truth table vs lookup table - Electronic Circuits Projects Diagrams Free

alamy · 2nd June 2004, 07:52 AM

hello all...

I am just starting out with PIC....i'm beginner.....

i just built jdm programmer and excited to buit a simple project...

i did simple led flasher...and its working..very happy

my question is...

i'm going to build the circuit that have 6 inputs and 4 outputs....
and i have my truth table to implement my circuit....

by using and gate and or gate ...i need 3 ics to implement my design...
so..i'm looking to replace these ics with pic16f84a...

how do i implement the truth table in PIC?...and how to use the lookup table in PIC?...

pls guide me....

thanks

StupidDum · 2nd June 2004, 08:50 AM

If possible, derive the equation for your logic.
Well, then, write a program to get the input, and use the equation, get the answer, and output it to your pins.

samcheetah · 2nd June 2004, 09:20 AM

the thing you are talking about is usually implemented using programmable logic devices PLD (PALs, GALs, SPLDs or CPLDs). although it could be implemented using PIC. using a PIC to work like 5 or 6 gates isnt that cost effective. u should use some type of PLD for this. u can program this very easily in Abel or similar programming languages.

motion · 2nd June 2004, 09:20 AM

This reminds me of an ancient application note of Microchip which does exactly what you intend to do. It's for the PIC16C5X which is an earlier version of the PIC16F84. You can easily port this to the '84 which still has the obsolete TRIS instruction.

http://www.microchip.com/stellent/id...pnote=en010976

RonH · 2nd June 2004, 04:26 PM

If you just want to play around you can use this as a lookup table. Just make sure you don't run over the 8-bit boundry, lookup up the microchip appnote that deals with implementing a table read for more info on that.

call the lookup with your data in Wreg

Code:

lookup	movwf	pcl,f
	retlw	0x01
	retlw	0x02
	retlw	0x03
	retlw	0x04
	retlw	0x05
	.
	.
	.

Ron

Exo · 2nd June 2004, 04:35 PM

It should be ADDWF PCL, F at the top

RonH · 2nd June 2004, 11:44 PM

Opps sorry yes you're right, typo. Should be addwf,pcl,f.

Also it doesn't have to be Retlw instructions either you could just as well place goto's in there. (as long as the subs you go to have returns at the end)

Ron

**Nigel Goodwin** · 2nd June 2004, 11:56 PM

Quote:

Originally Posted by RonH

Also it doesn't have to be Retlw instructions either you could just as well place goto's in there. (as long as the subs you go to have returns at the end)

Except it's no longer a 'lookup table', it's then a 'jump table', which is a very different structure.

I think the first action here is to study what actually needs to be done, it's all right saying a truth table was used originally - that isn't to say that's the best way to solve the problem - it may well just be a consequence of the logic chips used.

alamy · 4th June 2004, 09:12 AM

Quote:

Originally Posted by motion

This reminds me of an ancient application note of Microchip which does exactly what you intend to do. It's for the PIC16C5X which is an earlier version of the PIC16F84. You can easily port this to the '84 which still has the obsolete TRIS instruction.

http://www.microchip.com/stellent/id...pnote=en010976

thanks....i'll download this application note and play around with this...

regarding the RETLW instruction...

can i just simply.. ask pic to check the input then..look up to the table (retlw..) then ask pic to write to the output...?

thanks..

**Nigel Goodwin** · 4th June 2004, 10:04 AM

Quote:

Originally Posted by alamy

regarding the RETLW instruction...

can i just simply.. ask pic to check the input then..look up to the table (retlw..) then ask pic to write to the output...?

This type of table uses the W register as an index into the table, so if you had three inputs connected to bits 0, 1, and 2 of a port, they can represent the numbers 0 to 7 - you could use this as an index to a table with 8 entries, then write the entry to an output port.

Something like this:

Code:

movf   PortA, w       ; read PortA into W
andlw  b'00000111'  ; make sure value not larger than 7
call     Table            ; get entry from table
movwf PortB           ; and output to PortB

You could use different input pins if you want, but using the bottom ones makes the code nice and simple - and fast.

alamy · 14th June 2004, 02:27 AM

Quote:

Originally Posted by Nigel Goodwin

This type of table uses the W register as an index into the table, so if you had three inputs connected to bits 0, 1, and 2 of a port, they can represent the numbers 0 to 7 - you could use this as an index to a table with 8 entries, then write the entry to an output port.

Something like this:

Code:

movf   PortA, w       ; read PortA into W
andlw  b'00000111'  ; make sure value not larger than 7
call     Table            ; get entry from table
movwf PortB           ; and output to PortB

meaning that...if i have 6 inputs, my code will be like this?...

Code:

main         movf   PortA, w       ; read PortA into W
                andlw  b'00111111'  ; table limits?...6 inputs...64 entries
                call     Table            ; get entry from table
                movwf PortB           ; and output to PortB

Table        addwf pcl,f
                retlw 0x0
                .
                .
                .
                return

then ...if i'm going to try alternative way...using an equations...can PIC do it?..

my equation is

y0 = x5&x2 + x5&x1&x0 + x4&/x3&x2
y1 = x5 + x4&x3&x2
y2 = x2 + x5&x3 + x5&x4
y3 = x5 + x4&x3&/x2

...
pls guide me

thanks

**Nigel Goodwin** · 14th June 2004, 06:54 AM

Quote:

Originally Posted by alamy

then ...if i'm going to try alternative way...using an equations...can PIC do it?..

my equation is

y0 = x5&x2 + x5&x1&x0 + x4&/x3&x2
y1 = x5 + x4&x3&x2
y2 = x2 + x5&x3 + x5&x4
y3 = x5 + x4&x3&/x2

Yes, you can quite happily do this with a PIC, you just need to write the maths routines for it, there are plenty on the PICList you could look at.

However, it will be considerably slower than the table method!.

alamy · 16th June 2004, 05:14 AM

hi...

i'm sorry....i try to write the code to implement my 'lookup table'...actually this is my first time writting the code....

after click "build all" i got this message

Clean: Deleting intermediary and output files.
Clean: Done.
Executing: "C:\Program Files\MPLAB IDE\MCHIP_Tools\mpasmwin.exe" /q /p16F84A "digital.asm" /l"digital.lst" /e"digital.err"
Message[302] C:\PROGRAM FILES\MPLAB IDE\MYPROJ\DIGITAL.ASM 79 : Register in operand not in bank 0. Ensure that bank bits are correct.
Message[302] C:\PROGRAM FILES\MPLAB IDE\MYPROJ\DIGITAL.ASM 81 : Register in operand not in bank 0. Ensure that bank bits are correct.
Error[113] C:\PROGRAM FILES\MPLAB IDE\MYPROJ\DIGITAL.ASM 87 : Symbol not previously defined (pcl)
Halting build on first failure as requested.
BUILD FAILED: Wed Jun 16 12:03:59 2004

and my code is like this:

Code:

;**********************************************************************


	list      p=16F84A            ; list directive to define processor
	#include <p16F84A.inc>        ; processor specific variable definitions

	__CONFIG   _CP_OFF & _WDT_ON & _PWRTE_ON & _RC_OSC

; '__CONFIG' directive is used to embed configuration data within .asm file.
; The lables following the directive are located in the respective .inc file.
; See respective data sheet for additional information on configuration word.




;***** VARIABLE DEFINITIONS
STATUS      EQU     03h      ; variable used for context saving 
TRISA		EQU		85h	     ; variable used for context saving
PORTA		EQU		05H
TRISB		EQU		86h
PORTB		EQU		06h








;**********************************************************************
		ORG     0x000             ; processor reset vector
  		goto    main              ; go to beginning of program


		ORG     0x004             ; interrupt vector location
main	bsf		STATUS,5		  ; bank 0 -----> bank 1
		movlw   b'00111111'       ; set the port a as inputs
		movwf	TRISA             ; 
		movlw   b'11110000'       ; set the port b as ouputs
		movwf	TRISB    
		bcf		STATUS,5      	  ; bank 1 -----> bank 0
		movf	PORTA,w
		andlw	b'00111111'
		call	table
		movwf	PORTB
table	addwf 	pcl,f		; inputs   outputs
		retlw	b'0000'		; 0			
		retlw	b'0000'		; 1
		retlw	b'0000'		; 2
		retlw	b'0100'		; 3	
		retlw	b'0100'		; 4
		retlw	b'0100'		; 5
		retlw	b'0100'		; 6
		retlw	b'0100'		; 7
		retlw	b'0000'		; 8
		retlw	b'0000'		; 9
		retlw	b'0000'		; 10
		retlw	b'0100'		; 11
		retlw	b'0100'		; 12
		retlw	b'0100'		; 13
		retlw	b'0100'		; 14
		retlw	b'0100'		; 15
		retlw	b'0000'		; 16
		retlw	b'0000'		; 17
		retlw	b'0000'		; 18
		retlw	b'0100'		; 19
		retlw	b'0101'		; 20
		retlw	b'0101'		; 21
		retlw	b'0101'		; 22
		retlw	b'0101'		; 23
		retlw	b'1000'		; 24
		retlw	b'1000'		; 25
		retlw	b'1000'		; 26
		retlw	b'1000'		; 27
		retlw	b'0110'		; 28
		retlw	b'0110'		; 26
		retlw	b'0110'		; 30
		retlw	b'0110'		; 31
		retlw	b'1010'		; 32
		retlw	b'1010'		; 33
		retlw	b'1010'		; 34
		retlw	b'1011'		; 35
		retlw	b'1111'		; 36
		retlw	b'1111'		; 37
		retlw	b'1111'		; 38
		retlw	b'1111'		; 39
		retlw	b'1110'		; 40
		retlw	b'1110'		; 41
		retlw	b'1110'		; 42
		retlw	b'1111'		; 43
		retlw	b'1111'		; 44
		retlw	b'1111'		; 45
		retlw	b'1111'		; 46
		retlw	b'1111'		; 47
		retlw	b'1110'		; 48
		retlw	b'1110'		; 49
		retlw	b'1110'		; 50
		retlw	b'1111'		; 51
		retlw	b'1111'		; 52
		retlw	b'1111'		; 53
		retlw	b'1111'		; 54
		retlw	b'1111'		; 55
		retlw	b'1110'		; 56		
		retlw	b'1110'		; 57
		retlw	b'1110'		; 58
		retlw	b'1111'		; 59
		retlw	b'1111'		; 60
		retlw	b'1111'		; 61
		retlw	b'1111'		; 62
		retlw	b'1111'		; 63
		goto 	main


		END                     ; directive 'end of program'

when i double click at message the 'green cursor" point at the movwf TRISA and movwf TRISB and error at line table addwf pcl,f

actually what is the problem..? pls help me find the problem...

thanks

**Nigel Goodwin** · 16th June 2004, 06:55 AM

Try changing 'pcl' to 'PCL', the other messages are just warnings, not errors.

laroche73 · 16th June 2004, 08:48 PM

Just a comment on the topic of lookup tables vs. performing calculations in real time. It doesn't have to be all of one or the other. In general, if there are more than a few inputs, there's a noticable tradeoff between speed and storage space (lookup tables -> highest speed, large storage requirements, calculations -> lowest speed, minimal storage). For many non real time cases, it doesn't matter which method you choose, since the resources used (time, storage space) are minimal and the differences aren't visible to the end user. In some applications, making a tradeoff between tables and equations can result in a better use of resources.

Here's an example. Cubic interpolation often provides a better approximation to real world data than simple linear interpolation. The author claims that one quadrant of a sine wave can be approximated to better than 16-bits of precision using only 19 table entries. There is some simple math involved for the micro, IMO it's a great example of making good tradeoffs. - Claude

http://www.reed-electronics.com/ednm...1695/04di2.htm

here's the original, in forth :wink:

http://www.tinyboot.com/cubic.txt

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2nd June 2004, 07:52 AM	(permalink)
alamy	truth table vs lookup table hello all... I am just starting out with PIC....i'm beginner..... i just built jdm programmer and excited to buit a simple project... i did simple led flasher...and its working..very happy my question is... i'm going to build the circuit that have 6 inputs and 4 outputs.... and i have my truth table to implement my circuit.... by using and gate and or gate ...i need 3 ics to implement my design... so..i'm looking to replace these ics with pic16f84a... how do i implement the truth table in PIC?...and how to use the lookup table in PIC?... pls guide me.... thanks

2nd June 2004, 08:50 AM	(permalink)
StupidDum	If possible, derive the equation for your logic. Well, then, write a program to get the input, and use the equation, get the answer, and output it to your pins.

2nd June 2004, 09:20 AM	(permalink)
samcheetah	the thing you are talking about is usually implemented using programmable logic devices PLD (PALs, GALs, SPLDs or CPLDs). although it could be implemented using PIC. using a PIC to work like 5 or 6 gates isnt that cost effective. u should use some type of PLD for this. u can program this very easily in Abel or similar programming languages.

2nd June 2004, 09:20 AM	(permalink)
motion	This reminds me of an ancient application note of Microchip which does exactly what you intend to do. It's for the PIC16C5X which is an earlier version of the PIC16F84. You can easily port this to the '84 which still has the obsolete TRIS instruction. http://www.microchip.com/stellent/id...pnote=en010976 __________________ "Having to do with Motion Control"

2nd June 2004, 04:26 PM	(permalink)
RonH	lookup table If you just want to play around you can use this as a lookup table. Just make sure you don't run over the 8-bit boundry, lookup up the microchip appnote that deals with implementing a table read for more info on that. call the lookup with your data in Wreg Code: lookup movwf pcl,f retlw 0x01 retlw 0x02 retlw 0x03 retlw 0x04 retlw 0x05 . . . Ron

2nd June 2004, 04:35 PM	(permalink)
Exo	It should be ADDWF PCL, F at the top

2nd June 2004, 11:44 PM	(permalink)
RonH	Opps sorry yes you're right, typo. Should be addwf,pcl,f. Also it doesn't have to be Retlw instructions either you could just as well place goto's in there. (as long as the subs you go to have returns at the end) Ron

16th June 2004, 06:55 AM	(permalink)
Nigel Goodwin	Try changing 'pcl' to 'PCL', the other messages are just warnings, not errors. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

16th June 2004, 08:48 PM	(permalink)
laroche73	lookup tables vs. equations Just a comment on the topic of lookup tables vs. performing calculations in real time. It doesn't have to be all of one or the other. In general, if there are more than a few inputs, there's a noticable tradeoff between speed and storage space (lookup tables -> highest speed, large storage requirements, calculations -> lowest speed, minimal storage). For many non real time cases, it doesn't matter which method you choose, since the resources used (time, storage space) are minimal and the differences aren't visible to the end user. In some applications, making a tradeoff between tables and equations can result in a better use of resources. Here's an example. Cubic interpolation often provides a better approximation to real world data than simple linear interpolation. The author claims that one quadrant of a sine wave can be approximated to better than 16-bits of precision using only 19 table entries. There is some simple math involved for the micro, IMO it's a great example of making good tradeoffs. - Claude http://www.reed-electronics.com/ednm...1695/04di2.htm here's the original, in forth :wink: http://www.tinyboot.com/cubic.txt