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Document input.runningTimeMicros() overflows after 18 minutes #5372

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microbit-carlos opened this issue Oct 17, 2023 · 5 comments · Fixed by #6170
Closed

Document input.runningTimeMicros() overflows after 18 minutes #5372

microbit-carlos opened this issue Oct 17, 2023 · 5 comments · Fixed by #6170
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@microbit-carlos
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This caught me by surprised and it's a MakeCode specific limitation, as CODAL returns a 32 bit unsigned int and MakeCode drops the 2 most significant bits:

pxt-microbit/libs/core/control.cpp

Lines 239 to 245 in 975c890

/**
* Gets current time in microseconds. Overflows every ~18 minutes.
*/
//%
int micros() {
return system_timer_current_time_us() & 0x3fffffff;
}

I guess the signed bit makes sense, but don't know why the second bit is dropped as well.

It was done in commit 15b4b47 as part of PR #2572 (512b4dd).

This should probably be documented, as 18 minutes is a relatively short time that can be easily reached.

Docs:

pxt-microbit/docs/reference/input/running-time-micros.md

Lines 1 to 16 in 975c890

# Running Time Micros
Find how long it has been since the program started in micro-seconds.
```sig
input.runningTimeMicros();
```
## Returns
* the [Number](/types/number) of microseconds since the program started.
(One second is 1000000 microseconds.)
## See also
[show number](/reference/basic/show-number), [pause](/reference/basic/pause)

@microbit-carlos microbit-carlos changed the title Document input.runningTimeMicros() overruns after 18 minutes Document input.runningTimeMicros() overflows after 18 minutes Oct 17, 2023
@microbit-carlos
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@pelikhan out of curiosity, do you know why the 2 most significant bits from CODAL/DAL system_timer_current_time_us are masked out in MakeCode? (it was done in 512b4dd)

pxt-microbit/libs/core/control.cpp

Lines 239 to 245 in 975c890

/**
* Gets current time in microseconds. Overflows every ~18 minutes.
*/
//%
int micros() {
return system_timer_current_time_us() & 0x3fffffff;
}

The signed bit makes sense, but not sure why bit 30 is removed as well, was there a technical reason?

@pelikhan
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@mmoskal

@mmoskal
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mmoskal commented Oct 20, 2023

Integers up to 0x3fff_ffff are unboxed. Anything larger is boxed as a double. We could change this function to return a double (I think the codal call returns uint64_t).

@microbit-carlos
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Ah, that's interesting to know, thanks!

And yes, both DAL and CODAL return uint64_t (codal-microbit-v2 set CODAL_TIMESTAMP in target.json).

Not sure if changing the size range of input.runningTimeMicros() would be considered a breaking change (probably no, as it's still a number in JS/TS), so up to you, although if it's left as it is, it's worth noting the overflow in the docs.

@abchatra
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We need to document this. Not planning to fix.

@ganicke ganicke mentioned this issue Apr 30, 2025
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Update running time api reference doc with overflow note microsoft/pxt-microbit
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@pelikhan @abchatra @mmoskal @microbit-carlos