SOSC not working with a 32.768KHz crystal

E_Blue · Joined: 13 Apr 2011 Posts: 406

Scenario:
CCS v5.091
MPLAB X IDE v5.30
Device PIC24FJ1024GB606

With this setup Timer 1 runs but too slow, should count 1200 pulses in 300 seconds and is counting 1200 pulses in 474 seconds.

E_Blue · Joined: 13 Apr 2011 Posts: 406

I seems like is working but I can't get the signal wave in my oscilloscope even with a 10x probes, as soon as I touch the SOSCO it stop working.
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Electric Blue

temtronic · Posted: Mon Nov 25, 2019 11:43 am

I don't use that PIC, but I'd read the datasheet about what the design requirements for the xtal and caps are. Usually it's xx pfd for the ENTIRE osc section, so you have to add in PCB traces, layout, etc. Say the 'spec' say 20pfd. There could be 5pfd of 'board' capacitance, so 15pfd caps should be used.

If the PIC board and PSU is not grounded to the same ground as the scope, if could stop the oscillator as you're adding a LOT of C to the circuit.

Also, perhaps the xtal is not the correct 'cut'. You could use xtal, caps and a CMOS inverter (4069 ?) and test.

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

From the data sheet:

E_Blue · Joined: 13 Apr 2011 Posts: 406

At this moment is running ok but I can't see the wave in the oscilloscope.
The probe ground is connected half of an inch away from the oscillator, the PCB has 4 ground planes, one per layer.

I'm going to try to decrease the C values and see if stay running with the oscilloscope probe connected to it.
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Electric Blue

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

A scope probe is typically 30pF. If your crystal is already overloaded
with a bit too much capacitance on the board, with low drive, this just
kills the oscillation. Normally the OSC out pin is slightly less sensitive
to extra capacitance than the in pin.

E_Blue · Joined: 13 Apr 2011 Posts: 406

I have two 27pF to ground from each pin.

The tiny cylindrical clock crystal is low power right?
Or should use high power mode?
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Electric Blue

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

Ugh!...
What is the rated capacitance of your crystal?.
Understand that these capacitors should be twice the rated capacitance
for the crystal, less the sum of the pin & track capacitance.
So if your crystal is designed for (say) 12pF load capacitance, and
your PCB tracks have perhaps 5pf capacitance, then the capacitors
would need to be:

(12*2)-(5+4) = 15pF

If your tracking is even fractionally long, this will drop.

Now typical watch crystals are rated for load capacitances between
perhaps 6pF and 12pF. The former type is designed to run without
any added capacitance into circuits like the PIC & RTC chips. This is
much lower than crystals like the 4MHz used for the main oscillator,
These normally require capacitances like 20pF.
Typically the PIC pins will have perhaps 3 to 4pF capacitance.

I suspect you are overloading your crystal, possibly already having
perhaps twice the capacitance you really want....