In my 16C63JW circuit, I'm having trouble getting the oscillator to
oscillate (I think, based on observation of OSC pins with both scope and
CMOS logic probe).
First tried 20 MHz crystal with two 33pf caps, no resistor. Then tried 20
MHz ceramic resonator with built-in caps.
MCLR pin tied high. Analog meter says about 2 volts on OSC2 pin.
> In my 16C63JW circuit, I'm having trouble getting the oscillator to
> oscillate (I think, based on observation of OSC pins with both scope
> and CMOS logic probe).
>
> First tried 20 MHz crystal with two 33pf caps, no resistor. Then
> tried 20 MHz ceramic resonator with built-in caps.
>
> MCLR pin tied high. Analog meter says about 2 volts on OSC2 pin.
>
> Troubleshooting suggestions?
I have four suggestions, Tom:
1. Make sure that the Oscillator Type is set to HS (High Speed).
2. ONLY put the scope on Pin 15 (OSCOUT); unless you have a very
high-impedance probe, putting it on Pin 16 (OSCIN) will load
down the oscillator and can stop the clock. Note that the
frequency on OSCOUT is OSCIN/4, so don't be surprised if you
see "only" 5 MHz on that pin.
3. Check Vdd and Vss, and make sure the oscillator caps are tied
to Vss, not Vdd.
4. With the crystal, try using smaller caps (15 pF instead of 33
pF). With the ceramic resonator with built-in caps, make
sure that those internal caps are in the 10 pF range.
> .... Note that the frequency on OSCOUT is OSCIN/4, so don't be
> surprised if you see "only" 5 MHz on that pin.
and Kalle Pihlajasaari just reminded me that the OSCOUT
oscillates at the same speed as OSCIN when there's a crystal or
resonator attached; the OSCIN/4 frequency only appears on
OSCOUT if there's an RC oscillator attached.
Sorry for any confusion my lapse of memory may have caused.
At 2:37 AM 7/19/97, Andrew Warren wrote, concerning 16C63 oscillator problems:
> ONLY put the scope on Pin 15 (OSCOUT); unless you have a very
> high-impedance probe, putting it on Pin 16 (OSCIN) will load
> down the oscillator and can stop the clock.
Are we talking about the same PIC device? My crystal or resonator is
across pins 9 and 10, not 15 and 16.
> At 2:37 AM 7/19/97, Andrew Warren wrote, concerning 16C63 oscillator
> problems:
>
> > ONLY put the scope on Pin 15 (OSCOUT); unless you have a very
> > high-impedance probe, putting it on Pin 16 (OSCIN) will load
> > down the oscillator and can stop the clock.
>
> Are we talking about the same PIC device? My crystal or resonator
> is across pins 9 and 10, not 15 and 16.
Tom:
Thanks for catching that; I misread the question and thought
we were discussing the 16C83.
>1. Make sure that the Oscillator Type is set to HS (High Speed).
>2. ONLY put the scope on Pin 15 (OSCOUT); unless you have a very
high-impedance probe, putting it on Pin 16 (OSCIN) will load
down the oscillator and can stop the clock. Note that the
frequency on OSCOUT is OSCIN/4, so don't be surprised if you
see "only" 5 MHz on that pin.
I thought OSCOUT was OSCIN/4 only for RC not crystals. Also probing OSCOUT
can stop the clock or alter the frequency, unless it is high impedance, low
capacitance.
>3. Check Vdd and Vss, and make sure the oscillator caps are tied
to Vss, not Vdd.
Does connecting osc caps to vdd affect the clock? (as long as the supply is
decoupled well, I've not *yet* had a problem)
>4. With the crystal, try using smaller caps (15 pF instead of 33
pF). With the ceramic resonator with built-in caps, make
sure that those internal caps are in the 10 pF range.
MITVMA.MIT.EDU> wrote:
