Software Trim Idea - Electronic Circuits Projects Diagrams Free

Suraj143 · 2nd February 2009, 05:30 AM

My clocks time base is done with TMR0.

Can somebody explain me how to do a software trim for the TMR0.

Because my clocks timing is varying.

No need codes just tell me the idea

Thanks

blueroomelectronics · 2nd February 2009, 05:32 AM

What clocks, there is no trim for TMR0. What PIC are you using?

Suraj143 · 2nd February 2009, 05:37 AM

Quote:

Originally Posted by blueroomelectronics

What clocks, there is no trim for TMR0. What PIC are you using?

I'm using 16F88, I need TMR2 for another work.Thats why I used TMR0.

So I need the idea on how to do a software trim.

blueroomelectronics · 2nd February 2009, 05:50 AM

If you're presetting TMR0 it will be very difficult to keep it accurate, free running mode is best. Roman Black has an article on this.
Official Home Page of Roman Black
PS TMR1 has a wonderful connection to the CCP1 special interrupt (mode 14) which can form an excellent timebase for a clock.

Suraj143 · 2nd February 2009, 05:54 AM

Quote:

Originally Posted by blueroomelectronics

If you're presetting TMR0 it will be very difficult to keep it accurate, free running mode is best. Roman Black has an article on this.
Official Home Page of Roman Black
PS TMR1 has a wonderful connection to the CCP1 special interrupt (mode 14) which can form an excellent timebase for a clock.

Yes I'm resetting TMR0.I have every 10ms T0IF interrupts & I count 100 to generate 1 second.

On every T0IF interrupts I reload TMR0.

blueroomelectronics · 2nd February 2009, 06:21 AM

The problem with reloading TMR0 is it resets the prescaler and other such stuff. Allowing it to freerun is the better way to do it. It'll set TMR0IF on overflow and use Romans zero error clock algorithm for getting a 1 second timebase.

mister_e · 2nd February 2009, 06:22 AM

Usually you just don't reload a timer with a calculated value. You add the current Timer value + your calculated value + amount of cycle it need to reload your timer.

HTH

TMR0=TMR0 + ReloadValue + SomeCallItFudgeFactor

Quote:

Originally Posted by From the datasheet

If the TMR0 register
is written, the increment is inhibited for the following
two instruction cycles. The user can work around this
by writing an adjusted value to the TMR0 register.

Pommie · 2nd February 2009, 07:05 AM

If you set the prescaler to 32 then the timer will overflow every 8192 cycles - close to your current 10,000. Use Roman Blacks code to subtract 32 from the middle byte and add the 1,000,000 when the top bytes become zero. This will give you a very accurate clock.

Mike.

Suraj143 · 2nd February 2009, 07:19 AM

Quote:

Originally Posted by Pommie

If you set the prescaler to 32 then the timer will overflow every 8192 cycles - close to your current 10,000. Use Roman Blacks code to subtract 32 from the middle byte and add the 1,000,000 when the top bytes become zero. This will give you a very accurate clock.

Mike.

Hi thats a good way I'll try to like that.

Thanks

Pommie · 2nd February 2009, 08:33 AM

I just realised that adding 32 will not work with Roman Blacks code. It needs to be modified to,

Code:

		movlw	.32		;subtract 32*256 from accumulator
		subwf	AccHi,F
		skpnc
		goto	DoneTimer
		decfsz	AccUp,F
		goto	DoneTimer

;only gets here if 1 second has passed

		movlw	low(.1000000)	;add 1 million to the accumulator
		addwf	AccLo,f
		movlw	high(.1000000)
		skpnc
		addlw	1
		addwf	AccHi,f
		skpnc
		incf	AccUp,F
		movlw	upper(.1000000)
		addwf	AccUp,F
DoneTimer	bcf	INTCON,TMR0IF

The three variables AccLo, AccHi and AccUp can be initialised to 1.

Mike.

Mike, K8LH · 2nd February 2009, 01:43 PM

What are you using TMR2 for? You might be able to use it for both your current purpose and for the RTC 'heartbeat'.

There's a brief description of a software trimmer function here (for timer 2).

Regards, Mike

Suraj143 · 3rd February 2009, 02:55 AM

Hi Mike (Pommie) thanks for your new code thats the one I like to put.

Hi Mike, K8LH

I need a 2.5Khz separate frequency to drive a speaker.So I using PWM module.Thats why I cannot use TMR2.

Pommie · 3rd February 2009, 05:20 AM

Quote:

Originally Posted by Suraj143

I need a 2.5Khz separate frequency to drive a speaker.So I using PWM module.Thats why I cannot use TMR2.

If you set timer2s postscaler to 10 you will get an interrupt at 250Hz which you could use for the clock timebase. This will not effect your PWM output at all. To turn off the 2.5kHz signal set the pulse width to zero.

Mike.

Suraj143 · 3rd February 2009, 05:40 AM

Quote:

Originally Posted by Pommie

If you set timer2s postscaler to 10 you will get an interrupt at 250Hz which you could use for the clock timebase. This will not effect your PWM output at all. To turn off the 2.5kHz signal set the pulse width to zero.

Mike.

Hi Mike thats a clever method.It didn't come to my mind.I can't think like you all think.Because I'm very new to this subject.

So that means I can do both works (time base + speaker output frequency) from a single TMR2.Wow thats very nice.

Thanks for the help I'm going to try that as well.

Suraj143 · 3rd February 2009, 05:48 AM

At the moment I'm studying roman blacks original code.I have got stuck with that.Sorry about that.

First it loads accumulator to 100000+256=100256uS.
Every 256uS it generates interrupt.& every 256uS interrupts he subtract 256 from accumulator.

When continue subtracting theres a point coming you cannot subtract anymore.That is 100096uS.So balance is 160uS remaining (100256uS-100096uS =160uS) .

I'm not sure what he is doing for this 160uS

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2nd February 2009, 05:30 AM	#1
Suraj143	Software Trim Idea My clocks time base is done with TMR0. Can somebody explain me how to do a software trim for the TMR0. Because my clocks timing is varying. No need codes just tell me the idea Thanks

2nd February 2009, 05:32 AM	#2
blueroomelectronics	What clocks, there is no trim for TMR0. What PIC are you using? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

2nd February 2009, 05:50 AM	#4
blueroomelectronics	If you're presetting TMR0 it will be very difficult to keep it accurate, free running mode is best. Roman Black has an article on this. Official Home Page of Roman Black PS TMR1 has a wonderful connection to the CCP1 special interrupt (mode 14) which can form an excellent timebase for a clock. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

2nd February 2009, 06:21 AM	#6
blueroomelectronics	The problem with reloading TMR0 is it resets the prescaler and other such stuff. Allowing it to freerun is the better way to do it. It'll set TMR0IF on overflow and use Romans zero error clock algorithm for getting a 1 second timebase. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

2nd February 2009, 07:05 AM	#8
Pommie	If you set the prescaler to 32 then the timer will overflow every 8192 cycles - close to your current 10,000. Use Roman Blacks code to subtract 32 from the middle byte and add the 1,000,000 when the top bytes become zero. This will give you a very accurate clock. Mike.

2nd February 2009, 01:43 PM	#11
Mike, K8LH	What are you using TMR2 for? You might be able to use it for both your current purpose and for the RTC 'heartbeat'. There's a brief description of a software trimmer function here (for timer 2). Regards, Mike Last edited by Mike, K8LH; 2nd February 2009 at 01:43 PM.

3rd February 2009, 02:55 AM	#12
Suraj143	Hi Mike (Pommie) thanks for your new code thats the one I like to put. Hi Mike, K8LH I need a 2.5Khz separate frequency to drive a speaker.So I using PWM module.Thats why I cannot use TMR2.

3rd February 2009, 05:48 AM	#15
Suraj143	At the moment I'm studying roman blacks original code.I have got stuck with that.Sorry about that. First it loads accumulator to 100000+256=100256uS. Every 256uS it generates interrupt.& every 256uS interrupts he subtract 256 from accumulator. When continue subtracting theres a point coming you cannot subtract anymore.That is 100096uS.So balance is 160uS remaining (100256uS-100096uS =160uS) . I'm not sure what he is doing for this 160uS