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Graham,

I thnk with these processors if the internal fuses are not burned for an internal oscillator, then the default is an external one. I used an ATMEGA-8 for a freq counter project and it was the same with that. If you can get hold of a 1ppm ocxo or better, then you will be smiling.

73

John

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Digitally signed mail - M0ELS

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From: Graham <planophore@...>
To: John Randall <m0els@...>
Sent: Friday, 26 July 2013, 10:56
Subject: Re: [QRPLabs] Re: Nose to the windows

John,

Agreed.

My recollection of reading the appnote for the Atmega328 was that you "needed" program the correct fuse otherwise an external clock source would not work.

As Hans pointed out, that is indeed technically true however his experience was that it did not make that much difference and that when programmed to use an external crystal (or say 20Mhz), replacing the crystal with an external clock source would also work without having to reprogram any fuses.

I started down this path wanting to use a more precise clock source for the U2's micro just as you are intending but also with a though to using a higher precision oscillator (i.e. 10Mhz OCXO) to drive the micro for some timing experiments. Due to higher priorities, both of these little projects have been temporarily set aside but now having Hans' comments to hand, I will probably be able to move a bit further ahead with both a little sooner than anticipated.

I look forward to your report to the QrpLabs list your comments and results of replacing the U2's crystal with an external clock source.

cheers, Graham ve3gtc
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On 13-07-26 05:15 AM, John Randall wrote:

I know this cn be donw with the ATMEGA 8 OR 16 processor chip has an internal clock but can be switch on/off. I think an external crystal oscillator block? block will be more stable as it can be temp controlled or covered to keep it reasonably stable.. Worth looking into !
GPS locking can also be used

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Digitally signed mail - M0ELS

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From: Graham
To: QRPLabs@...
Cc: John Randall
Sent: Thursday, 25 July 2013, 22:28
Subject: Re: [QRPLabs] Re: Nose to the windows

Mmmm, might not as easy as that.

I had the same idea. I haven't yet rechecked but I seem to recall in my perusal of the Atmega data sheet that in order to use an external oscillator rather than a crystal you need to set a specific bit or two for that purpose when the chip is programmed.

Something else became a priority before I pursued the U2 kit built any further. It is still patiently waiting it's turn on the workbench.

cheers, Graham ve3gtc



On 13-07-25 02:56 PM, John Randall wrote:

I will also change the 20Mhz xtal for a more stable oscillator.

John