Re: [ECOS] CDL define_proc: Unable to put "extern" in an include files

[Nick Garnett <nickg@ecoscentric.com>]

Jim Seymour <ecosjim@cipher.com> writes:

> 
> What I'm trying to do is allow for our system clock speed to be a
> run-time variable, instead of a fixed constant.  This means that the
> #define's that get generated for CYGNUM_RTC_PERIOD (and the like) will
> refer to a global variable.  Without an "extern" statement, these
> references will cause a compiler error.

There are other ways to achieve this.

The way I have done this in the past is not to make CYGNUM_RTC_PERIOD
variable. Too much code assumes that this value is a constant; it uses
its value on startup to calculate some scaling factors, or whatever,
and then doesn't read it again. So even if CYGNUM_RTC_PERIOD were to
vary, the original derived values would still be used.

Instead, where I want to run the system at different clock speeds I
express CYGNUM_RTC_PERIOD is terms of a fictional fixed rate clock;
usually around 1MHz, but you can go as low as 1KHz if you want. Then,
in HAL_CLOCK_INITIALIZE(), I calculate the real clock period using the
current system clock rate. The selected period is saved in the HAL, so
when I change the system clock rate I can recalculate the real period
(just recalling HAL_CLOCK_INITIALIZE() does the trick usually).

Obviously HAL_CLOCK_READ() also needs to do some arithmetic to
translate system clock ticks back into ticks of the fictional clock.


-- 
Nick Garnett                                     eCos Kernel Architect
eCosCentric Limited     http://www.eCosCentric.com/   The eCos experts
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