Added support for external clock source and systick user-defined clock value for ARM

[valexandru]

Pull Request Overview

This pull request adds support for setting external clock source for systick
and extends the support for setting a user-defined clock speed for the
systick.

This pull request is needed for #1918 to work because for the i.MX RT 1052 EVKB
board, even though syst_calib.SKEW = 0 and syst.calib.NOREF = 0, the
calibration value is not correct and causes thesystick to overflow too fast.

Also, if I select external clock source in syst_csr, which causes an external
24 MHz clock source to be prescaled to 100KHz and used as systick, the
system will work.

Testing Strategy

This pull request was tested using the i.MX RT 1052 EVKB Board.

Documentation Updated

• No updates are required.

Formatting

• Ran make prepush.

[ppannuto]

I believe @alevy wrote the original code here, so I'd appreciate his sign-off on the revised hertz() just in case there was something magic about reading the TENMS that I don't know about. Otherwise I think this is good to go.

[hudson-ayers]

Though it would lead to some duplicated code, I would lean towards just creating a new ExternalSystick type and reimplementing the SysTick trait for it.

Many of these functions are called on every single iteration of the scheduler, so it seems a shame to have to make these decisions dynamically on every call to enable() and hertz()

[valexandru]

Though it would lead to some duplicated code, I would lean towards just creating a new ExternalSystick type and reimplementing the SysTick trait for it.

Many of these functions are called on every single iteration of the scheduler, so it seems a shame to have to make these decisions dynamically on every call to enable() and hertz()

The hertz() function was not significantly changed. If a value was configured in the chip, self.hertz would be different than 0, otherwise everything remains the same. Would there be a problem for checking the value for external_clock in enable()?

[hudson-ayers]

I thought about it some more and given that hertz was already checked at runtime instead of compile time I suppose this approach is consistent with what we were already doing, so I will retract my requested changes.

[alevy]

Looks good to me

[hudson-ayers]

bors r+

[bors]

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