More on 8254: Re: Timer resolution et al : Implementation of Read_timer()
svasn_rtems at yahoo.com
Thu Mar 11 11:11:06 UTC 2004
> 1. The formula for "total" is probably correct,
> because the 16-bit
> timers from 8254 are decremented at each clock and
> when they reach zero
> they are reloaded with the maximum value.
> Therefore, in order to get
> the number of 8254 ticks from the beginning of the
> current period, you
> must subtract the value read from 8254 from the
> maximum value.
Very sorry for wrong statement.
> Pay attention to the mode used for initializing
> 8254, in some modes
> (the "square-wave" modes), each period is composed
> of two half-periods
> during which the timers are reloaded than
> decremented by two, in such
> modes it is more complicated to interpret the values
> read from 8254.
In clock driver, timer0 is initialized as:
outport_byte(TIMER_CNTR0, count >> 0 & 0xff);
outport_byte(TIMER_CNTR0, count >> 8 & 0xff);
and in timer driver, it is initialized as follows:
timerOn(const rtems_raw_irq_connect_data* used)
>> 0 & 0xff);
>> 8 & 0xff);
I think this routine in timer driver is never
called or should never be called, because it conflicts
with clock driver. Is it correct?
> 2. You said that "clicks" is read from timer0. That
> is not the timer
> used for interrupts, which is timer1. The period of
> timer0 (used in
> ancient PC's for refreshing the dynamic memory)
> would not be related to
> the BSP_Configuration.
> Check in the code how is updated "Ttimer_val".
> If it is incremented
> by tick interrupts, then maybe the "clicks" are
> actually read from
> timer1, not from timer0, and in that case
> "US_PER_ISR" should be equal
> to the period of the tick interrupts, as configured.
A fragment from Read_timer is here:
clicks = (msb << 8) | lsb;
total = (Ttimer_val * US_PER_ISR) + (US_PER_ISR -
It looks that same timer is used in both
clock and timer drivers. clicks is read from timer0.
Ttimer_val is incremented in 'timerisr' routine.
Since we have a clock ISR, this timerisr is not
probably not active. Does it mean that Ttimer_val is
not incremented at all? If I am correct, US_PER_ISR
in the above expression should be replaced by
Ttimer_val should get the value of difference in value
of clock_driver_ticks between Timer_initialize() and
Read_timer(). Please tell me whether I am correct.
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