What kind of external clock is needed for PIC32?

Question

Nearly all PIC32 errata I have read (Item 41 - page 5) have the same issue -

A crystal oscillator cannot be used as an input to the Primary Oscillator (OSC1/OSC2 pins).

An internal oscillator will not do, so I will need an external oscillator, but I am unsure what I should be looking for when searching for a suitable part. I am basically looking for an 8MHz clock that I can pump into the internal PLL to boost up.

How can I select a suitable (and near same price as common xtal equiv) external clock for this device ?

Are there any gotcha's for someone who has never used an external clock before that they should be aware of ?

Answer 1

This errata is a bit confusing. They are saying that you cannot use an external crystal (not crystal oscillator) for rev A3 and A4 but for A5 you must stick to the particular specified crystal manufacturer and part number to assure operation.

To use an external oscillator (crystal or otherwise) there should be no special requirements, just drive the oscillator input as any other digital input.

Match the voltage range you plan to run the MCU at, make sure the power drain (sometimes large) is tolerable, and check the other requirements including output drive voltage (should be a square wave output not sine or clipped sine) and that should be it. For low price, shop around pick the cheapest part that is highly available and meets or exceeds you requirements, as always.

It is doubtful it will be as cheap as a crystal since the module contains a crystal, load capacitors and an oscillator chip. I see about $0.65 in 1k quantity, no doubt possible to do better. Also some frequencies are more popular than others. Note the power draw (16mA in this case) which may be higher than your MCU chip average draw.

Answer 2

That only applies to silicon revision A3 and A4.

As I read it, that issue was introduced in 'Rev C Document (4/2014)', then last updated in 'Rev E Document (9/2014)', and AFAICT has not been changed since, so I assume the update in 'Rev E Document (9/2014)' is when it says the issue does not apply to A5. So, presumably silicon revision A5 has been available for almost two years.

So it is likely that you will be buying A5 silicon, or you can ask your supplier to guarantee it is A5.

Having said that, A5 silicon is only guaranteed to work with a
ABLS-LR-12.000MHZ-18-D-R-T crystal, so use one of them.

This doesn't mean other crystals with the same spec won't work, just that it is the only one they guarantee will. If this is a product, use the guaranteed part. If it is a one-off, or the part is very hard to get, try to find a very similar spec. alternative, and test it. It will likely work.

The quote for A5 silicon is on page 19:

Module: Primary Oscillator
To use the Primary Oscillator with a crystal, only the following crystal is supported for PIC32MZ EC family devices:
XTAL = ABLS-LR-12.000MHZ-18-D-R-T
No other crystals are guaranteed to function 100% over voltage, temperature,
and population. Adequate testing required in your application to ensure reliable operation.

From that, I assume that an external crystals can't be used with A3 or A4 silicon. However, unless you already have those chips, it is unlikely to be relevant.

The quote for the overall issue is:

Module: Oscillator
Depending on the revision of silicon, a crystal oscillator cannot be used as the input to the Primary Oscillator (OSC1/OSC2 pins).
Work around
For Revision A3 and A4 silicon: Use an external clock or an internal FRC.

So I read that as being clear, A3 and A4 silicon can't use an external crystal.