PIC Delay calculator

**Nigel Goodwin** · 16th January 2008 10:27 PM

Originally Posted by wmmullaney

I'm just going to use the code I'v got.

I read the datasheet. How do I do a config word? Is it done in hex? Can it be done in binary?

Check any PIC code examples (like mine above), you can set it in hex (as I do), in binary, or (as many do) using long text strings full of AND'd from the include files.

What resistor value do you suggest as a current limiting resistor for high brightness blue leds with a pic running at 5v?

Use ohms law subtract the voltage drop of the LED from 5V to get the voltage, and decide what current you want, then apply ohms law to get the resistor.

But it depends if you're multiplexing or not?.

**wmmullaney** · 19th January 2008 01:00 AM

I'm not sure the voltage drop of the led, will 350 ohms be okay?

Here's my clock code, how does it look?

Code:

	;********************
	; Written by,       *
	; William Mullaney  *
	; On                *
	; January 14, 2008  *
	;                   *
	; Binary Clock      *
	; PIC16F628A        *
	;********************
	
	LIST	p=16F628A
	include "p16f628a"
	__config 0x2118
	
	HOUR equ 20h
	MIN  equ 21h
	R1   equ 22h
	R2   equ 23h
	R3   equ 24h
	
	MOVLW    07h
	MOVWF    CMCON
	BSF      STATUS,	RP0
	MOVLW    b'11111111'
	MOVWF    TRISA
	MOVWF    TRISB
	BCF      STATUS,	RP0
	
SHR BTFSS    PORTB,     08h
	GOTO     SHR
	MOVF     PORTA,     HOUR
	
SMN BTFSS    PORTA,		08h
	GOTO	 SMN
	MOVF	 PORTB,		MIN
	
CHK	BTFSC	 PORTA,		08h
	GOTO	 CHK
	
	BSF		 STATUS,	RP0
	MOVLW	 b'00000000'
	MOVWF	 TRISA
	MOVWF	 TRISB
	BCF		 STATUS,	RP0
	
CLK CALL	 DLY
	INCF	 MIN,		1
	MOVF	 MIN,		W
	BCF		 STATUS,	C
	MOVLW	 C3h
	BTFSS	 STATUS,	C
	GOTO	 PRT
	CLRF	 MIN
	INCF	 HOUR,		1
	MOVF	 HOUR,		W
	BCF		 STATUS,	C
	MOVLW	 F3h
	BTFSS	 STATUS,	C
	CLRF	 HOUR
	INCF	 HOUR,		1
	
PRT MOVF	 HOUR,		PORTA
	MOVF	 MIN,		PORTB
	GOTO	 CLK
	
DLY CBLOCK
	R1
	R2
	R3
	ENDC
	
	MOVLW	 22h
	MOVWF	 R1
	MOVLW	 CBh
	MOVWF	 R2
	MOVLW	 83h
	MOVWF	 R3
	
DY0	DECFSZ	 R1,		F
	GOTO	 $+2
	DECFSZ	 R2,		F
	GOTO	 $+2
	DECFSZ	 R3,		F
	GOTO	 DY0
	RETURN

sorry it looks bad, something with my text editor.

**wmmullaney** · 19th January 2008 03:50 PM

Originally Posted by wmmullaney

I'm not sure the voltage drop of the led, will 350 ohms be okay?

Here's my clock code, how does it look?

Code:

	;********************
	; Written by,       *
	; William Mullaney  *
	; On                *
	; January 14, 2008  *
	;                   *
	; Binary Clock      *
	; PIC16F628A        *
	;********************
	
	LIST	p=16F628A
	include "p16f628a"
	__config 0x2118
	
	HOUR equ 20h
	MIN  equ 21h
	R1   equ 22h
	R2   equ 23h
	R3   equ 24h
	
	MOVLW    07h
	MOVWF    CMCON
	BSF      STATUS,	RP0
	MOVLW    b'11111111'
	MOVWF    TRISA
	MOVWF    TRISB
	BCF      STATUS,	RP0
	
SHR BTFSS    PORTB,     08h
	GOTO     SHR
	MOVF     PORTA,     HOUR
	
SMN BTFSS    PORTA,		08h
	GOTO	 SMN
	MOVF	 PORTB,		MIN
	
CHK	BTFSC	 PORTA,		08h
	GOTO	 CHK
	
	BSF		 STATUS,	RP0
	MOVLW	 b'00000000'
	MOVWF	 TRISA
	MOVWF	 TRISB
	BCF		 STATUS,	RP0
	
CLK CALL	 DLY
	INCF	 MIN,		1
	MOVF	 MIN,		W
	BCF		 STATUS,	C
	MOVLW	 C3h
	BTFSS	 STATUS,	C
	GOTO	 PRT
	CLRF	 MIN
	INCF	 HOUR,		1
	MOVF	 HOUR,		W
	BCF		 STATUS,	C
	MOVLW	 F3h
	BTFSS	 STATUS,	C
        GOTO PRINT
	CLRF	 HOUR
	INCF	 HOUR,		1
	
PRT MOVF	 HOUR,		PORTA
	MOVF	 MIN,		PORTB
	GOTO	 CLK
	
DLY CBLOCK
	R1
	R2
	R3
	ENDC
	
	MOVLW	 22h
	MOVWF	 R1
	MOVLW	 CBh
	MOVWF	 R2
	MOVLW	 83h
	MOVWF	 R3
	
DY0	DECFSZ	 R1,		F
	GOTO	 $+2
	DECFSZ	 R2,		F
	GOTO	 $+2
	DECFSZ	 R3,		F
	GOTO	 DY0
	RETURN

sorry it looks bad, something with my text editor.

Fixed a bug.

**David / W9GF** · 8th April 2008 10:30 PM

I created a small excel spreadsheet which i'm attaching that may be helpful in calculating the various delays available with a given clock frequency. All you need to enter is your clock frequency in mhz - everything else is calculated based on that.

Enjoy!

David

Edit: An updated attachment is here: PIC Delay calculator

**mike50** · 9th April 2008 04:16 PM

Here is a little program that will calculate the settings (prescale, postscale and PR2) for TMR2 to generate interrupts for whatever time interval you want. If it can generate settings for exactly what you want, it shows all the ways that can be achieved. If there are no settings to get exactly what you want it shows the closest values (one greater and one less than what you want).

You run it as:

TMR2 n

where n is the timer interval you want in instruction cycles.

examples:

Code:

C:>TMR2 10000
10000 Prescaler 1:16 Postscaler 1:5  PR2 124
10000 Prescaler 1:4  Postscaler 1:10 PR2 249

C:>TMR2 1234
 1233 Prescaler 1:1  Postscaler 1:9  PR2 136
 1235 Prescaler 1:1  Postscaler 1:13 PR2  94

The maximum value for n is 65536

---updated with program that doesn't require the C-runtime DLL---

Mike

**ericgibbs** · 9th April 2008 04:29 PM

Originally Posted by David / W9GF

I created a small excel spreadsheet which i'm attaching that may be helpful in calculating the various delays available with a given clock frequency. All you need to enter is your clock frequency in mhz - everything else is calculated based on that.

Enjoy!

David

Hi,
Tried runs OK, BTW 'OPen Office' will run the program.

**ericgibbs** · 9th April 2008 04:31 PM

Originally Posted by mike50

Here is a little program that will calculate the settings (prescale, postscale and PR2) for TMR2 to generate interrupts for whatever time interval you want. If it can generate settings for exactly what you want, it shows all the ways that can be achieved. If there are no settings to get exactly what you want it shows the closest values (one greater and one less than what you want).

You run it as:

TMR2 n

where n is the timer interval you want in instruction cycles.

examples:

Code:

C:>TMR2 10000
10000 Prescaler 1:16 Postscaler 1:5  PR2 124
10000 Prescaler 1:4  Postscaler 1:10 PR2 249

C:>TMR2 1234
 1233 Prescaler 1:1  Postscaler 1:9  PR2 136
 1235 Prescaler 1:1  Postscaler 1:13 PR2  94

The maximum value for n is 65536

Mike

Hi Mike,
Unzipped, keep getting this error msg in Win XP.

**mike50** · 9th April 2008 04:44 PM

Originally Posted by ericgibbs

Hi Mike,
Unzipped, keep getting this error msg in Win XP.

Hmmm. The program is written in Visual C++, and shouldn't require .NET. But that is the error you can get for .NET applications trying to run on machines without .NET (or with the wrong version).

Actually, the problem was that it was compiled to need the C-runtime library as a DLL. I've rebuilt it with the runtime contained within the .exe.

This updated version should work okay.

Mike

**savnik** · 9th April 2008 05:39 PM

Originally Posted by ericgibbs

Hi Mike,
Unzipped, keep getting this error msg in Win XP.

Is a DOS programm.
You must to runn this from CMD

**ericgibbs** · 9th April 2008 05:50 PM

Originally Posted by savnik

Is a DOS programm.
You must to runn this from CMD

Hi,
Thought it was a Windows program, will try from DOS/cmd.

Thanks.

**mike50** · 9th April 2008 06:34 PM

Originally Posted by ericgibbs

Hi,
Thought it was a Windows program, will try from DOS/cmd.

Thanks.

It is a window's program, but you really need to run it from a command prompt so you can supply it with the parameter.

The error that ericgibbs was seeing is due to not being able to find a .dll that it wanted. The new version doesn't require any .dll's.

Mike

**David / W9GF** · 9th April 2008 06:58 PM

Saw your output mike50 with postscaler values so i corrected a couple of little bugs with mine, added a place to put in "Delay I need" - and it shows you all of your options - even converts the 8 bit stuff to hex for ya (guess microsoggy didn't think anyone would need a DECTOHEX32() function in Excel Grrrrr)...

Also added some timer notes i found in the Data sheet for the 18F4550 at the bottom.

Hope this stuff is accurate! I'll be testing & working with it for the next couple of weeks so i'll update my .xls if i find anything out of kelter!

David

The most current version of the attachment is here: PIC Delay calculator

(today anyway

)

**mike50** · 9th April 2008 07:17 PM

David,

I'm not sure I understand the meaning of the Postscaler in your spreadsheet. TMR0 doesn't have a postscaler. If you are doing this for TMR2 then you want to include the PR2 register as part of the calculation.

Mike

**David / W9GF** · 9th April 2008 07:21 PM

I may have used the wrong terminology - but that's the value to write in the TMR0 & TMR1 register instead of 0 ... and now that i've typed this the value i have is incorrect - i forgot this thing is counting up and not down - so the value should be subtracted from 255 for the 8 bit mode and subtracted from 65535 for the 16 bit mode.

That making sense or am i way off base?

**mike50** · 9th April 2008 07:41 PM

Oh, I see what you are getting at. What you are calling "postscaler" is more like the PR2 register of TMR2. It defines how many counts before TMR0 overflows.

There is a problem with that technique though. Some time elapses from the point where the TMR0 overflows and where your interrupt service routine can write the new value into TMR0. In some circumstances you could correct for that time, but in others (if there are other interrupts enabled, or the mainline sometimes disables interrupts) you can not accurately compensate.

That is why TMR2, with a hardware postscaler and the PR2 (match) register, is so nice. It is all in hardware. Your timing has no dependency on the time it takes to get into your interrupt service routine. The interrupts come like, well , "clockwork"

Mike

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Excel Spreadsheet to help in calculating TIMER0 delays

Made some changes to the Excel Spreadsheet

postscaler

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