1
AVR108: Setup and Use of the LPM Instruction
Features
• Use of the LPM (Load Program Memory) Instruction with the AVR
®
Assembler
• Load Constants from Program Memory
• Use of Lookup Tables
Introduction
This application note describes how to access constants saved in Flash Program
memory of the AVR microcontrollers. The AVR is based on a Harward architecture,
this means that Address and Data memory use separate busses. This is necessary to
achieve single cycle instructions execution speed. To be able to save constants in
Flash memory the Load Program Memory (LPM) instruction is included in the instruc-
tion set.
Use of the LPM
The LPM instruction is included in the AVR Instruction Set to load a data byte from the
FLASH Program memory into the Register File.
The Flash Program memory of the AVR microcontroller is organized as 16 bits words.
The Register File and SRAM Data memory are organized as eight bits bytes. Special
consideration must therefore be taken when loading data from Program memory to
Data meory.
The Z-register in the Register File is used to access the Program memory. This 16 bits
register pair is used as a 16 bits pointer to the Program memory. The 15 most signifi-
cant bits selects the word address in Program memory. Because of this, the word
address is multiplied by two before it is put in the Z-register.
Figure 1. Z Address Register
The least significant bit of the Z Address Register selects either Low byte (0) or High
byte(1) of the Program memory word. To calculate the Low (ZL) an High (ZH) part of
the address, use the LOW() and HIGH() functions.
To load data from random places in program memory, the Z-register must be set up
with the proper address each time a new address is accessed.
In Program memory the data is organized with one byte in the low part of a program
word and the next byte in the high part. Because of this, the message string will
appear as if every pair of characters has been swapped, when viewed in the memory
view in AVR Studio
®
.
15 8 7 1 0
Program Word Adress
Low/High
Byte Select
8-bit
Microcontroller
Application
Note
Rev. 1233B–AVR–05/02
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