07 Feb 2017 08:59:25 GMT

Hi Bjørn Spockeli, I think nRF51x22 goes to System OFF then RTC also STOP! Are you sure it can run?

RE: Using NRF51822 for full RTC and low power

bjorn-spockeli — Tue, 07 Feb 2017 08:49:59 GMT

If the product requires to accurately keep track of time over a long period of time then I recommend that you use an external Real-Time Clock and not the the Real-Time Counter(RTC) of the nRF51x22. If you put the nRF51x22 in System ON with 16kb RAM enabled and keep the RTC running then you should see about 2.6uA (SYSTEM-ON base current with 16 kB RAM enabled.) + 0.8uA (RC32k Run current)+ 0.1uA (RTC Run Current)= ~3.5uA.

RE: Using NRF51822 for full RTC and low power

tuyenpham — Tue, 07 Feb 2017 08:38:12 GMT

I mean "power off" = deep sleep mode. I think it also the same remove power and reset with NRF51822. If must use external RTC then we have 2 coin cell and it occupy more space on board. I think this design not perfect such as wearable device...

RE: Using NRF51822 for full RTC and low power

bjorn-spockeli — Tue, 07 Feb 2017 08:28:00 GMT

@tuyenpham: You should also take into consideration that the internal 32kHz RC oscillator has default tolerance of plus/minus 2% (250ppm when calibrated at a regular interval), i.e. it will not be very accurate over a long period of time.

RE: Using NRF51822 for full RTC and low power

Roger Clark — Tue, 07 Feb 2017 05:39:43 GMT

If you completely remove the power from the nRF51822 (or the nRF52) RTC and all other functions will stop.

Either you need an external RTC (which has its own battery),

Or.. you have a battery to power your nRF5 processor and put it into low power mode, when you are not using it for its main function.