rtc_isr_tmr2

Real Time Clock running from ISR on TMR2

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

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
TMR2 runs on internal clock speed and interrupts on overflow
every TMR2 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
.
This RTC runs on TMR2. You must setup TMR2 yourself
an example how to do this for a PIC16F876A is given here:
prescaler = 1, internal clock, TMR2 = on
PIE_TMR2IE = on, INTCON_GIE = on
.
T2CON_TOUTPS   = 0b0000    -- 1:1 postscaler
T2CON_TMR2_ON  = on        -- TMR2 on
T2CON_T2CKPS   = 0b00      -- 1:1 prescaler
.
PIE_TMR2IE     = on        -- TMR2 interrupt enabled
INTCON_PEIE    = on        -- peripheral interrupt enabled
INTCON_GIE     = on        -- global interrupt enabled


Dependencies

No dependency found



Summary

Global variables/contants

Procedures

Functions


API details

Global variables/contants

Procedures

  • RTC()

    No documentation found

Functions


Related samples

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