Hi,
I'm using NXP imx6ul-evk(single core cortex-a7 processor) and I'm trying to operate CPU at different frequencies(642MHz, 480MHz, 100MHz, 12MHz) and experiencing time drift on certain frequencies (Generic Timer's time lags Real Time), I'm using virtual timer. I've certain confusions which are following:
* Which clock is going to Generic timer,Why it has range, as I read it operates in range [1-50 MHz], if so then what is its relation with system's core clock which has range up to 696 MHz?
* On system side I've three memory mapped system_counter block (SYSTEM_COUNTER_CTRL = CNTControlBase), SYSTEM_COUNTER_CMP and (SYSTEM_COUNTER_RD = CNTReadBase).
* I consider the timer clock rate 8000000 Hz, load 80000 in CNTV_TVAL and it gives me 10 ms interval but changing the system frequency cause time drift?
sajjad,
To answer your other questions:
The clock going to the Generic Timer is some kind of counter source out in the system. It has little to no bearing on core frequency, in fact it absolutely must count at an absolutely constant 'rate' - the drift you experience is usually the time taken to service the interrupt, which the Generic Timer deals with by treating the TimerView of the counter as a signed value (therefore it shows how many ticks in the negative you are..). While it is possible that you could measure actual system counter drift, it is probably swamped by other factors.
On the i.MX6UL it seems the "System Counter" you're referencing is that Counter Module from Martin's post. It's probably not a good idea to actually use the memory-mapped system counter if you are able to use the system register interface. What that counter is doing is offering a programmable interface which controls up to 64 bits of counter value - either natural binary or greycoded to get around the system. The Generic Timer doesn't actually keep time, nor can you disable the counter from counting - the Generic Timer enable has no effect on the Counter Module. It is just two comparators against an input value, provided by that counter module, and the enable is simply whether the comparators would create an event.
Would it not be better to do:
time_to_event = 10ms - cntv_tval
cntv_tval = time_to_event
and use the TimerValue view of the timer? It seems like a lot less work..
If the time taken to countdown the same specific TVAL changes on CPU clock speed then NXP have broken the system counter (or given it the wrong clock root.. cross your fingers that it is configurable!)
Ta,
Matt
Thanks Matt, I used the same flow as you mentioned.
thanks, helpful. We are using 528MHz imx6ul on MYS-6ULX board.