# Tuning-fork clock sharing

I'm considering a design that uses an MCP7940M RTC and a PIC16F15223. The clock section for the RTC shows

I'm considering using a CFS-20632768EZBB 32.768 kHz tuning fork style crystal with CL = 6 pF.

I'm deciding whether to either drive the PIC from the RTC clock, or have it run its own low-frequency, low-power internal clock. LFINTOSC is much less accurate but that wouldn't actually matter.

If I understand this correctly:

and I set Cx1 = Cx2 = Cx, then Cx would be at most 12 pF, less accounting for board stray capacitance. If I use this value for the circuit shown in 4-2, and also connect X2 to the PIC's CLKIN, would there be any adverse effects? In the PIC's spec I'm unable to find a clear definition for the input capacitance of CLKIN, but maybe it's 5-50 pF if it's the same as parameter D380 $$\C_{IO}\$$. If this is true and 50 pF is added to the load capacitance equation, it probably puts the RTC clock driver way out of spec, right? (The RTC asks for CL <= 9 pF.) Does this imply that the only practical clock option for the PIC is its LFINTOSC?

• Oh, man, I'm probably fighting a losing battle here, because I'm seeing a lot of this lately. "The oscillator is a CFS-20632768EZBB". No. No no no no no no no. The crystal is a CFS-20632768EZBB. An oscillator is an active circuit that you give power and possible control signals, and which generates a periodic signal (i.e. sine or square wave). A crystal is just a resonator unless you make it part of an oscillator. Please please please unconfuse yourself, and work to unconfuse the world on this point. Aug 26, 2021 at 1:01
• @TimWescott Fine? I guess I wasn't confused on the matter and already understand the crystal to not be an oscillator on its own; there's no need to panic. I've edited the question. Aug 26, 2021 at 1:05
• Thank you for the edit. I'm leaving the above comment stand because, well, there are four lights. Aug 26, 2021 at 1:06
• This is the third time I've run across it this week. It's either a blip and I'm overreacting, or it's a trend and I'm too late. But I'm gonna try. Aug 26, 2021 at 1:07
• @TimWescott lol ... your first comment made me think of this vimeo.com/338492037 Aug 26, 2021 at 1:07