1ms Elapsed Timer Demo with DT's Instant Interrupts

[Heckler]

Hi Tabsoft (and others who might have some insight)

I have incorporated your 1ms timer (top of this thread) as a basis for my nixie clock. ( I had wanted to implement Darrells' instant interrupts so this was perfect)

I needed a 5 ms interrupt to update my nixie display and was able to modify your "elaptimer_interrupt.pbp" include file to give me the 5 ms interrupt by changing the include as shown below...

                                     ; Added 04/03/2015 "IntTime"
IntTime = 200                  ;  200 = 5ms
                                     ; Change IntTime in calc for interrupt time
                                     ; 1000=1ms, 100=10ms, 10 = 100ms, etc.
                                     ; Needs to be a multiple of 10.

But I am finding that my clock gains about 2 minutes (give or take) in about 18 hours.

I wonder if the problem might be in this part of your include...

if Ticks = 200 then          ' Modified 04/03/2015 from 100 to 1000
                                  '  change Ticks = x to right interrupt time
                                  '  1000=1ms, 100=10ms, 10=100ms, etc.
Ticks = 0

when Ticks go from 0 to 200, wouldn't that be 201 steps.
Shouldn't it be from 0-199 or 1-200 to get the 200 steps??

But in my case where the clock is running fast I should probably start increasing this count to slow the clock down?

somewhere in the thread or in the comments in the include there is alluded to that the code is somehow self calibrating?? or am I misreading that??

What is the best way to arrive at a more accurate time base?

I am just using the internal 8mhz osc in my 16F1828 so I know it won't be super accurate.
My clock does read NTP time every morning so I don't need long term accuracy.
I will probably change the code to read the NTP time twice a day if necessary but I would like to fine tune it a bit if possible.

thanks (in advance) for any guidance from the forum

[Tabsoft]

Heckler,

I'll take a look when I get a few moments. See if I can set it up on some hardware and give it a test run for a while.

[Heckler]

Hi TabSoft,

Possibly I am expecting too much from the internal OSC. The documentation states "Factory Calibrated to within+/- 1%"
1% of 24 Hours is +/- 14.4 minutes so my complaint regarding 2-4 minutes in 24 hours is well within the 1%

I took a look at the elap_timer include file and can't quite tell how I might make fine adjustments to the timing routine other than adding or subtracting in "Ticks" in the following code segment...

if Ticks = 200 then          ' Modified 04/03/2015 from 100 to 1000
                                  '  change Ticks = x to right interrupt time
                                  '  1000=1ms, 100=10ms, 10=100ms, etc.
Ticks = 0

Am I on the right track??

Is it the case that this include file is somehow self calibrating??
If so how does that happen? and against what standard?

This is all somewhat low priority as it is just for a real time clock that only displays hours:minutes and I can opt to check NTP time (more than once a day) or go to an external crystal oscillator.

Thanks for any guidance or explanation you (or others) might be able to offer.