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PICkit ? 1 Flash Starter Kit User’s Guide
C.7
LESSON 5 – PROGRAM MEMORY LOOK-UP ROUTINES
There is often a need to store constants or strings in a PIC microcontroller. Storing this
information in program memory is the best solution as long as this data never needs to
change and program memory is available. Program memory is non-volatile, therefore,
it will maintain information regardless of V DD voltage levels or PIC microcontroller reset.
This lesson demonstrates how to retrieve data from Program Memory. It covers the
following topics:
? What a program memory look-up table is
? How a program memory look-up table is implemented
? Why a program memory look-up table is useful
C.7.1
Design
Constants stored in program memory are accessed via look-up tables. A look-up table
is similar to a computed goto (see Lesson 2 ) only instead of a list of goto statements,
after incrementing the program counter, there is a list of retlw instructions. Each
retlw instruction is followed by one byte of the information. Example C-3 shows a
look-up table for the string “Microchip” written in assembly language.
EXAMPLE C-3:
LOOK-UP TABLE WRITTEN IN ASSEMBLY
LookupProgramMemory
movlw high StartTable
movwf PCLATH
movlw low StartTable
addwf index, w
btfsc STATUS, C
incf PCLATH, f
movwf PCL
StartTable
retlw
retlw
retlw
retlw
retlw
retlw
retlw
retlw
retlw
" M "
" i "
"c"
"r"
"o"
"c"
"h"
"i"
"p"
For this lesson, constants corresponding to a sequence of LEDs are stored in a
program memory look-up table. For instance, the first constant stored is 5F. This
corresponds to the binary number “ 01011111 ”. Each bit corresponds to an LED,
therefore, LEDs D0, D1, D2, D3, D4 and D6 will be lit when this constant is accessed.
After each look-up table, the value is displayed on the LEDs in the same fashion as
discussed in Lesson 3 , using the Timer0 interrupt. The appearance of more than one
LED being lit at a time is possible because the LEDs are multiplexed fast enough that
the human eye can not detect the LEDs being turned on and off. When SW1 is pressed,
an interrupt-on-change is generated to increment the lookup to the next location.
Figure C-14 shows the flowchart for this example.
DS40051D-page 60
? 2004 Microchip Technology Inc.
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