Making a timer, 32.768kHz+555+DT_INT. Good idea?

[financecatalyst]

That would gain about 2.5 seconds per hour, which is not good.

Have you set LP oscillator mode in your configs?
Since it worked for you at 4mhz, it's probably set to XT and over driving the crystal.

How are you measuring that frequency?
Is it a high quality frequency counter?
Accurate measurements of low frequencies can be difficult, with large errors.

Yes, I change it to LP when driving it with 32 kHz and your advised process to run timer 1 from internal clock (loading TMR1H=%11100000) & XT when 4MHz (with 32kHz attached 4 TMR1).

For frequency, unfortunately I am not equipped with those luxuries, I am just using an Timer Calculator I found in the Forum only. Screenshot attached.

[financecatalyst]

Even with loading 32768 into TMR1, the calculator shows the interrupts 1.00027 Hz.
This is just to seek clarification, it is unlikely that I will be able to use 4MHz for OSC & 32kHz for TMR1.

[Darrel Taylor]

That calculator is wrong.

Here's the way it works ...
The main OSC is running at 32768 hz.
The CPU is running a 8192 hz (32768 / 4).

Timer1 is loaded with the highbyte of 57344 and will overflow at 65536.
65536 - 57344 = 8192

So the timer overflows exactly 1.0000 seconds every time (depending on the crystal tolerance).

[financecatalyst]

That calculator is wrong.

Here's the way it works ...
The main OSC is running at 32768 hz.
The CPU is running a 8192 hz (32768 / 4).

Timer1 is loaded with the highbyte of 57344 and will overflow at 65536.
65536 - 57344 = 8192

So the timer overflows exactly 1.0000 seconds every time (depending on the crystal tolerance).

OK, I am now running everything as advised. I synchronised the time with my phone last night. Today appx after 12 Hrs, the phone is appx 6 seconds ahead than the clock. What could be the possible problem?

[Dave]

Financecatalyst, I would first check that the crystal is indeed operating at the proper frequency... (Frequency counter)

Dave Purola,
N8NTA

[Bruce]

With Timer1 you'll normally want to stop it, add in your reload value to the existing count (+ whatever time is involved for instructions after stopping it), then restart it. This can throw you off if you're not right on.

This could be a lot easier with Timer2 since it gets automatically reset - so you don't need to mess with reloading the timer.

Here's an example that keeps spot-on with my PC time clock to the second;

DEFINE OSC 4  ' using a 4MHz crystal
 
' setup vars for clock
Time    VAR BYTE   ' accumulates TMR2 to PR2 match counts
Minutes VAR BYTE   ' minutes
Hours   VAR BYTE   ' hours
Seconds VAR BYTE
Match   VAR PIR1.1 ' TMR2 to PR2 match interrupt flag bit
 
CMCON = 7          ' disable comparators
INTCON = 0         ' not using interupts. Just monitoring int flag bits
 
Time = 0           ' clear TMR2 to PR2 match counter
Hours = 11         ' set clock starting hour here
Minutes = 52       ' set clock starting minutes here
Seconds = 0
 
PR2 = 249           ' 249 +1 extra cycle for TMR2 reset = 250*5*16*1uS=20mS
Match = 0           ' clear match flag
 
' setup & start TMR2
T2CON = %00100110   ' 1:5 postscale, 1:16 prescale, TMR2 on
 
Main:
   ' every 20mS the TMR2IF flag is set, and this routine is entered.
   ' Plenty of time to do other stuff.
   IF Match THEN          ' has TMR2 matched PR2? (should happen every 20mS)
      Match = 0           ' yes. clear TMR2 to PR2 match flag bit
      Time = Time + 1     ' increment 20mS count     
      IF Time = 50 THEN   ' 50 x 20mS = 1 second
         Time = 0         ' show time in 1 second intervals
         HSEROUT ["Time = ", DEC2 Hours,":",DEC2 Minutes,":",DEC2 Seconds,13,10]
         Seconds = Seconds + 1
      ENDIF    
      IF Seconds = 60 THEN     ' has 60 seconds passed?
         Seconds = 0
         Minutes = Minutes + 1 ' increment minute count
         IF Minutes = 60 THEN  ' have 60 minutes passed?
            Minutes = 0        ' yes. roll-over minutes 00
            Hours = Hours + 1  ' update hours
            IF Hours = 24 THEN Hours = 0 ' roll-over hours from 24 to 00
         ENDIF ' end IF Minutes = 60                
      ENDIF    ' end IF Seconds = 60
   ENDIF       ' end IF Match
 
   GOTO Main
   END

Of course you'll want a 4MHz osc that's right on the money if you need really precise timing.

[Darrel Taylor]

Of course you'll want a 4MHz osc that's right on the money if you need really precise timing.

He's using a 32768 crystal for the primary OSC.