It can be dangerous to wait for a specific timer value in case an interrupt happens at just that moment and you miss the required count. So it is better to wait until the counter has reached at least the target value. But as noticed, comparing the timer with a target value creates a problem when the target is lower than the initial value.
One way to avoid this problem is to consider the interval that has elapsed with unsigned
variables and arithmetic. Their behaviour is well defined when values wrap.
A hardware counter is almost invariably of size 8, 16, 32 or 64 bits, so choose a variable type to suit. Suppose the counter is 32-bit:
void delay(uint32_t period)
{
uint32_t mark = TIMER_read(0);
uint32_t interval;
do {
interval = TIMER_read(0) - mark; // underflow is well defined
} while(interval < period);
}
Obviously, the required period must be less than the counter's period. If not, either scale the timer's clock, or use another method (such as a counter maintained by interrupt).
Sometimes a one-shot timer is used to count down the required period, but using a free-run counter is easy, and using a one-shot timer means it can't be used by another process at the same time.
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