# 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

- `var volatile dword _rtc_bresenham = _rtc_cycles_per_second`
- `var volatile byte seconds`
- `const _rtc_cycles_per_second = (target_clock / 4)`

### Procedures

- `RTC()`
- `init_timer3()`

### Functions

## API details

### Global variables/contants

- `var volatile dword _rtc_bresenham = _rtc_cycles_per_second`

  No documentation found

- `var volatile byte seconds`

      global variable updated by ISR

- `const _rtc_cycles_per_second = (target_clock / 4)`

  No documentation found

### Procedures

- `RTC()`

  No documentation found

- `init_timer3()`

  No documentation found

### Functions

## Related samples

Here are the list of samples which use this library:

18f2620 | 18f2620_rtc_tmr3.jal