When is crystal standby current drawn? Not when ON and all blocks idle?

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

TIE Fighter — Fri, 13 Sep 2013 03:03:54 GMT

This is correct as as per measurements done and as stated in the datasheet. The nRF8001 draws 2uA or lower in idle mode.

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

Erland Lewin — Wed, 04 Sep 2013 16:01:29 GMT

Thank you for both your answers!

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

Ole Morten — Wed, 04 Sep 2013 04:32:13 GMT

This current is, as Audun says, consumed when the external high-frequency crystal is running, but no module is actually requiring it. Control of the external crystal is normally handled automatically by the softdevice, and turned off in between connection intervals. However, there is an API function to request the external HFCLK to be on, in case the application needs it for something else than the radio (for example the peripherals indicated in table 24 in the PS). If you call this function, but there isn't any peripheral active that actually requires this oscillator, you will see this current draw. Normally, in between connection intervals, you will therefore see I_ON,IDLE + I_RTC + I_X32k/I_RC32k, as given in section 8.1 in the SoftDevice Specification, which equals 2.3 + 0.2 + 0.4/0.8 µA, depending on 32.768 kHz clock source used.

What Audun is trying to explain is however that the internal RC consumes more current than the external crystal. If you have a lot of application processing (i.e. run the CPU for long periods), there is a point where the 16 MHz RC is running for so much of the time anyway that it is actually beneficial to instead always run the external crystal, since it has lower run current.

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

Erland Lewin — Tue, 03 Sep 2013 18:04:47 GMT

The nRF8001 doesn't have this issue, does it? It actually draws 2 uA in idle mode, despite having a 16 MHz crystal? I didn't find anything in its product specification about this.

Thanks,

Erland

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

Erland Lewin — Tue, 03 Sep 2013 16:55:30 GMT

So, in a BLE application, is it always neccessary to have at least one 16 MHz oscillator in standby? The minimum current possible to draw in "ON, all blocks idle" in a BLE application is therefore 37 uA?

If this is the case, then the 2.3 uA figure in the product specification sounds grossly misleading.

RE: When is crystal standby current drawn? Not when ON and all blocks idle?

Audun — Tue, 03 Sep 2013 13:39:52 GMT

Hi Erland,

the standby current for the external crystal oscillator will come on top of those 2.3 µA, as you suggest. 2.3 µA idle current is when internal RC oscillator is used.

In addition to having a higher standby current, the crystal oscillator has a significantly lower running current than the internal RC oscillator (400 µA vs 750 µA). This means that when 16M usage rise above X %, it is beneficial to keep the external crystal running. Assuming that the internal RC oscillator consumes ~0 µA in standby:

400 µA * X/100 + 35 µA * (100 - X)/100 = 750 µA * X/100
X ~= 9 %