Programming a microcontroller in assembly languages takes time and to reduce that time, embedded systems engineers often have tricks up their sleeves. One of those tricks is the use of lookup tables. In this post, I will share how I use lookup tables in assembly language for PIC microcontrollers.
The RETLW Opcode
Lookup tables are possible in assembly primarily because of the RETLW opcode. This instruction means “return with a literal in w” and is used like this:
Here, 0x04 is the literal value placed inside the W register before returning. Since the opcode returns to something, it is usually inside a subroutine.
Take note that RETLW reads the stack for the address to which to return. So a GOTO followed by RETLW will result in a stack underflow error.
Lookup Table Example
The most common lookup table is the one for seven segment displays. Frugal engineers don’t want to use decoders in using seven segment displays and so connect them directly to PIC pins. A lookup table is used to convert the numbers to their equivalent for seven segment displays.
Below is an example table:
table addwf PC retlw b’00111111’ retlw b’00000110’ retlw b’01011011’ retlw b’01001111’ retlw b’01100110’ retlw b’01101101’ retlw b’01111100’ retlw b’00000111’ retlw b’01111111’ retlw b’01100111’
The binaries in this code are the seven-segment equivalent for digits zero to nine, assuming a common cathode displays and LSB at segment ‘a’.
Now if I want to display the digit seven (connected to PORTB), for example, I would do it like this:
movlw 0x07 call table movwf PORTB
What this does is move a literal value 7 to W then call the lookup table. On the table, the value 7 adds to the current value of the program counter which is the first line of the table. The program then jumps 7 lines towards
The program now returns to the caller with W equal to the seven segment equivalent for the digit 7.
Example Project: Decade Counter
Let’s say we use two seven segment displays to count from zero to ninety-nine. One way to implement this is to assign variables to the ones and tens digits. As the ones digit go past 9, the tens digit increments by one. Using lookup tables make this project easier.
#include <P16F877A.inc> __CONFIG _FOSC_XT & _WDTE_OFF & _PWRTE_OFF & _BOREN_OFF & _LVP_OFF & _CPD_OFF & _WRT_OFF & _CP_OFF RES_VECT CODE 0x0000 ; processor reset vector GOTO START ; go to beginning of program INT_VECT CODE 0x0004 ; interrupt vector GOTO ISR ; go to interrupt service routine MAIN_PROG CODE ; let linker place main program CBLOCK 0x20 micros ones tens count ENDC START bsf STATUS,RP0 movlw 0x00 movwf TRISB movlw 0x00 movwf TRISC movlw 0x07 movwf OPTION_REG bcf STATUS,RP0 bsf INTCON,GIE bsf INTCON,TMR0IE clrf ones clrf tens clrf micros MAIN movlw 0x02 movwf PORTC movf ones, W call TABLE movwf PORTB call DELAY movlw 0x01 movwf PORTC movf tens, W call TABLE movwf PORTB call DELAY goto MAIN ISR bcf INTCON,GIE bcf INTCON,TMR0IE incf micros,1 movf micros,0 sublw 0x0F btfsc STATUS,Z goto inc_ones goto ret inc_ones clrf micros incf ones, 1 movf ones, 0 sublw 0x0A btfsc STATUS,Z goto inc_tens goto ret inc_tens clrf ones incf tens, 1 movf tens, 0 sublw 0x0A btfsc STATUS,Z clrf tens goto ret ret bcf INTCON,TMR0IF bsf INTCON,GIE bsf INTCON,TMR0IE retfie DELAY LOOP DECFSZ count,F GOTO LOOP RETURN TABLE addwf PCL retlw b'00111111' ;digit 0 retlw b'00000110' ;digit 1 retlw b'01011011' ;digit 2 retlw b'01001111' ;digit 3 retlw b'01100110' ;digit 4 retlw b'01101101' ;digit 5 retlw b'01111101' ;digit 6 retlw b'00000111' ;digit 7 retlw b'01111111' ;digit 8 retlw b'01101111' ;digit 9 END
Very helpful. I thank you.