PIT initializing in MPC860

[Joel Sherrill <joel@OARcorp.com>]

Thomas Doerfler wrote:
> Hello Leon,
> 
> I also got stuck at these lines. These lines were initially 
> coded for a mpc860 system, where the PIT was fed by an 
> external 4 MHz crystal, not the 25MHz oscillator. For a 4MHz 
> crystal, you set 
> 
> Cpu_table.clicks_per_usec=4
> Cpu_table.microseconds_per_tick=10000
> 
> and get a resulting PIT initalization value of 40000, which is 
> just a bit below 2^16. This works fine.
> 
> If your PIT input clock gets increased (to 25MHz) then the 
> resulting divider value simply does not fit into the 16 PIT 
> reload register and then you get odd system tick frequencies.
> 
> You may activate the prescaler between the clock input and the 
> PIT input (divide by 128), but in that case you get very odd 
> timer tick frequencies. 
> 
> We came over this some weeks ago on newer MBX860 boards, and 
> due to certain reasons I only could hack together a solution 
> for demo purposes, but I did not yet find time to implement it 
> cleanly. (Yes, Joel, I know you are waiting for it aswell...)

:)  And thanks again for your hospitality. :)

If I remember the problem correctly, you have to know the
clock frequency to select the proper divider and this is
all done inside the BSP at compile time.  Is there a way to
probe for the clock frequency being used?

If not, would it be feasible to have a BSP specific configure
option to specify the clock information?  And use some default
value otherwise based upon the BSP model variant?

> I hope this helps a bit.
> 
> wkr,
> 	Thomas.
> 
> 
>>Hello to MPC860 gurus :-)
>>
>>	I am stuck with the following small issue:
>>The file ../libcpu/powerpc/mpc8xx/clock/clock.c contains:
>>
>> pit_value = (rtems_configuration_get_microseconds_per_tick() *
>>               rtems_cpu_configuration_get_clicks_per_usec()) - 1 ;
>>
>>and this value is calculated incorrectly in my case - MPC860 running at 25MHz 
>>I put Cpu_table.microseconds_per_tick=10000 (10ms per RTEMS clock) and 
>>Cpu_table.clicks_per_usec=25.
>>
>>This means that I incorrectly understand the meaning of 
>>Cpu_table.clicks_per_usec. As it is also used in several other places, I do 
>>not want to change it by deriving the required value from the formula in 
>>MPC860 manual.
>>
>>I tried to find something in documentation, but was not able to find this 
>>Cpu_table, which is used in mpc8xx.
>>
>>I shall be very thankful for clarification.
>>-- 
>>Leon M.Pollak
>>leonp at plris dot com
> 
> 
> --------------------------------------------
> IMD Ingenieurbuero fuer Microcomputertechnik
> Thomas Doerfler           Herbststrasse 8
> D-82178 Puchheim          Germany
> email:    Thomas.Doerfler at imd-systems.de
> PGP public key available at: http://www.imd-
> systems.de/pgp_keys.htm
> 


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