cpu/cortexm_common: implement SysTick timer, add option to use it for XTIMER #14553

benpicco · 2020-07-18T19:04:17Z

Contribution description

SysTick is a 24 bit down-counting timer that is implemented on every Cortex-M processor.
It runs at the same frequency as the CPU and can generate an interrupt when it reaches 0.

It allows to free up hardware timers for other uses and accelerates bring-up of new Cortex-M platforms since SysTick is implemented the same everywhere.

To comply with the RIOT timer interface (xtimer & ztimer) that expect an up-counting, monotonic timer,
SysTick is augmented by software to fulfill these requirements.

This also adds a software prescaler to simulate arbitrary frequencies and introduces a virtual, 32 bit counter.
That allows for longer intervals than would be possible with the SysTick hardware peripheral alone and makes it possible to use it as a backed for xtimer.

I initially hoped to see some power savings by not running a timer peripheral, but on examples/timer_periodic_wakeup power draw went up from 2.35 mA with the periph timer to 2.37 mA with the SysTick timer as source for the xtimer clock.

Testing procedure

A basic test is included with tests/periph_systick.
But it is possible to run any test or application that uses xtimer off SysTick by adding

USEMODULE += xtimer_on_systick

to the application Makefile.

This should work on any Cortex-M processor.

Issues/PRs references

bergzand · 2020-07-19T17:50:22Z

Last time I tried something with the systick timer, I found out that the timer is stopped during CPU standby. Probably because the CPU clock is stopped during those standby times. Is that the reason why power consumption increased when using the systick timer?

maribu

See inline comments

cpu/cortexm_common/include/systick.h

cpu/cortexm_common/periph/systick.c

cpu/cortexm_common/include/systick.h

tests/periph_systick/main.c

benpicco · 2020-07-20T09:15:32Z

Last time I tried something with the systick timer, I found out that the timer is stopped during CPU standby.

But that's true for most periph timers too, usually only RTT will keep running during standby.
In the WFI state, SysTick should still be running.

But we are getting more interrupts as SysTick will overflow after 350ms at 48 MHz.

maribu · 2020-07-20T10:07:02Z

Last time I tried something with the systick timer, I found out that the timer is stopped during CPU standby.

But that's true for most periph timers too, usually only RTT will keep running during standby.
In the WFI state, SysTick should still be running.

But we are getting more interrupts as SysTick will overflow after 350ms at 48 MHz.

IMO, power saving is just broken with xtimer anyway. So I don't think we need to worry about this. Having a widely available timer backend for xtimer is IMO a nice addition when porting new CortexM based MCUs, as it allows testing and merging those boards without timer support.

With ztimer we can use an RTT for long timeouts and power saving. Having in addition a high resolution timer can complement the setup in some cases, when short but high resolution timeouts are needed e.g. in device drivers. With #13722 being merged now, the power consumption would only be higher while the high resolution timer is actually used (e.g. during an interaction sequence of a driver with the hardware).

maribu · 2020-08-05T19:39:33Z

Needs a rebase. Feel free to squash, btw.

maribu · 2020-08-05T19:42:14Z

I still think that frequency conversion is better left to the upper layer. At least ztimer should work with this. And if not, I'd say this is a bug worth fixing :-)

benpicco · 2020-08-05T21:34:45Z

I've moved the use of the prescaler to the systick_prescaler pseudo-module.
This way we don't have to layer xtimer on ztimer on systick if we use xtimer and can let ztimer do the prescaling once it's hooked up.

maribu

I think the compiler won't be able to detect that now_offset is only needed when module systick_prescaler is used. So maybe the suggested changes would result in slimmer code in that case?

cpu/cortexm_common/periph/systick.c

kaspar030 · 2020-08-06T12:24:27Z

cpu/cortexm_common/include/systick.h

+ *
+ * To comply with the RIOT timer interface that expects an up-counting, monotonic
+ * timer, SysTick is augmented by software to fulfill these requirements.
+ * This also adds a software prescaler to simulate arbitrary frequencies and introduces


Is this necessary and / or desirable?

We already have two higher level timer implementations that do exactly that, and more.

There are quite some challenges to overcome to get this right.

As is, I'd probably cut out the guts and re-implement as ztimer backend.

The issue was that xtimer cannot handle arbitrary frequencies: #14553 (comment)

I'm personally fine with having this a ztimer only. IMO, xtimer is on the way out anyway.

I found it useful to just being able to use xtimer_on_systick to free up the periph timer and not having to configure ztimer as well.
I don't think the conditional overflow handling adds much to the code, and it's pretty minimal when enabled.

ztimer still expects the timer to be up-counting, right?

talih0 · 2020-08-18T15:05:28Z

In cpu/cortexm_common/vectors_cortexm.c:476, there is a comment saying that SysTick interrupt is unused. You want to remove or update it.

benpicco · 2020-08-18T15:25:03Z

Thank you, updated.

This introduces the `xtimer_on_systick` pseudo-module that can be used to source the xtimer clock from SysTick, available on every Cortex-M CPU.

tests/periph_systick/app.config.test

kconfigs/Kconfig.features

cpu/cortexm_common/Kconfig

Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>

tests/periph_systick/Makefile

tests/periph_systick/app.config.test

hugueslarrive · 2021-03-03T14:47:58Z

Why not handle the hardware prescaler by 8 (CLKSOURCE=0) ?

It seems confusing to me to see an option as xtimer_on_systick or filenames as systick.c or systick.h in a tickless operating system so why not use another name as "core_timer" for exemple ?

stale · 2022-03-02T11:18:41Z

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.

benpicco added Type: new feature The issue requests / The PR implemements a new feature for RIOT Area: cpu Area: CPU/MCU ports labels Jul 18, 2020

benpicco requested review from maribu, kaspar030, bergzand and dylad July 18, 2020 19:04

benpicco requested review from DipSwitch, haukepetersen, MichelRottleuthner, MrKevinWeiss and thomaseichinger as code owners July 18, 2020 19:04

benpicco force-pushed the cpu/cortexm_common-SysTick branch 3 times, most recently from 3670475 to 8df5548 Compare July 18, 2020 19:11

dylad mentioned this pull request Jul 18, 2020

cpu/sam0_common: add support to run PWM off TC timers #14478

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keestux mentioned this pull request Jul 19, 2020

SAMD21 can lockup and not wake from sleep #14545

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maribu reviewed Jul 20, 2020

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benpicco requested review from cgundogan, jia200x and leandrolanzieri as code owners September 9, 2020 21:02

cpu/cortexm_common: implement SysTick timer

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benpicco force-pushed the cpu/cortexm_common-SysTick branch from bb6789d to 333143f Compare December 3, 2020 19:47

benpicco mentioned this pull request Dec 3, 2020

cpu/qn908x: Initial minimal support for NXP QN908x CPUs. #13855

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benpicco added 2 commits December 3, 2020 22:40

tests/periph_systick: add test for SysTick

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sys/xtimer: add option to run xtimer off SysTick

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This introduces the `xtimer_on_systick` pseudo-module that can be used to source the xtimer clock from SysTick, available on every Cortex-M CPU.

benpicco force-pushed the cpu/cortexm_common-SysTick branch from 333143f to 9b16b84 Compare December 3, 2020 21:41

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Update cpu/cortexm_common/Kconfig

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Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>

fixup! Update cpu/cortexm_common/Kconfig

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fixup! fixup! cpu/cortexm_common: implement SysTick timer

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jukrebs mentioned this pull request Jun 11, 2021

add a systick interface for the ztimer #16543

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MrKevinWeiss added this to the Release 2021.07 milestone Jun 21, 2021

benpicco mentioned this pull request Jul 8, 2021

cpu/rpx0xx: port RIOT to the Raspberry Pi RP2040 MCU #16609

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cpu/cortexm_common: implement SysTick timer, add option to use it for XTIMER #14553

cpu/cortexm_common: implement SysTick timer, add option to use it for XTIMER #14553

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benpicco commented Jul 18, 2020

Contribution description

Testing procedure

Issues/PRs references

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