Atmel AVR AT90S8515-4 Manual do Utilizador descarregar pdf (Página 13)

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

has to be fetched from one of the 32 (R0 to R31) General

Purpose Registers. Anyway, this instruction sets the Data

Direction Register PORTB (DDRB) register to all high. By

setting this register to 0xFF, all IO pins on PORTB are

configured as outputs.

Loop:

Another label...

out PORTB, Temp

We Now write the value 0xFF to PORTB, which would give

us 5V (Vcc) on all PORTB IO pins if we where to measure it

on a real device. Since the IO ports is perhaps the most

used feature of the AVR it would be a good idea to open

the Datasheet on the PORTB. Notice that PORTB has 3

registers PORTB, PINB and DDRB. In the PORTB register

we write what we want written to the physical IO pin. In

the PINB register we can read the logic level that is

currently present on the Physical IO pin, and the DDRB

input or output. (The reason for 3 registers are the "Read-

Modify-Write" issue associated with the common 2 register

approach, but this is a topic for the Advanced class.)

dec Temp

This Decrement (DEC) instruction decrements the Temp

(R16) register. After this instruction is executed, the

contents of Temp is 0xFE. This is an Arithmetic instruction,

and the AVR has a wide range of Arithmetic instructions.

For a complete listing of available instruction: Look in the

Instruction Set Summary in the Datasheet!

rjmp Loop

Here we make a jump back to the Loop lable. The program

will thus continue to write the Temp variable to PORTB

decrementing it by one for each loop.

I guess you have figured out what our masterpiece is doing. We have made a

counter counting down from 255 to 0, but what happens when we reach zero?

To find out this, we will simulate the behaviour in AVR Studio 4.

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Atmel AVR AT90S8515-4 Manual do Utilizador Página 13