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I am experimenting with a PIC16F84A at the moment.

I can get this working fine using an RC network for the clock.

When I switch to using a crystal (the usual xtal across pins 15&16 with 2 caps going to ground from the same pins) I am not getting any clock at all.

The PIC is the 20MHz version, and I have tried all the crystals I have - 4MHz, 14.3MHz and 25MHz, and numerous different capacitors rangeing from 22pf right up into the uF range, but all I get is what appears to be a logic high on pin 15 and a logic low on pin 16.

Yes, I have changed the program config to be _XT_OSC (I have tried them all), but to no avail.

I am using gpasm to assemble the program and pk2cmd to flash the PIC in circuit under Ubuntu.

Any clues?

Edit: diagram:

            22pf
16 o----+---||----+
        |         |
        -         |
       |=|Xtal    +---+
        -         |   |
        |         |   _
15 o----+---||----+   - GND
            22pf

(a little crude, I know ;) )

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    \$\begingroup\$ 22pF may be even too high. Try 12-18pf range. Also, are you using a breadbord? \$\endgroup\$
    – Hans
    Commented May 14, 2011 at 15:34
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    \$\begingroup\$ Ohkay - if I run it in HS oscillator mode, and ditch the capacitors (just have a crystal linked between pins 15 and 16) I get it working. If I then add the capacitors it stalls. Well, I say stalls - it appears to possibly be working at about 0.1Hz. \$\endgroup\$
    – Majenko
    Commented May 14, 2011 at 16:27
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    \$\begingroup\$ Solderless breadboards can be troublesome, especially for circuits involving crystal oscillators; I never use them. Try putting it on a PCB. \$\endgroup\$
    – Leon Heller
    Commented May 14, 2011 at 16:48
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    \$\begingroup\$ Do not stay too long with the 16F84. It is obsolete, expensive, limited in capability. Try 16F88 instead: finitesite.com/d3jsys/16F88.html \$\endgroup\$
    – markrages
    Commented May 14, 2011 at 16:55
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    \$\begingroup\$ @MattJenkins, it is actually the input capacitance and inductance of the scope probe that often kills the xtal when you touch it. Second, as pointed out, too much capacitance can often be a larger issue then too little. \$\endgroup\$
    – Kortuk
    Commented May 15, 2011 at 6:41

3 Answers 3

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You might have too much capacitance on the oscillator pins this can be caused by:

  1. Using the wrong capacitors for the selected oscillator/crystal.
  2. Building your circuit on a breadboard which increases capacitance in the circuit.
  3. Connecting a scope probe to the oscillator circuit.

When measuring the oscillator frequency it is best to connect the probe to the oscillator through a buffer. The PIC microprocessors often have a CLKOUT pin that serves this function, but it can only be used in some oscillator modes.

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Add a 1Meg Ohm resistor in parallel with your xtal

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    \$\begingroup\$ can you elaborate on your answer? \$\endgroup\$
    – jsolarski
    Commented Jun 15, 2011 at 12:14
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    \$\begingroup\$ I think what he's thinking of is a bias resistor to keep the CMOS inverter near its threshold region. This is necessary when using a CMOS inverter as the gain element in a crystal oscillator. PICs already have this built in. I've done over 100 PIC projects and never needed to add a external bias resistor. However, I have seen Microchip mention it on occasion. I don't think this was a dumb suggestion deserving a downvote, so I'm upvoting it to cancel that out. \$\endgroup\$
    – Olin Lathrop
    Commented Jun 25, 2011 at 12:42
  • \$\begingroup\$ @Olin - I guess it was downvoted for lacking detail, not for being wrong. \$\endgroup\$
    – stevenvh
    Commented Jun 25, 2011 at 13:30
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I've always been able to solve this problem by reducing, or often totally eliminating the capacitors. I've had this problem more times than I care to count, but it's never failed to fix the problem.

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  • \$\begingroup\$ Hm, I don't know if I can recommend this. My experience is that on most oscillators this has an adverse effect. \$\endgroup\$
    – stevenvh
    Commented Jun 25, 2011 at 12:27
  • \$\begingroup\$ I agree with stevenvh. Without any caps it could start oscillating at overtone frequencies. Also, without the right load on the crystal, the frequency will likely be a little out of spec, even if it oscillates at the fundamental. \$\endgroup\$
    – Olin Lathrop
    Commented Jun 25, 2011 at 12:39

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