RTC 32.768khz oscillator stuck

[Doomguy42]

Hi,
Myself and a buddy have remade some old boards. The RTC on the 25year old original boards doesn't seem to be great often losing its time and date.

Initially on our remake boards I couldn't get the 32.768khz clock signal using new crystal resonators... I worked out that my oscillating caps (22pf on the schematic) are overloading the crystal. I Fitted an existing crystal from an original old board and it worked... although it has stopped briefly in cold weather.

Using the new crystals with 4pf and 12.5pf I still couldn't get the new crystals to resonate.

This should be an easy job because it's a small circuit... I think I'm missing something...
The PIC RTC is on a socket and works in old boards... Is there anything that the PIC RTC or Vbat circuit can do to mess up the clock out (xtal)? I'm measuring around 4v at the oscillating cap.

 

[Doomguy42]

Crystal data sheet... 'tuning fork?'

 

[rjenkinsgb]

What are the exact crystals you are using? 32 KHz ones can vary tremendously from type to type, with loading and drive requirements.

Also, how long are the traces from the MCU to the crystal?

 

[rjenkinsgb]

OK, you beat me to it!

It should have a total capacitance on each leg of the circuit, of around twice the load spec, so I'd try two 18pF caps?

It is very low drive so may also need a resistor between the osc out pin and the crystal / load cap?

Edit - A 100K resistor is probably reasonable.

 

[Doomguy42]

Hi the traces are short the caps are 10mm from the pic and 10mm from the crystal (same as the original).
The crystal I selected is added above.
The 3 other clocks and boards all work perfectly... I'm missing something with the RTC not working or poorly

 

[Nigel Goodwin]

I use 2 x 15pF on my 32KHz crystals, and they work perfectly.

 

[Doomguy42]

OK, you beat me to it!

It should have a total capacitance on each leg of the circuit, of around twice the load spec, so I'd try two 18pF caps?

It is very low drive so may also need a resistor between the osc out pin and the crystal / load cap?

Edit - A 100K resistor is probably reasonable.

Thanks very much, I'll try that

 

[Tony Stewart]

Beware of stuck input or output. Measure out with pull up or down on input & ESD safe

 

[ZipZapOuch]

Those are some really old PICs in the schematic. Did you substitute a newer PIC? If so, newer PICs need to have firmware set to use the external crystal.