Minimal Hardware Questions (8MHz Oscillator)

Created Mon, 29 Sep 2014 10:01:19 +0000 by Cosford

Cosford

Mon, 29 Sep 2014 10:01:19 +0000

Hi,

Simple question really, what is the 8Mhz oscillator on the various chipkit boards actually used for, as I was under the impression that the chip used the internal oscillator to run the chip?

Basically, I'm looking to make a PCB based on the Uno32 (and later the max32) and working out what hardware is actually required on-board to meet my needs. I need to be able to reprogram the chip, but I assume I can do that through the ICSP header using a PICKit 3?

Thanks, Cosford.

mikes

Mon, 29 Sep 2014 13:14:32 +0000

The chip uses a Phase Locked Loop to turn the 8Mhz oscillator into the 80MHz clock that the chip runs on.

If you are making your own board it is possible to remove it but you will need to modify the boot-loader. I will also make your timing less accurate

Cosford

Mon, 29 Sep 2014 13:39:49 +0000

Ah, thanks for that. I'd like to try and avoid modifying the bootloader at all costs, so will include that in the PCB.

What is the purpose of IC4 (An op-amp?) on the Uno32? It's visible in the lower right corner of page 2 of the schematics: http://www.digilentinc.com/Data/Products/CHIPKIT-UNO32/ChipKIT%20Uno32_bysa_c_sch.pdf

Thanks in advance, Cosford.

majenko

Mon, 29 Sep 2014 14:39:29 +0000

It is part of the power selection system. It compares the incoming voltage from the barrel jack to a threshold voltage, and if it is below that voltage it turns on the P-channel MOSFET that allows power from the USB port.

It's a way of having two power sources yet not having the voltage drop of a diode in there - allows the full 5V from the USB to be used.

Cosford

Mon, 29 Sep 2014 14:48:18 +0000

Ah, yes of course, that would make sense. I can omit that for my purposes then, as it will always be fed via a linear regulator.

Thanks for your help. I think that covers everything for now. =)

Cosford

Wed, 01 Oct 2014 10:32:32 +0000

Hi again, I have another question I hope you may be able to help with.

X1 (The 8MHz crystal), has a 680 ohm resistor in series with it and the OSC2 pin. Why is this resistor present here (R33) whilst not in the X2 circuit (for the RTC?).

What purpose does this resistor serve? And should the value of this be tailored in any particular way to the particular choice of crystal used?

Thanks again

majenko

Wed, 01 Oct 2014 12:28:06 +0000

It's in section 6 of the PIC32 manual:

A series resistor, RS, may be required for AT strip cut crystals.

I myself have never used one on my circuits, and they have never had any problems. I don't know if the crystal they're using is an "AT strip cut" one, but according to Wikipedia:

Most (estimated over 90%) of all crystals are this variant.

so it probably is,

The manual doesn't say under what circumstances a series resistor would be required...

Mind, I have switched to clock chips instead of crystals recently, since they are simpler to work with and only require 1 IO pin.

Cosford

Wed, 01 Oct 2014 12:53:14 +0000

Great, thanks once again. I scanned the manual but couldn't find any reference to it. I guess that's what I get for scanning it instead of reading it properly.

majenko

Wed, 01 Oct 2014 12:59:29 +0000

The manual is rather large and takes a lot of skimming - it's about 30 PDFs ;)

That bit is a footnote in PDF "06 - Oscillators" under the block diagram of the oscillator system.