Hello David,
On 10/12/2013 11:13 AM, David Wallis wrote:
> Hi all,
>
> I'm trying to use the watchdog timer on an ATMega324P for power saving by
> sleeping for 8 seconds, and then having the watchdog wake the device.
> However, I'm struggling to set the prescaler flags in WDTCSR.
>
> According to the datasheet, the 'Change Enable' flag (WDCE) must be set to
> change the prescaler bits, but this clears after 4 clock cycles*. I'm
> thinking that perhaps, this can't be done in forth because changing the
> bits will take more than 4 clock cycles (I don't know, I'm guessing).
>
> Has anyone done this before and have a solution? If not, I guess I have to
> write the word in assembler, which I haven't done before. Any pointers on
> how to do this?
>
Are you sure, you want to do this with the watchdog timer? Isn't the
watchdog timer only able to generate a reset?
I have a system which sleeps for 8 seconds, then wakes up to do
a little work, and then go back to sleep. This requires the
sleep function (trunk/core/devices/atmega328p/words/sleep.asm)
to be included. It also needs a 32kHz crystal connected to Pins
C6,C7. Timer2 is then initialized to
a. have the external crystal as clock source (ASSR)
b. have a prescaler of 1024 (TCCR2)
my code examples are from atmega32!
2variable uptime
: .uptime_sec uptime 2@ d.i [char] s emit ;
: .uptime
uptime 2@
2dup decimal d.i [char] s emit space
&60 ud/mod &60 ud/mod &24 ud/mod
d.i [char] d emit space \ days
2 u0.r [char] : emit \ hours
2 u0.r [char] : emit \ minutes
2 u0.r \ seconds
;
: ++uptime 1 s>d uptime 2@ d+ uptime 2! ;
: tick_isr_uptime
1 ticks +!
8 s>d uptime 2@ d+ uptime 2!
;
: +ticks-uptime
$07 TCCR2 c! \ clock_ts2/1024
$08 ASSR c! \ source: 32 kiHz crystal
\ register interupt service routine
['] tick_isr_uptime TIMER2_OVFAddr int!
\ enable timer2 interupt
TIMSK c@ $40 or TIMSK c!
;
The main loop in my Forth programm looks somewhat
like this:
: run
\ <== initialize everything
...
0 s>d uptime 2!
idle.ticks 1+ ticks ! \ run job immediately
+ticks-uptime
begin
\ <== do work only every idle.ticks rounds
\ <== e.g. 4 rounds of 8 seconds each
ticks @ idle.ticks > if
1 ticks !
\ run job
once \ <== here is where the work is done
then
\ back to power down mode
$03 sleep \ <== here we go back to sleep
repeat
;
I know, I'm not answering your question directly,
but hopefully the above can serve as inspiration.
Cheers,
Erich
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