long countdown timer, how to save power?

[skimask]

Any idea what Darrel was talking about when he was saying it could be done without using an interrupt?

I think what he was talking about is that you can use the WDT to wake up the PIC here and there and check the interrupt flag bits for Timer1 manually rather than have Timer1 provide the wakeup. Then reset the interrupt flag bits and go back to sleep.

[Darrel Taylor]

I think something like this would work. (untested)

<font color="#000000"><b>TMR1IF  </b><font color="#008000"><b>VAR </b></font><b>PIR1</b>.<font color="#800000"><b>0

</b></font><font color="#0000FF"><b><i>;Countdown from
</i></b></font><b>Years     </b><font color="#008000"><b>CON </b></font><font color="#800000"><b>0
</b></font><b>Days      </b><font color="#008000"><b>CON </b></font><font color="#800000"><b>30
</b></font><b>Hours     </b><font color="#008000"><b>CON </b></font><font color="#800000"><b>4
</b></font><b>Minutes   </b><font color="#008000"><b>CON </b></font><font color="#800000"><b>12

</b></font><b>TheCount  </b><font color="#008000"><b>VAR WORD</b></font>[<font color="#800000"><b>2</b></font>]

<font color="#008000"><b>ASM  
</b></font><font color="#000080">TotalSeconds = _Years*31536000 + _Days*86400 + _Hours*3600 + _Minutes*60
    MOVE?CW TotalSeconds &amp;&amp; 0xFFFF, _TheCount
    MOVE?CW TotalSeconds &gt;&gt; 16, _TheCount + 2
</font><font color="#008000"><b>ENDASM

</b></font><b>T1CON </b>= <font color="#800000"><b>%00001011                  </b></font><font color="#0000FF"><b><i>; T1 OSC ON, Timer ON

</i></b></font><b>Main</b>:
    <b>TMR1IF </b>= <font color="#800000"><b>0                     </b></font><font color="#0000FF"><b><i>; clear the Timers IF flag
    </i></b></font>@ <font color="#008000"><b>SLEEP
    </b></font>@ <b>NOP                        
    TheCount </b>= <b>TheCount </b>- <font color="#800000"><b>2        </b></font><font color="#0000FF"><b><i>; subtract 2 seconds
    </i></b></font><font color="#008000"><b>IF </b></font><b>TheCount </b>= <font color="#800000"><b>0 </b></font><font color="#008000"><b>THEN           </b></font><font color="#0000FF"><b><i>; if low Word is 0
        </i></b></font><font color="#008000"><b>IF </b></font><b>TheCount</b>(<font color="#800000"><b>1</b></font>) = <font color="#800000"><b>0 </b></font><font color="#008000"><b>THEN    </b></font><font color="#0000FF"><b><i>;   check high Word
            </i></b></font><font color="#008000"><b>HIGH </b></font><b>PORTB</b>.<font color="#800000"><b>0           </b></font><font color="#0000FF"><b><i>;   count complete, turn a pin on
            </i></b></font><font color="#008000"><b>STOP
        ENDIF  
        </b></font><b>TheCount</b>(<font color="#800000"><b>1</b></font>) = <b>TheCount</b>(<font color="#800000"><b>1</b></font>) - <font color="#800000"><b>1  </b></font><font color="#0000FF"><b><i>; not finished dec high Word
    </i></b></font><font color="#008000"><b>ENDIF
GOTO </b></font><b>Main
</b>

Make sure the WDT, VREF and anything that draws current is turned off.

HTH,
  DT

[Kamikaze47]

Thanks Darrel. Nice work as usual.

I assume that GIE and TMR1IE have to be set in order for this to work?

The only thing i might add is TMR1H.7=1 so that it will wake up every second instead of every 2 seconds.

*edit* Was just looking at the data sheet, and half answered my own question. So now i'm assuming that I need GIE clear and TMR1IE set.

[Darrel Taylor]

I assume that GIE and TMR1IE have to be set in order for this to work.

The only thing i might add is TMR1H.7=1 so that it will wake up every second instead of every 2 seconds.

You need to leave GIE OFF, since there's no Handler for it to jump to.
But I think you're right, you may have to turn TMR1IE on to get it to Wake-Up.

TMR1H.7=1 will work too. But it's waking up twice as often as needed, and will use twice the power.

And after a month, a 1 second difference is less than the error rate, so attempting to get 1 second accuracy is futile at best.

DT

[Kamikaze47]

True enough. The only reason i may want to do it is so that i can watch the seconds count down when the LCD is switched on (will have a pushbutton in series with the LCD power so its only on when u press the button). I guess its a weigh-up between battery life and pretty counting seconds

[skimask]

True enough. The only reason i may want to do it is so that i can watch the seconds count down when the LCD is switched on (will have a pushbutton in series with the LCD power so its only on when u press the button). I guess its a weigh-up between battery life and pretty counting seconds

Should be easy enough to change the counting rate/wake up rate dynamically if you end up using the TMR1 method as described above.

[Kamikaze47]

good idea... if i tie the lcd's power pin to an input pin i can tell if the button is pressed and switch to counting seconds.

Thanks guys, u've been a big help.