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Oscillator Crystal 20Mhz

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@polo-g40
I'm not too sure about the term crystal resonator, I'm saying this due to alternative solutions I can think off right now since PIC16F887 datasheet HS mode mentioned at there.

@Raza
Hi,
UPS board ? I need 20Mhz for interfacing my RFID reader RS232. with baud rate of 9600.

Regards
Linspire
 

What do you think about scope probe capacitance?

When you connect the probe tip and probe gnd to your crystal, you are actually killing the oscillator with induced parallel capacitance of your probe. Probes have at least 10 pF capacitance, and that is more than enough to stop the oscillator from working.

If you followed the data sheet recommendation and connected the 15 to 33 pF capacitors from your crystal to gnd, it should be working. I'm using the 887's connected to some ZigBit modules and communicating on 38400 bps.
 

ZigBit Modules ?
What's that ? Is it expensive product ?

Regards
Linspire
 

Good point probe capacitance.... I was going to say "does the chip still carry on processing if you've got LEDs flashing", good call...

You could still make a "Crystal Oscilator" module (5v in SqWave Out) using real cheap (but fast) 2 x 7 series inverters and a couple of caps, I did once using tied-input nand gates... If you've got a crystal (resonator) and access to chips, https://www.edaboard.com/threads/180003/ seems pheasable!!
Best regards
NEAL
 

polo-g40 said:
Good point probe capacitance.... I was going to say "does the chip still carry on processing if you've got LEDs flashing", good call...

You could still make a "Crystal Oscilator" module (5v in SqWave Out) using real cheap (but fast) 2 x 7 series inverters and a couple of caps, I did once using tied-input nand gates... If you've got a crystal (resonator) and access to chips, https://www.edaboard.com/threads/180003/ seems pheasable!!
Best regards
NEAL
Click to expand...

Do you have proper steps to construct it ?

Regards
Linspire
 

1st step is find a TTL or CMOS inverter on a chip if you can... Quad-Nand-Gate chip'd by my favorite.. And some verboard
NEAL
 

Err...are you saying MAX232 ?
I'm not expertise on these field.
Further information provided will be appreciated.

Regards
Linspire
 

Something like this:

74HC04CLOCK.jpg
 
I've just realised where I used to see *thousands* of 20Mhz XTal Oscilator blocks, in *very* old 3com network cards (the ones with the AUI port on, as well as BNC and TP, and I think also the early Intel EtherLink's (the 10mbit ones)... I knew someone who swapped these modules on a pair of cards for 1Mhz Modules, and got it them transmit (one way) over CB Radio...! But they were easily removable blocks, I wish I'd kepto some!
NEAL

---------- Post added at 16:24 ---------- Previous post was at 16:19 ----------

**broken link removed**

The silver thing on the top!!!
NEAL
 

polo-g40 said:
I've just realised where I used to see *thousands* of 20Mhz XTal Oscilator blocks, in *very* old 3com network cards (the ones with the AUI port on, as well as BNC and TP, and I think also the early Intel EtherLink's (the 10mbit ones)... I knew someone who swapped these modules on a pair of cards for 1Mhz Modules, and got it them transmit (one way) over CB Radio...! But they were easily removable blocks, I wish I'd kepto some!
NEAL

---------- Post added at 16:24 ---------- Previous post was at 16:19 ----------

**broken link removed**

The silver thing on the top!!!
NEAL
Click to expand...

What's this thing ? Is it kind of lan/internet module ?


Linspire
 

You may want to study the following lessons:





The lesson discuss the various clocking options, including the design of an external standalone oscillator and the appropriate configuration bit settings for each option.

Oscillator circuits are particularly sensitive to stray capacitance, so steps must be taken when attempting to measure the oscillator waveform with normal test equipment.

Reference Microchip Appnote AN1288 Design Practices for Low-Power External Oscillators, pg1, Section Probing the Circuit:

Oscillator circuits are highly sensitive to capacitance;
therefore, special care needs to be taken when
examining signals. A regular oscilloscope probe has
10-12 pF of capacitance, which can be sufficient to stop
oscillations. It is recommend that low-capacitance
probes be used, preferably with a JFET input, and that
the OSC2 pin be probed instead of OSC1.

Many new devices incorporate Automatic Gain Control
(AGC) for the crystal oscillator drive circuit; where, to
conserve power, the amplitude of the signal is reduced
when the circuit is operating as intended. When
examining the waveforms, this needs to be considered,
as the AGC may be attempting to compensate for an
imperfect circuit by increasing the peak to peak drive
signal. When adding additional load to the circuit, such
as an oscillator probe, the amplitude of the signal will
initially be reduced. The AGC will then compensate and
increase the amplitude back to its earlier level. This
response occurs slow enough to be visible on an
oscilloscope.
Click to expand...

Hope the info helps,

BigDog
 
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    FvM

    FvM

    Points: 2
    Helpful Answer Positive Rating
Its an OLD ISA PC Network card

---------- Post added at 10:49 ---------- Previous post was at 10:47 ----------

I'm going to try putting a JFET link to one of the chip's comparator to see if can get square wave, just for fun
CheersBigdog!
NEAL
 

polo-g40 said:
Its an OLD ISA PC Network card

---------- Post added at 10:49 ---------- Previous post was at 10:47 ----------

I'm going to try putting a JFET link to one of the chip's comparator to see if can get square wave, just for fun
CheersBigdog!
NEAL
Click to expand...

Share result here, after you tried it.

Linspire
 

Will Do!! Works out cheaper than the RS's £100/module if it does work! a 16F877 I've got on verobard'll be an easy test.. I found "said" Jfet on a blown-up-monitor board and knew usedful one day! I'm wondering now how I'm going to measure 20Mhz, so I'll try it with a 4 to start I think, I can borrow a handheld scope... I can tell easily if its going up and down, 2LEDS either way round
**broken link removed**

I'll let you know if anything happens, just for fun.... Search ebay for *really* old ISA Network cards, see if any of those above...
Neal
:)
 

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