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extmod/utime_mphal: Fix implementation of new semantics of ticks_diff().

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Now the function properly uses ring arithmetic to return signed value
in range (inclusive):
[-MICROPY_PY_UTIME_TICKS_PERIOD/2, MICROPY_PY_UTIME_TICKS_PERIOD/2-1].

That means that function can properly process 2 time values away from
each other within MICROPY_PY_UTIME_TICKS_PERIOD/2 ticks, but away in
both directions. For example, if tick value 'a' predates tick value 'b',
ticks_diff(a, b) will return negative value, and positive value otherwise.
But at positive value of MICROPY_PY_UTIME_TICKS_PERIOD/2-1, the result
of the function will wrap around to negative -MICROPY_PY_UTIME_TICKS_PERIOD/2,
in other words, if a follows b in more than MICROPY_PY_UTIME_TICKS_PERIOD/2 - 1
ticks, the function will "consider" a to actually predate b.
This commit is contained in:
Paul Sokolovsky 2016-10-30 03:07:22 +03:00
parent 76146b3d9a
commit e429daa572
1 changed files with 7 additions and 1 deletions
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8
extmod/utime_mphal.c
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@ -89,7 +89,13 @@ STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
// we assume that the arguments come from ticks_xx so are small ints
uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
int32_t diff = end - start;
if (diff < (signed)-(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
diff += MICROPY_PY_UTIME_TICKS_PERIOD;
} else if (diff >= (signed)(MICROPY_PY_UTIME_TICKS_PERIOD / 2)) {
diff -= MICROPY_PY_UTIME_TICKS_PERIOD;
}
return MP_OBJ_NEW_SMALL_INT(diff);
}
MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);