PIC32MX Reference Clock module

[dannyf]

I'm no fan of PIC32 but just noticed a seldom mentioned feature of its oscillator module: ***user programmable*** reference clock out. The module allows you to select a reference source (both internal and/or external), and then it will run it through a 24-bit fractional divider (consisting of a 15-bit integer divider and a 9-bit fractional divider).

This is essentially a full-featured digitally controlled oscillator. with 24-bit of resolution.

Pretty cool to use it as a signal generator, sweeper, ...

[Howardlong]

I'm no fan of PIC32 but just noticed a seldom mentioned feature of its oscillator module: ***user programmable*** reference clock out. The module allows you to select a reference source (both internal and/or external), and then it will run it through a 24-bit fractional divider (consisting of a 15-bit integer divider and a 9-bit fractional divider).

This is essentially a full-featured digitally controlled oscillator. with 24-bit of resolution.

Pretty cool to use it as a signal generator, sweeper, ...

Depending on the application, it will exhibit significant jitter, it's similar in concept to the NCOs used in some PiC16s.

[hans]

It's probably similar to an I2S PLL clock as seen on STM32F4. Because the multiplier is presumably fractional , it's nice to generate the 64x, 128x or 256x bit clocks required for audio.

[Buriedcode]

Not only that, you can generate a rather clean 12.288MHz MCLK for DAC's using a 12MHz clock for USB.  This makes life much easier for USB DACs running in either isochronous or async mode.  Some PIC24's have this also. 

[MT]

I'm no fan of PIC32 but just noticed a seldom mentioned feature of its oscillator module: ***user programmable*** reference clock out. The module allows you to select a reference source (both internal and/or external), and then it will run it through a 24-bit fractional divider (consisting of a 15-bit integer divider and a 9-bit fractional divider).

This is essentially a full-featured digitally controlled oscillator. with 24-bit of resolution.

Pretty cool to use it as a signal generator, sweeper, ...

Depending on the application, it will exhibit significant jitter, it's similar in concept to the NCOs used in some PiC16s.

long time ago i used MX32 but i recall the NCO's in PIC16 is a phase accumulators  "bit skip" while REFOCON in MX32 is PLL
feeding a divider so initial jitter is from PLL while the divider acts as a filter, ie the larger the div ratio the lesser of phase jitter.
Disclaimer for being utterly wrong.

STM32 obviously outperform a MX32 with its multiple 16 by 16 bit divider for every TIMx there is.

However i wonder how much worse the RC clock on STM32 are compared to MX32 for USART apps, i recall MX32 RC clock
had really god specs not needing a crystall for high baud rate USART apps, i recall i havent seen any ppm specs for STM32 RC clk?!

[Howardlong]

Nah, while there is a PLL available to derive the system/USB clocks, there is no PLL+VCO inside the reference oscillator config, just a phase accumulator, dressed up in a slightly different way to the NCOs of some PIC16s (which ironically tend to have longer phase accumulators, I guess that's the difference between what Microchip term a reference clock and an NCO).

This is not to denegrate the peripheral, it has real applications, but be aware of the limitations, these are not low jitter DC-to-daylight pure frequency sources, very far from it in fact.

[MT]

What i meant was the REFOCON clk was derived from the PLL among other sources. So the REFOCON is actually a phase acc, interesting.