Real Time Clock

I have created a simple clock program which uses the following settings:
External Osc = 4mhz xtal (1000000 operations per second)
Tmr0 = 250
Prescaler = 32
Counter (my variable) = 125

I use Disbale and resume as instructed in the help files

My counter is incremented each time the timer and prescaler trip over to 0. This mathematically gives exactly 1.0000 second per as a time base.
However, each time I run this code, it is out by as much as 1 second per minute.

Can any one suggest why I get a varience of such a magnitude. Perhaps show mw a reliable time kleepting function or code.

Thanks for any help rendered.

Read this site for some explainations: http://www.romanblack.com/one_sec.htm

I have looked at the sie for Roman Black but no good.
I have checked his code and found that he uses a simple count to tack a little bit onto the end of a second every 4 clocks. This works with the asm code but does not translate into picbasic without the same amount of error.

Thanks anyway.
Maybe I just need to use assembler only and forget the picbasic stuff.

Hi Bob I will see you later with a working code.
From Aaron ;-)

Quote:

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Originally Posted by Bob Spencer

I have created a simple clock program which uses the following settings:
External Osc = 4mhz xtal (1000000 operations per second)
Tmr0 = 250
Prescaler = 32
Counter (my variable) = 125

I use Disbale and resume as instructed in the help files

My counter is incremented each time the timer and prescaler trip over to 0. This mathematically gives exactly 1.0000 second per as a time base.
However, each time I run this code, it is out by as much as 1 second per minute.

Can any one suggest why I get a varience of such a magnitude. Perhaps show mw a reliable time kleepting function or code.

Thanks for any help rendered.

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In real time application ,the picbasic pro not good...becuse we cant calculate the time exactly in this cases the assemply will be better..

Why won't DT interrupts work? I don't really understand all the code but it's fairly simple to plug in values to get any time base you'd like.

Just my 2 pennies here but I'd use at least 100 Hz and count pulses until you get to a second. From what I've read that further reduces timing errors by averaging the error over time.

Best regards

if you want very good accuracy you can use ds 1302 or ds1307 rtc time ic

but you can try this code


' LCD clock program using On Interrupt
' Uses TMR0 and prescaler. Watchdog Timer should be
' set to off at program time and Nap and Sleep should not be used.
' Buttons may be used to set hours and minutes

Define LCD_DREG PORTD ' Define LCD connections
Define LCD_DBIT 4
Define LCD_RSREG PORTE
Define LCD_RSBIT 0
Define LCD_EREG PORTE
Define LCD_EBIT 1

hour var byte ' Define hour variable
dhour var byte ' Define display hour variable
minute var byte ' Define minute variable
second var byte ' Define second variable
ticks var byte ' Define pieces of seconds variable
update var byte ' Define variable to indicate update of LCD
i var byte ' Debounce loop variable

ADCON1 = 7 ' PORTA and E digital
Low PORTE.2 ' LCD R/W low = write
Pause 100 ' Wait for LCD to startup

hour = 0 ' Set initial time to 00:00:00
minute = 0
second = 0
ticks = 0

update = 1 ' Force first display

' Set TMR0 to interrupt every 16.384 milliseconds
OPTION_REG = $55 ' Set TMR0 configuration and enable PORTB pullups
INTCON = $a0 ' Enable TMR0 interrupts
On Interrupt Goto tickint


' Main program loop - in this case, it only updates the LCD with the time
mainloop:
PORTB = 0 ' PORTB lines low to read buttons
TRISB = $f0 ' Enable all buttons

' Check any button pressed to set time
If PORTB.7 = 0 Then decmin
If PORTB.6 = 0 Then incmin ' Last 2 buttons set minute
If PORTB.5 = 0 Then dechr
If PORTB.4 = 0 Then inchr ' First 2 buttons set hour

' Check for time to update screen
chkup: If update = 1 Then
Lcdout $fe, 1 ' Clear screen

' Display time as hh:mm:ss
dhour = hour ' Change hour 0 to 12
If (hour // 12) = 0 Then
dhour = dhour + 12
Endif

' Check for AM or PM
If hour < 12 Then
Lcdout dec2 dhour, ":", dec2 minute, ":", dec2 second, " AM"
Else
Lcdout dec2 (dhour - 12), ":", dec2 minute, ":", dec2 second, " PM"
Endif

update = 0 ' Screen updated
Endif

Goto mainloop ' Do it all forever


' Increment minutes
incmin: minute = minute + 1
If minute >= 60 Then
minute = 0
Endif
Goto debounce

' Increment hours
inchr: hour = hour + 1
If hour >= 24 Then
hour = 0
Endif
Goto debounce

' Decrement minutes
decmin: minute = minute - 1
If minute >= 60 Then
minute = 59
Endif
Goto debounce

' Decrement hours
dechr: hour = hour - 1
If hour >= 24 Then
hour = 23
Endif

' Debounce and delay for 250ms
debounce: For i = 1 to 25
Pause 10 ' 10ms at a time so no interrupts are lost
Next i

update = 1 ' Set to update screen

Goto chkup


' Interrupt routine to handle each timer tick
disable ' Disable interrupts during interrupt handler
tickint: ticks = ticks + 1 ' Count pieces of seconds
If ticks < 61 Then tiexit ' 61 ticks per second (16.384ms per tick)

' One second elasped - update time
ticks = 0
second = second + 1
If second >= 60 Then
second = 0
minute = minute + 1
If minute >= 60 Then
minute = 0
hour = hour + 1
If hour >= 24 Then
hour = 0
Endif
Endif
Endif

update = 1 ' Set to update LCD

tiexit: INTCON.2 = 0 ' Reset timer interrupt flag
Resume

End