[TriEmbed] ESP8266 Programming tip

[Peter Soper]

John,

   Forgive me if I'm missing something about your questions and this is 
insulting your intelligence.

    But here's what's really happening. Your app code *except your 
callback function* is executing "above" the interrupt level. So it's 
statements 1, 2, 3, ... N are executing. As MIke said, *DURING* one of 
your statements executing the timer interrupt can happen. Your callback 
function is then called from the timer interrupt handler. It can see 
your program's variable, etc, and when the callback's statements are 
completed and the callback function returns control comes back out of 
the interrupt handler and the CPU picks back up executing your program's 
statements, starting with completion of any current one that's "in the 
middle" of completion for an a/rbitrary definition of "middle"/. This 
includes being in the middle of updating the two halves of a variable's 
value that can't be updated with a single memory write. So, for example, 
if your callback code was manipulating double precision floats along 
with your non-interrupt code, then it would be very possible to have one 
half of the float value glued into another half by messed up order of 
memory operations. But again, as Mike described, there are many other 
ways where there can be trouble if your non-interrupt code is making a 
library call and then your callback code tries to use the same library 
but there is no awareness that both threads of control are sharing some 
memory.

   I'm still not sure what you mean by "sequential" below, but hope this 
helps.

-Pete

On 1/27/21 4:28 PM, John Vaughters via TriEmbed wrote:
> Rodney,
>
> Ok so does the os_timer_arm use interrupts? That is the part that confuses me. If so than it is not sequential as I expected. But can be made practically sequential with the bool flag concept. In my case the tasks are definitely simple now. What kind of time frame would one expect to be acceptable in an interrupt task?
>
> Thanks,
>
> John Vaughters
>
>
>
>
>
>
> On Wednesday, January 27, 2021, 3:55:49 PM EST, Rodney Radford <ncgadgetry at gmail.com> wrote:
>
>
>
>
>
> I just looked at the Ticker library routines and they are only a few lines and basically a wrapper around os_timer_arm which is supposed to have 1 ms accuracy, plus or minus 1ms.
>
> Arduino/Ticker.cpp at master · esp8266/Arduino (github.com)
>
> The os functions are documented here:
>
> 2c-esp8266_non_os_sdk_api_reference_en (espressif.com)
>
> Three is also an os_timer_arm_us that is supposed to be accurate to 500us (so basically 0.5 ms) so it doesn't really get you much better resolution
>
> But it should be accurate enough for most operations as long as you keep your interrupt handlers short/simple
>
>
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