Minimum RTEMS system clock frequency

[Joel Sherrill]

On March 19, 2015 4:06:08 AM CDT, Alexander Krutwig <alexander.krutwig at embedded-brains.de> wrote:
>Hello,
>
>currently I am working on the implementation of FreeBSD timecounters 
>into RTEMS as illustrated in Ticket #2271 of the developer's board 
>(https://devel.rtems.org/ticket/2271).
>In the progress, I found out that the minimum RTEMS system clock 
>frequency should at least be 20Hz to guarantee a correct functionality.
>Are there users that are troubled by this minimum system clock
>frequency?

I have been recommending 1 ms as a good starting spot for a clock tick quantum for years. It is a pretty safe least common divisor. And it minimizes the 1/2 tick average error more than larger values.

I think the default it 10 ms so this should not impact that.

This shouldn't be a problem. Just add a test to confdefs.h if it is above that and a warning or error.  And update the manual for the configuration parameter about the recommended range.

FWIW I have worked on production systems with ticks of 250 ns with no issue. I tested a mpc5200 and got down to 20ns ticks before the tests wouldn't make forward progress. The isr overhead was too much.

>Best regards,
>
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--joel