Atmel AVR AT90S8515-4 Manual do Utilizador descarregar pdf (Página 12)

AN INTRODUCTION INTENDED FOR PEOPLE WITH NO PRIOR AVR KNOWLEDGE - AVRFREAKS.NET

;My Very First AVR Project

Lines beginning with " ; " (semicolon) are comments.

Comments can be added to any line of code. If comments

are written to span multiple lines, each of these lines much

begin with a semicolon

.include "8515def.inc"

Different AVR devices have e.g. PORTB placed on different

location in IO memory. These .inc files maps MNEMONICS

codes to physical addresses. This allows you for example

to use the label PORTB instead of remembering the

physical location in IO memory (0x18 for AT90S8515)

.def Temp = R16

The .def (Define) allow you to create easy to remember

labels (e.g. Temp) instead of using the default register

Name (e.g. R16). This is especially useful in projects

where you are working with a lot of variables stored in the

general purpose Registers (The Datasheet gives a good

explanation on the General Purpose Registers!)

.org 0x0000

This is a directive to the assembler that instructs it to place

the following code at location 0x0000 in Flash memory. We

want to do this so that the following RJMP instruction is

placed in location 0 (first location of FLASH). The reason is

that this location is the Reset Vector, the location from

where the program execution starts after a reset, power-

on or Watchdog reset event. There are a also other

interrupt vectors here, but our application does not use

interrupts, so we can use this space for regular code!

rjmp RESET

Since the previous command was the .org 0x0000, this

Relative Jump (RJMP) instruction is placed at location 0 in

Flash memory, and is the first instruction to be executed.

If you look at the Instruction Set Summary in the

Datasheet, you will see that the AT90S8515 do not have a

JMP instruction. It only has the RJMP instruction! The

reason is that we do not need the full JMP instruction. If

you compare the JMP and the RJMP you will see that the

JMP instruction has longer range, but requires an

additional instruction word, making it slower and bigger.

RJMP can reach the entire Flash array of the AT90S8515,

so the JMP instruction is not needed, thus not

implemented.

RESET:

This is a label. You can place these where you want in the

code, and use the different branch instructions to jump to

this location. This is quite neat, since the assembler itself

will calculate the correct address where the label is.

ldi Temp, 0xFF

Ah.. finally a decent instruction to look at: Load Immediate

(LDI). This instruction loads an Immediate value, and

writes it to the Register given. Since we have defined the

R16 register to be called "Temp", this instruction will write

the hex value 0xff (255 decimal) to register R16.

out DDRB, Temp

Why aren't we just writing "ldi DDRB, Temp"? A good

question, and one that require that we take a look in the

Instruction Set Manual. Look up the "LDI" and "OUT"

instructions. You will find that LDI has syntax : "LDI Rd, K"

which means that it can only be used with General Purpose

Registers R16 to R31. Looking at "OUT" instruction we see

that the syntax is "OUT A, Rr" Which means that the

content that is going to be written by the OUT instruction

1 2 ... 7 8 9 10 11 12 13 14 15 16 17

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