SPI Clock Frequency

[espruino-discuss3]

Posted at 2015-03-16 by user53771

Hi,

i would like to match the SPI clock frequency to some given value. From the STM32F103RCT6 hardware manual it looks like the SPI clock is generated from the AHB clock (PCLK) using a prescaler (divide by /2, /4, /8, ...). To get a more exact match to my required SPI clock, I can only think of changing the AHB clock...

I have the following questions:

1. Is it possible to change the AHB clock frequency from Javascript (or change the main PLL/clock)? If this is possible, how would that affect the other peripherals (like serial port baudrate)?
2. How is the PLL set up during build, i.e. would it work to recompile everything after changing chip.speed from 72 to e.g. 68 in boards/ESPURINOBOARD.py? Would then the other peripheral clocks get calculated correctly?

Thanks!

[espruino-discuss3]

Posted at 2015-03-16 by @gfwilliams

@user53771 yes, you're right - the F103 can only get the SPI clock right to +/- 50%.

You should be able to change the clock using peek32 and poke32 functions, but USB on the F103 is really picky, so you probably wouldn't be able to keep that going.

It looks like USART would be fine though - just set it up again with Serial1.setup and it'll use the updated clock speeds. You may find that getTime() behaves strangely afterwards though - it depends which board you're using.

As you say the other option is to do it at compile time. I'm not sure that changing the clock speed in the .py file will be good enough - you may also need to change this file. But after that it should work.

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Posted at 2015-03-19 by user53771

Hi @gfwilliams,

thanks for the feedback - I will try to go the first way first (maybe add external USB<=> Serial for accessing the board without USB).

Regarding the compile time way: I do not really understand what the chip.speed setting in the .py file is good for? Is it used somewhere during the build to e.g. generate header files or code? Or is it only used to generate the (really nice) web pages?

Thanks!

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Posted at 2015-03-19 by @gfwilliams

I can't honestly remember if the .py file is used in compilation or not. The idea was that eventually it would be, but currently there's still a mix of that, stuff defined in the Makefile, and even #defines in the code.