> Paul Chicofsky wrote:
> >
> > Hello everybody,
> > I am having deficalty in getting the right dely to the clock i am
> > using for my pic16f877. which i am using 20MHz clock. I have
> > calculated a 1msec as follows:
>
>
> Hi Paul, nobody seemed to notice my last post re this
> but here is a good way to get any timer period you
> need with very little code:
>
>
>
> > Roman Black wrote:
> >
> > Hi Guys, I've implemented a version of Bob's
> > zero-error one second timer. I added two changes
> > and the whole thing fits in just a handful of
> > instructions. It will give perfect results with
> > any crystal speed (that divides evenly by 4).
> >
> > Basic theory: (example 16MHz crystal, 4MHz instruction)
> > * put one second value in 24bit var (4,000,000 ticks)
> > * the TIMER0 int is every 256 instructions
> > * every int we subtract 256 from value
> > * when value gets <0 we generate the one second event,
> > and add another 4,000,000 ticks to var
> >
> > It has zero cumulative error and zero short-term
> > average error. There will be a max error per second
> > of 256 instructions (64uS at 16MHz crystal).
> >
> > The two changes I made from Bob's idea are;
> > * subtracting a set value of 256 (1x mid-byte)
> > * adding an extra 256 ticks at the start.
> >
> > The first change means that instead of doing a
> > proper 24bit subtract every int, you just need
> > to dec the mid byte of the var, (and do carry test
> > and dec msb if needed). Very fast and small code.
> >
> > The second change means the var never goes below
> > zero, instead our <0 test simply becomes a <256
> > test.. This is very quick, we just test that
> > msb=0 AND mid=0.
> >
> > This condition means that now to add the 4,000,000
> > ticks to the variable again we are guaranteed that
> > msb and mid bytes are 0, so we can simply load
> > the two values from literals straight into the
> > msb and mid bytes. We no longer need a proper
> > 24bit add! We must do an add on the lsb byte,
> > but this is only one byte add and one carry test.
> > If carry, we can just inc the mid byte, as it's
> > value is guaranteed.
> >
> > The whole thing is very small and quick as there
> > are no real 24bit add/subtracts needed. :o)
> >
> > -----------------------------------------------------
> > My code is specific to my app but here is the
> > psuedo code:
> >
> > initialising: (assuming 16MHz crystal, 4MHz clock)
> > set TIMER0 to generate int every 256 clocks
> > put 4,000,000 in 24 bit var
> > add another 256 to var (inc mid byte)
> >
> > int handler:
> > save off status/w
> >
> > dec mid byte (subtract 256 from total value)
> > check carry and dec msb if needed
> > (that was our entire 24bit subtract!)
> >
> > now test for msb=0 AND mid=0 (detect <256)
> > if not, goto int exit (very quick int in most cases!)
> >
> > (here we have detected 1 sec event)
> > generate 1 second event
> >
> > (now we do the add 4,000,000 again)
> > load msb value into msb (msb was guaranteed 0)
> > load mid value into mid (mid was guaranteed 0)
> > add lsb value to whatever is left in lsb
> > if carry, inc mid
> > (that's our whole 24bit add done!)
> >
> > int exit:
> > restore status/w
> > -----------------------------------------------------
> >
> > There are a lot of good things about Bob's system.
> > Any crystal can be used, if it gives x instructions/sec.
> >
> > It will also work great for other timing periods,
> > like my datalogger that needs an event at 2Hz, 4Hz, 8Hz,
> > selectable. I just change the total value of ticks
> > added to the 24bit var. This can be done on the fly
> > with no problems.
> >
> > Quirky thing? With my 16MHz crystal, the clock is 4MHz.
> > So a one second period is 4,000,000 ticks. This just
> > happens to be 3D 09 00 in 24bit hex. Notice the lsb
> > is zero. This means that there will be NO ERROR on a
> > one second period. No jitter, as the only jitter with
> > this system is the lsb averaging. That was good luck,
> > I found it by accident! :o)
> >
> > The jitter is not a big problem as with most crystals
> > it will be a short term average. With a 8MHz crystal,
> > (2,000,000 ticks) one second will be 128 ticks late,
> > the next second will be perfect. A 2-cycle average
> > with zero average error. A 4MHz crystal gives 4-cycle
> > average with every 4th second perfect.
> >
> > Hope all this makes sense. Thanks Bob for the cool
> > idea!:o)
> > -Roman
> >
> > PS. If you just need to update a real time clock each
> > second and don't want to spend time in an interrupt
> > every 256 instructions, you could change the TIMER0
> > prescaler to 256. This gives an int every 256x256
> > instructions. Then just subtract one msb every int,
> > using the same principle. Just as quick. Remember
> > to add an extra 256x256 ticks at the start instead
> > of just an extra 256 ticks. Error per second (jitter)
> > is larger, but cumulative and average errors are still
> > zero. Max jitter per second becomes 16ms. Averaging
> > will take 24 seconds. An almost perfect real-time clock
> > with extremely low processing time requirement.
> >
> > --
> >
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