Hello,
I am designing the Artemis module into a PCB and would like to know if the 32.7 kHz crystal is required for the Artemis to operate normally.
I am not planning to use any RTC functionality and would like to save some board space.
Looking at the Artemis design files, I see that there is a 32 MHz source connected to the Apollo3, so I presume that the 32.7 kHz crystal is not required.
If anyone can confirm, it would be much appreciated.
Thanks!
Check out the Apollo3 data sheet section “Clock Generator and Real Time Clock Module”. There is a nice diagram of the Apollo3 clock tree showing all the various clock sources and what they can drive (mini version attached). You can see that the 32Mhz crystal is ONLY connected to the BLE processor core, and is not actually available for use by the regular Apollo3 processor core. The Apollo3 processor ONLY runs off its HFRC (high frequency RC) oscillator and has no other source of clocking. The HFRC can run at 48 MHz or 96 MHz, with an optional divide by two step to generate 24 MHz which is not actually shown in the diagram.
To answer your question, the 32768 XTAL is used by more than just the RTC. As far as the processor itself is concerned, the 32768 crystal is used to dynamically calibrate the 48 MHz RC clock (if desired) as seen in the clock tree diagram in the block marked ‘HFADj’ (High Frequency Adjust). Also, the peripherals can use the 32768 crystal if they need more timing accuracy than the main HFRC oscillator can supply. Specifically, the IOM blocks (the SPI/I2C/UART serial units) down at the bottom of the diagram only get clocked with HFRC, potentially divided down. So if you have no 32768 crystal, then your serial clock sources will not be crystal accurate. If your system is OK with that, then you don’t need the crystal.
MattHoworko:
Hello,I am designing the Artemis module into a PCB and would like to know if the 32.7 kHz crystal is required for the Artemis to operate normally.
I am not planning to use any RTC functionality and would like to save some board space.
Looking at the Artemis design files, I see that there is a 32 MHz source connected to the Apollo3, so I presume that the 32.7 kHz crystal is not required.
If anyone can confirm, it would be much appreciated.
Thanks!
Hi buddy, am trying similar stuff, which one did you finally opt for?
Regards,
Scott M Smith
This post about loading capacitors (or their omission) for the crystal is relevant:
And to confirm for Scott - the short answer is the external 32.7kHz crystal is not required for normal operation.
