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chipkit max32 crystal oscillator


metso

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hi,

We have been happy to use chipkit max32 for quite some time now. Now we wanted to experiment a design that uses parts of the max32 circuitry. It turned out that when we have the crystal oscillator FQ7050B-8.000 (*1) connected to PIC32MX795F512, it uses an internal oscillator instead, and it runs slow. The model for the crystal oscillator is the one specified in chipkit schematics file (*2). If we replace it with the physical oscillator circuit removed from one of our chipkit max32 boards, controller performance becomes just as fast as in max32, and it is using the external oscillator the way it should.

Is the external oscillator X1 on max32 something other than the one mentioned in the schematics (*2)? If so, could someone please specify its make and model.

Best,

metso

*1 https://www.digikey.com/product-detail/en/fox-electronics/FQ7050B-8.000/631-1029-1-ND/1024734

*2 https://reference.digilentinc.com/_media/reference/microprocessor/max32/chipkit_max32_sch.pdf

 

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hi Jon,

Thank you for your reply. To me it seems there is still some info missing here. When I look at pins on the datasheet of FQ7050B-8.000 (*1) and compare them with pin ordering of Chipkit Rev D Eagle design (*2), they are different (see attached images). Crystal is connected between pins 1 and 3 on one chip, but it is using pins 2 and 4 on the other. The chips also look different: Text on our FQ7050B-8.000 is 'FOX 8.000' whereas our chipkit oscillator reads '8.000 MEC AL4JD'. How is this??

Best,

metso

*1 http://www.foxonline.com/pdfs/C7BQ.pdf

*2 https://reference.digilentinc.com/chipkit_max32/chipkit_max32_revd

 

FQ7050Bpins.png

chipkitOscPins.png

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Hello @metso,

I learned a little bit more from our hardware engineer; there are a few things that could be going on in this situation, although they did confirm that the oscillator is a different part than indicated on the schematic; the crystal used on the Max32 is from Mercury Electronic. It is an MQ series set to 8.000 MHz - http://www.mercury-crystal.com/upload/product/file1_150604828411.pdf

As you likely know, the PIC32 has some registers that set the configuration to use or not use an external clock source (in this case the crystal). However, the fault condition for the PIC32 if anything is detected to be wrong with the crystal is to use the internal oscillator. I don't know how you have your setup done on your PCB, but on ours you'll note that we have a pair of capacitors (C22 and C23) that adjust the phase from the crystal to compensate for the internal PIC32 MX feedback amplifier circuit. There is a resistor (R32) that acts as a gain adjust for the feedback system that is likely sub-optimal for this particular board; you may need to adjust this resistor to be 680 Ohms, like it is on the Uno32, which has a very similar circuit. Note though that the PCB itself adds capacitance to the system so if your own board has a different layout, those three components may need to be adjusted as well. Note that if the resulting gain is not greater than one, the PIC32MX will fault to using the internal oscillator rather than the external crystal.

Let me know if you have any questions.

Thanks,
JColvin

 

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