Could I move the external 32.768kHz crystal on 51822 for keyboard application???

[jimlin]

Hi,

As the title said, I need more GPIOs for application, and could I use the XL1 and XL2 ?
What happen when use RC oscillator instead?

Thanks.

[FormerMember]

For a BLE application, you'll need at least 250 ppm accuracy for the LF clock and this cannot be achieved with RC oscillator. The other option that you have is to use the LF clock synthesized from the HF clock. The advantage with this option apart from having 2 additional GPIOs is that you wouldn't have to wait for 0.3s for the LF OSC to settle if you stop it and you'll have predictable and low latency from waking up. But the con is that the sleep power required would be higher.

[jimlin]

Hi,

How much current will it raise when sleep, if the synthesized LFCLK used?

[FormerMember]

You'll have to add the current required for the HF_CLK. The datasheet says that HF_CLK oscillator will consume around 470uA when it needs to drive for any peripheral, 25 uA if it is idle. I guess (not sure) HF_CLK providing clock to LF_CLK accounts to it being active, so 470 uA more, which is quite a bit. I hope someone can clarify this.

[syntroniks]

In my experiments running with LFCLKSRC_RC_OSC_4000 I notice a few microamps higher current consumption relative to the 32khz crystal. Perhaps the hf clock is turned off inbetween connection events. The 16MHz osc is not a requirement of the internal 32khz osc. The synthesized lf clock, however, does require the HFCLK to be running. I don't know why there is even a synthesized clock :D Limited uses I guess

[FormerMember]

Yeah, couldn't come up with a realistic low power use case where the synthesized clock would be useful.