the scope probe gambit
I was fiddling with an old prototype project which used the Morse Watch/Timer PIC chip.
photo from discontinued kit: http://wb9kzy.com/morwatch.htm
But it didn't work, I was trying to do a sleep timer which wouldn't turn off as it should after 10 minutes.
I finally realized that I was being misled by the dual clock oscillators in the PIC chip. The internal 4 MHz RC oscillator worked fine with menus and selecting/starting the timer delay. But the 32,768 Hz crystal oscillator wasn't running which was needed for the actual timing.
Oh no, a bad 32,768 Hz crystal ? I tried grounding the crystal case, no help. Then I tried probing the crystal oscillator pins and voila, it started to work.
The magic of scope probe capacitance had saved the day. When the probe was attached the additional capacitance added to the 10 pF to ground cap started the oscillator. So I snipped out the 10 pF cap and soldered in a 22 pF cap.
It seems to work. In the actual Morse Watch/Timer kit the caps on the crystal pins are 15 pF each so I must have figured out in 2009 that 10 pF caps wouldn't work reliably.
Luckily I've never had access to super low capacitance active probes !
I even encountered this when working on IC testing at GTE, sometimes just hanging the scope probe on a device under test pin was enough to quell an oscillation or slow down a rise/fall time.
Mark it good and ship it !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
photo from discontinued kit: http://wb9kzy.com/morwatch.htm
But it didn't work, I was trying to do a sleep timer which wouldn't turn off as it should after 10 minutes.
I finally realized that I was being misled by the dual clock oscillators in the PIC chip. The internal 4 MHz RC oscillator worked fine with menus and selecting/starting the timer delay. But the 32,768 Hz crystal oscillator wasn't running which was needed for the actual timing.
Oh no, a bad 32,768 Hz crystal ? I tried grounding the crystal case, no help. Then I tried probing the crystal oscillator pins and voila, it started to work.
The magic of scope probe capacitance had saved the day. When the probe was attached the additional capacitance added to the 10 pF to ground cap started the oscillator. So I snipped out the 10 pF cap and soldered in a 22 pF cap.
It seems to work. In the actual Morse Watch/Timer kit the caps on the crystal pins are 15 pF each so I must have figured out in 2009 that 10 pF caps wouldn't work reliably.
 |
| 15 pF caps in red box |
Luckily I've never had access to super low capacitance active probes !
I even encountered this when working on IC testing at GTE, sometimes just hanging the scope probe on a device under test pin was enough to quell an oscillation or slow down a rise/fall time.
Mark it good and ship it !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
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