Hi Guennadi,
Guennadi Liakhovetski schrieb:
> On Thu, 7 Aug 2008, Jens Gehrlein wrote:
>
>> Hi Guennadi,
>>
>> Guennadi Liakhovetski schrieb:
>>
>>> diff --git a/cpu/arm1176/s3c64xx/interrupts.c
>>> b/cpu/arm1176/s3c64xx/interrupts.c
>> [snip]
>>> +void udelay(unsigned long usec)
>>> +{
>>> + unsigned long long tmp;
>>> + ulong tmo;
>>> +
>>> + tmo = (usec + 9) / 10;
>>> + tmp = get_ticks() + tmo; /* get current timestamp */
>>> +
>>> + while (get_ticks() < tmp)/* loop till event */
>>> + /*NOP*/;
>>> +}
>> I tried to follow the interrupt init function, but it's difficult.
>> What is the least possible delay with the udelay function?
>> I remember that i.mx31 udelay cannot currently be below 30 microsseconds due
>> to the low input clock. If someone uses udelay(1) in polling loops with
>> timeout, the actual duration is much more longer than assumed.
>
> Actually, the "/ 10" and this comment
>
> /*
> * We use the following scheme for the timer:
> * Prescaler is hard fixed at 167, divider at 1/4.
> * This gives at PCLK frequency 66MHz approx. 10us ticks
> * The timer is set to wrap after 100s, at 66MHz this obviously
> * happens after 10,000,000 ticks. A long variable can thus
> * keep values up to 40,000s, i.e., 11 hours. This should be
> * enough for most uses:-) Possible optimizations: select a
> * binary-friendly frequency, e.g., 1ms / 128. Also calculate
> * the prescaler automatically for other PCLK frequencies.
> */
>
> shall make it clear, that the timer resolution is configured to 10us,
> which is also the minimum udelay. The timer could also do better, but then
> we get other disadvantages, e.g., shorter wrap-around time.
Okay. Thank you.
I think, it's very useful for a developer to know the limits of high
resolution timers and to see it immediately, so he can consider it in
his code.
Kind regards,
Jens
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