Low latency (direct) interrupt handlers

[avtolstoy]

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Problem

See #1168 #1333

Solution

This PR adds two wiring functions attachInterruptDirect() and detachInterruptDirect() that allow the user code to directly insert interrupt handlers into the interrupt vector table.

Steps to Test

• wiring/no_fixture: INTERRUPTS_04_attachInterruptDirect

Example App

void pvd_irqn_handler() {
    // Do something
}

void setup() {
    attachInterruptDirect(PVD_IRQn, pvd_irqn_handler);
}

void loop() {

}

References

• Closes #1168
• Closes #1333
• [CH7827]

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• TODO: Add documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[pkourany]

Reminder to review!

[m-mcgowan]

Are you psychic...I was literally reviewing this when you commented!

[pkourany]

I'll try and test with SparkIntervalTimer and a scope. This is an important one to release as soon as its ready IMO.

[pkourany]

I tried this feature branch with my SparkIntervalTimer library coupled with my RGBPongClock and I get a hard fault. I will test with a more basic app (blink LED) along with the SparkIntervalTimer library next.

[pkourany]

I tried a simple app with SparkIntervalTimer (using attachInterruptDirect()) and I get hard fault immediately after flashing over the code. If I compile with regular attachInterrupt() the code works fine.

To test, I simply change the original code using attachInterrupt() such as this line:
attachSystemInterrupt(SysInterrupt_TIM3_Update, Wiring_TIM3_Interrupt_Handler_override)
to:
attachInterruptDirect(TIM3_IRQn, Wiring_TIM3_Interrupt_Handler_override)

[avtolstoy]

@pkourany I've just built SparkIntervalTimer with minimal changes:

        if (!attachInterruptDirect(TIM3_IRQn, Wiring_TIM3_Interrupt_Handler_override)) ; //error
        if (!attachInterruptDirect(TIM4_IRQn, Wiring_TIM4_Interrupt_Handler_override)) ; //error
        if (!attachInterruptDirect(TIM5_IRQn, Wiring_TIM5_Interrupt_Handler_override)) ; //error
        if (!attachInterruptDirect(TIM6_DAC_IRQn, Wiring_TIM6_Interrupt_Handler_override));  //error
        if (!attachInterruptDirect(TIM7_IRQn, Wiring_TIM7_Interrupt_Handler_override));  //error

and used the example provided with it, and it does not crash. Don't have a scope at hand, but at the very least D7 is flashing as intended.

EDIT:
Tested on both Electron and Photon.

[pkourany]

@avtolstoy, as I read your post I realized that with all my testing, I failed to update the system firmware parts! I'll test again just to confirm. Thanks!

[pkourany]

@avtolstoy, I must be missing something. I checked out the branch, go to the firmware/modules directory and build/flash the system firmware and app using:

make clean all PLATFORM=photon APP=rgbpcFastInt program-dfu

Everything compiles and flashes but same hard fault. What am I missing?

[pkourany]

Tested with SparkIntervalTimer (with some overdue mods) and it works. Good to go!

[pkourany]

Tested successfully with SparkIntervalTimer library.

[m-mcgowan]

Paul, what was the cause of the hard fault? Anything we need to look into?

[m-mcgowan]

Who's going to write up the docs for this? We need to make clear the interplay between the normal wiring interrupts and the direct interrupts, and provide guidance on when to use which.

[ScruffR]

Will there be a version of attachInterruptDirect() that also allows to provide a priority parameter as attachInterrupt() already does?