clkfreq timing ?

$WMc%

Hello all

·I'm following the SIRCS,Propeller style article in the Jan.2010 issue of Nuts&Volts by Jon Williams. The sircs start bit "as stated" is 2.4ms and the statement "· START_BIT := clkfreq / 1_000_000 * 1_920· " is used.
My·question is how does this equal 2.4ms ?

I've looked at the· clkfreq·explanation on pg.175 of the Prop.Manuel, But I seem to be missing something.

Thanks in advance for any info/help on this topic.

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The Truth is out there············___$WMc%___···························· BoogerWoods, FL. USA

Post Edited ($WMc%) : 1/1/2010 6:58:43 PM GMT

Mike Green

CLKFREQ is the number of system clock ticks in one second, so CLKFREQ / 1_000_000 is the number of clock ticks in 1us. If you multiply this by 1_920, you'd get the number of clock ticks in 1.92ms.

In the code archives for the article (downloadable from Parallax's Nuts and Volts webpage), the constants look like "CLKFREQ / 1_000_000 * 2400". Where did you get the 1_920 value?

$WMc%

Thanks Mr. Green

· 2400 sounds more like it.

Heres the code I downloaded from the Prop.Obj.Exc.

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The Truth is out there············___$WMc%___···························· BoogerWoods, FL. USA

Mike Green

It looks like Jon is allowing for +-20% error in pulse width. 1920us is 80% of the nominal start pulse width of 2400us.

$WMc%

Mr.Green

Now I really understand the 1920 value. I hadn't thought of a short start bit. I see he did the same thing with "ONE_BIT".
I'll add this 80% of value to the REM code so I'll remember later.

Thanks again

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The Truth is out there············___$WMc%___···························· BoogerWoods, FL. USA