Hi Gordon,
Gordon Henderson schrieb:
> void mathTest (uint16_t x)
> {
> uint32_t y ;
> uint32_t i = 0 ;
>
> for (;;)
> {
>
> y = (uint32_t)x * 9668 + 500 ;
> if (y != 4225416L)
> {
> serialPuts ("Curruption @ ") ;
> ...
> }
> ++i ;
>
> }
>
> works fine with interrupts off, or with the math code in the interrupt
> routine commented out, but sees random coruptions with them on:
[..]
> That's the first I've noticed int and I've not even dived into floating
> point land yet...
This does not really come as a surprise:
The manual has a special section (3.9.1.4) Common interrupt pitfall
which specifically mentions 16 bit and 32 bit arithmetics...
> Any thoughts, anyone?
First thought would be not to use math code within interrupts:)
Second thought, if you just need a multiply within interrupt,
you could (taking multiply with 9668 (= 0x25c4) and add with 500 as example)
get to the intended y by:
uint32_t xx = x;
y = 500;
xx <<= 2; // 0x0004
y += xx;
xx <<= 3; // 0x0040
y += xx;
xx <<= 1; // 0x0080
y += xx;
xx <<= 1; // 0x0100
y += xx;
xx <<= 2; // 0x0400
...
(bugs are mine, also check that the addition does not cause diving
into a nonreentrant function)
Greetings,
Frieder
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