Sharing Crystal with 2 IC's

[ShaneMichael]

Sorry, not a code issue, it's a circuit issue, but I'm at a loss so here I am, begging for guidance:

I have a setup with the pic talking with DTMF IC's, the HT9170B and HT9200B from Holtek (DTMF Generator and DTMF Receiver). Both use the 3.579545 MHz crystal and 20 pF caps.

All is good when the IC's have their own crystal, when I try to share the crystal, the receiver doesn’t work correctly. I connected X1 to X1 and X2 to X2, so the crystal is parallel, didn't work. Other combinations also don't seem to work.

What happens is pin 15 (DV) on the receiver should drive high when a valid tone is received. It will then decay at a rate set by the RC circuit on pins 16/17. This works with a dedicated crystal. Sharing a crystal, DV drives low when a tone is received, and stays high the rest of the time. It’s almost inverted from normal operation.

When the pin drives high I know a new tone has been received and is ready to be read into the pic from 4 data ports D0-D3. With the pin driving high, that breaks the code.

It seems to me that the two chips should be able to share, I've Googled this every way I can think (Try googling "sharing crystal" and see what you get! :) I did find one article that said it should be in series, X2 to X1 and nothing connected to X2 of the last IC in the series.

Is sharing the crystal a dead end and I should give up while I'm behind and just use separate crystals?

Thanks,
Shane

[Darrel Taylor]

I did find one article that said it should be in series, X2 to X1 and nothing connected to X2 of the last IC in the series
That's how it should be.

1 chip has the crystal and capacitors on both X1 and X2.
X2 from the chip with the crystal goes to X1 of the other chip.
X2 of the second chip will be unused.
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[ShaneMichael]

ok, I'll go back and try that set up again, glad I'm not crazy to think that should work.

Also I'll try leaving the crystal across the reciever and take X2 to X1 of the generator.

Thanks,
Shane

[Sirdeevo]

Hi Shane,

Did you get this working in the end?

As it happens I'm using the same two chips, and have had the same thought! :)
I searched for "sharing crystals between IC", but I could take a guess where "sharing crystal" might get you ;)

I was talking to a friend and they recommended I use a series resistor, and tune it up to minimise reflections off the IC1-X2(out) and IC2-X1(IN) as well.

Cheers,

[ShaneMichael]

Yes, this did work, just as Darrel Taylor posted in his reply. I didn't use any additional hardware, just the two capacitors and crystal. I do remember having trouble with one of the IC's, seems like when I toggled the Enable/Disable it would shut down the crystal. I tested the other IC it the crystal worked Enabled or not. So I used it and took X2 of it to X1 of the other IC.

Hope that helps,
Shane

[bBrian]

Hi,

old Topic but great solution and answer!

Just to add, as Shane pointed out:
The HT9170 has got a PWDN pin, which would inhibit the oscillator, so one will have to attach the crystal to the HT9200.


Brian