16F684 Timer1 Issues

[Mark Slovacek]

I am trying to figure out how to use a overflow interrupt on TMR1 to semi accurately measure pulses per time period for a future project.

The posted program was made to signal a TMR1 interrupt after 100ms, and update Sec, Min, and Hours.
Additionally, the program was designed to start at a specific time so I could track the microcontroller time versus real time.

The program loses about 14min per hour.

Some of the code came from suggestions found on the GCBASIC forums. 
I have racked my brains, and studied the forums trying to figure out why there is so much difference between the microcontrollers time and real time.

' This is a program to test the accuracy of the internal timer1
' Hardware configuration
#chip 16F684, 4
#config osc = int
' LCD connection settings
#define LCD_IO 2 'Set up the LCD to use 4 bit operation
' Define the 2 LCD Clock and Data Lines
#define LCD_DB PORTC.2
#define LCD_CB PORTC.1
Dim Counter As Word
Hour = 8: Min = 30: Sec = 0: Counter = 0
'Clock pulse frequency is 4/4 = 1 MHz
'Divided by 1 by prescaler (PS1_1/2), timer incremented at frequency of .5 MHz
' = every 2 us
' If cleared, timer overflows after 65536 * 2 us = every 131.072 ms
' Timer will overflow after 131.072 ms
' We need it to overflow after 31.072, so the timer must have a value loaded into it
' Need to load TMR1H, TMR1L with value that they will have after 31.072 ms - then it will count to 131.072, taking 100.00 ms
' Value = 65536 * (31.071 / 131.072) = 15535.5
' High byte of this is 60, low byte is 175 (60 * 256 + 175 = 15535)
' So, we need to set TMR1H to 60 and TMR1L to 175.
' If you're not comfortable with high and low bytes, put 3035 into a word variable (say TempVar)
' Then, set TMR1L to TempVar, and TMR1H to TempVar_H - this makes the compiler split the number up
' Set up Timer1
InitTimer1 Osc, PS1_1/2
StartTimer 1
On Interrupt Timer1Overflow Call IncTime
TMR1L = 0 'Need to do this according to the PIC datasheet
TMR1H = 60
TMR1L = 175
do 'Main part of Program
    cls
    Print HOUR
    Locate 0,2: Print ":"
    Locate 0,3: Print Min
    Locate 0,5: Print ":"
    Locate 0,6: Print Sec
    Wait 300 ms
    
Loop
SUB IncTime
'Increment the counter
Counter ++
If Counter = 10 Then '10 x 100ms 1sec
    Sec ++
    Counter = 0
End If
If Sec = 60 Then
    Min ++
    Sec = 0
End If
If Min = 60 Then
    Hour ++
    Min = 0
End If
If Hour = 12 Then
    Hour = 0
End If
End Sub

Can anybody tell me what I am doing wrong?

[gcha44]

Hi,
The 16F84 has only a 8 bits Timer0 and you can't use Tmr1H and Tmr1L!
So , you can try  another PIC or if you don't , you must use Timer0 .

Then Timer0 overflow after 256 pulses or 256µS with a 4Mhz cristal  .
So , to use TMR0 interrupt , you must preload Timer0 with 6 .
Then ,Timer0 will overflow every 250 µS . With a 1/16 prescaller  , then it'll owerflow every 250 * 16 = 4000 µs or 4 ms
For one  second (= 1000 ms) , Timer0 will overflow 1000 /4 = 250 times .

Each time Timer0 will overflow , you'll must write :
Timer0=6
Counter +
if Counter>=250 then
   sec +
   counter=0
end if
And so on
Regards
GC

[Mark Slovacek]

GC,

I am a little confused, according to the 16f684 spec sheet there is a 16 bit TMR1 with the capacity to set Tmr1H and Tmr1L.

http://ww1.microchip.com/downloads/en/devicedoc/41202c.pdf

Is GCBASIC not set up to utilize the TMR1 features in the 16F684?

I will try your suggestion in regards to using TMR0.

Thanks for your help

[gcha44]

Oops , I 'm sorry , I 've read 16F84 , but it was 16F684 !!
So , you can use Timer1 .
But , to have One second interrupt ., you can do :
With 1/2 prescaller , Timer1 must be preloaded to 65536-50000= 15536 because 50000 ( 50000µs=50ms *2=100ms)
Then you can do the same thing as in previous exemple
Timer1 must overflow 10 times to have one second delay
and you have to preload TImer1H=60 , Timer1L=176 at each interrupt
So , it seams that your code is correct, but I think you  must  put   ( TImer1H=60 , Timer1L=176)   in sub  IncTime , because these values must be preloaded after each interrupt , but if you leave them in main program , Timer1 will be reloaded even is there is no interrupt 
Regards
GC

[gcha44]

I realize that I was very tired yesterday , indeed my  last explanation wasn't good . lol
Because , if you lose about 14 mn , it's evident that TMR1 isn't preloaded each time interrupt occurs , and TMR1 starts again with it's complete value : 65536 . Because   15536/ 65535 =23.7   and   14mn/60mn=23,3  !!
So , your program is correct . I think you must only put  ( Timer1H=60 , Timer1L=176) in sub IncTime and your problem will be solved.
Regards
GC