What's the benefit of having an external 32 kHz crystal?

RE: What's the benefit of having an external 32 kHz crystal?

jjifa — Sat, 15 Feb 2020 19:46:00 GMT

Is it only about 10uA of current dissipated by not using the 32.768Khz crystal?

I do not want to use crystal, but this should not cause any processor delay.

Current consumption is not a problem for my development.
Of course, we won't use the RTC timer either.
It is just important to make the PCB lighter.
Is it right to remove the 32k crystal in this case?

RE: What's the benefit of having an external 32 kHz crystal?

vit bernatik — Tue, 23 Aug 2016 09:17:18 GMT

@CR - I assume that during sleep the 32MHz clock is turned off and 32KHz is turned on. After wakeup the time is recomputed. But due to RC oscilator being less precise - the recomputed time is less precise. So even though HF (32MHz) xtal is started for receiving BLE operation. The RX windows has to be start sooner (in case RC osc was slower) and end later (in case RC osc was faster). Therefore RX windows is wider. And chip has increased RX currrent for longer time - Which no-one wants... If you use small battery and want to run it significantly longer just use 32Khz xtal... Also you will save extra time for calibration. Basically what they most likiely do is that they run precise HF xtal and LF low precise RC. Then they know how many HF clocks is per one LF clcock. And can recompute LF->HF. But the thing is that RC oscilator changes in time due to temperature and what not.

RE: What's the benefit of having an external 32 kHz crystal?

energy — Mon, 18 Jul 2016 11:39:53 GMT

Does this means that we just have to add 10 uA to all scenarios described on the following link to compute the impact of not using external 32khz ?

devzone.nordicsemi.com/.../

Thanks

RE: What's the benefit of having an external 32 kHz crystal?

FI — Fri, 20 May 2016 02:48:43 GMT

Just curious Ole, which blocks, especially of the radio, use the 32K?

RE: What's the benefit of having an external 32 kHz crystal?

davman — Thu, 19 May 2016 15:10:22 GMT

Hello Team Nordic

Is there a similar power-comparison for the new nrf52-series?

RE: What's the benefit of having an external 32 kHz crystal?

CR — Wed, 20 May 2015 10:16:27 GMT

Why must the receive window be widened? Is the radio clock generated from the RC timer?

RE: What's the benefit of having an external 32 kHz crystal?

Ole Morten — Fri, 07 Mar 2014 12:17:35 GMT

I'm not absolutely certain whether the calibration interval is the same for the nRF8001, but any needed calibration of the nRF8001's RC oscillator is anyway handled internally in the chip, and no external actions are needed to make it happen.

RE: What's the benefit of having an external 32 kHz crystal?

patrick — Fri, 07 Mar 2014 12:09:59 GMT

Is this also applicable for the nRF8001 BLE chip ? Just want to be sure this mechanism of using the internal RC oscillator and the 4 sec. calibration interval is also done in the nRF8001 chip.

RE: What's the benefit of having an external 32 kHz crystal?

Ole Morten — Thu, 02 Jan 2014 13:57:03 GMT

Calibration every 4 s should be sufficient to keep the clock stable for all normal temperature variations, so even outside there should be no problems with this.

RE: What's the benefit of having an external 32 kHz crystal?

s. hoang — Thu, 26 Dec 2013 22:52:10 GMT

We have an app that the device will be outside. Therefore, there can be a significant, but not rapid, temperature swing. If we go with the internal RC oscillator, is there a temperature range that the calibration will be able to keep the clock stable or like most RC's it is too unstable.

RE: What's the benefit of having an external 32 kHz crystal?

Ole Morten — Wed, 26 Jun 2013 04:23:35 GMT

If you take a look in the nRF51822 PS, you can see that it is specified to be less that 250 ppm when calibrated every 4 seconds as long as the temperature is constant. This is within the requirements given in the Bluetooth Core Specification.

RE: What's the benefit of having an external 32 kHz crystal?

Ole Morten — Tue, 25 Jun 2013 18:21:09 GMT

The calibration is only needed as long as the chip is actually running, so the calibration will still run when needed, no matter if you power off the chip externally.

RE: What's the benefit of having an external 32 kHz crystal?

Bret Foreman — Tue, 25 Jun 2013 18:19:50 GMT

The reason I ask is the we are not currently using an external 32KHz crystal and we power down the chip externally when we are not expecting BLE activity. So the calibration you speak of can never happen. Is this OK?

RE: What's the benefit of having an external 32 kHz crystal?

Bret Foreman — Tue, 25 Jun 2013 18:15:00 GMT

In the reply "The RC oscillator must be calibrated every 4 seconds to be accurate enough." Accurate enough for what?

RE: What's the benefit of having an external 32 kHz crystal?

Ole Morten — Mon, 10 Jun 2013 21:41:37 GMT

An external 32 kHz crystal will give lower average current consumption than using the RC oscillator, at the expense of cost and board space. There are primarily 3 reasons the current consumption is lower:

The RC oscillator must be calibrated every 4 seconds to be accurate enough. Calibration normally takes about 17 ms, and requires the 16 MHz to be running, giving an average current consumption increase of about 6-7 µA.
The receive window must be widened to account for the worse accuracy. Since the RC is 250 ppm, compared with typically 20-30 ppm for an external crystal, the radio must receive for a slightly longer time, giving an increase of 1-2 µA average
The run current or the RC is 0.8 µA instead of 0.4 µA for a typical crystal.

In total, this gives 8-10 µA extra average current consumption. Depending on application, this may be enough to justify the cost and board space for a crystal.