rtc_isr_tmr3

Real Time Clock running from ISR on TMR3

Author Eur van Andel, Copyright © 2010, all rights reserved.
Adapted-by
Compiler 2.4o

Description

This Interrupt Service Routine updates the variable seconds
about once per second. The precision is 0.2 ppm, the accuracy depends
on the Xtal used. The main program should check if seconds are > 60 and
if so, increase minutes, hours, days, months, etc. calendar.jal does this


Sources

http://www.romanblack.com/one_sec.htm
         http://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm


Notes

This is the Bresenham Line Algorithm, invented at IBM in 1962,
which gets an accurate end result by summing the small errors resulting of
taking discrete steps and correcting when the error gets too large. This
means that individual second lengths may show some jitter, but that long-term
timekeeping is accurate. 
.
For instance: if xtal=20 MHz, then 5 MHz clock, so 5 000 000 clocks/second
TMR3 runs on internal clock speed and interrupts on overflow
every TMR3 interrupt decreases _rtc_bresenham by 0x1_00_00, or 2^16
if _rts_bresenham < 0x1_00_00 then 5_000_000 is added
so the jitter is 2^15/5e6 = 0.6%
Total second count is accurate, since the lower 15 bits stay intact and add up
.
Example of use:
init_timer3 ()               ;initialize TMR3
INTCON_GIE = on             ;enable interrupts
PIE2_TMR3IE = on            ;enable TMR3 interrupts
forever loop
                            ; use seconds for whatever you need
end loop


Dependencies

No dependency found



Summary

Global variables/contants

Procedures

Functions


API details

Global variables/contants

Procedures

Functions


Related samples

Here are the list of samples which use this library:

18f262018f2620_rtc_tmr3.jal