Incorrect PIT value inititalization?

[Joel Sherrill <joel@OARcorp.com>]

Leon Pollak wrote:
> Hello, all.
>         We have some problem with the PIT initialization code for MPC860. As 
> it 
> resides in the common directory, we think that it is worth asking the 
> question here.
>         Our board has the 5MHz XTAL clock. The file
> /opt/rtems/rtems-src/c/src/lib/libcpu/powerpc/mpc8xx/clock/clock.c:72
> 
> in function ClockOn the code that initializes the PIT value (see below)
> seems to be incorrect in case of a slower XTAL (not 50MHz as mbx860 has), 
> yielding float values for clicks_per_usec.
> For example a 5MHz XTAL divided by 4 produces a 1.25MHz frequency, which
> means that clicks_per_usec = 1.25. When it is assigned to an unsigned32
> member it is truncated to 1, and the resulting value is only a rough
> estimate of the value actually desired.
> 
> Original code:
> pit_value = (rtems_configuration_get_microseconds_per_tick() *
>                rtems_cpu_configuration_get_clicks_per_usec()) - 1 ;
> 
> We suggest the following modification:
> 
> /* PIRTCLCK is XTAL / 4. XTAL is computed from the clock speed and
> PLPRCR. */
> unsigned long pitrtclk = (Cpu_table.clock_speed / ((m8xx.plprcr >> 20) +
> 1)) >> 2;
> pit_value = pitrtclk / 1000 * rtems_configuration_get_milliseconds_per_tick() 
> - 1;
> 
> 
> Do MPC8xx gurus agree?

Hopefully someone will confirm this solution.  When you get a patch, 
please file it as a PR.

> Thanks
> Leon Pollak
> 


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Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
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