1ms Elapsed Timer Demo with DT's Instant Interrupts

[Tabsoft]

I think we may be beginning to cloud up this thread.
Do you want to start another?

[Art]

There might not be a need to if I fix the second channel on my scope, and do what you did,
except look at the generated pulse on one ch, and the GPS pulse on the other.
Then if I can’t sort it out I’ll start another one.
Cheers, Brek.

[Tabsoft]

Art,

Just curious if you got this to work.
I would think that you would have to define a DT interrupt source for the EXT int and set the handler label.

[Art]

Hi Tabsoft
No, but I had a moment of inspiration. Wouldn’t even need the interrupt, but it will help.
The idea is to sync it roughly, with port.0 interrupt, then turn off ext int and the clock will be somewhere close.

Then although port.0 int is disabled, the IO pin still works... so next, increment or decrement the so called “constant value”
until the timer cycle you check port.0 straight after the interrupt and find the GPS PPS pin is high.
Then subtract as many instructions from the constant value as it took to check port.0 after the timer reload.

For 10MHz pic, that might have it down to the 400ns instruction time, but I’m not sure of that.

[Tabsoft]

What is the interval and duration of your input pulse on PORTB.0 from the GPS?

[Art]

Hi, looks like I ignored you there..... We took it to email!

[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