avra <YOUR FILE>.s
avrdude -p m168 -P /dev/ttyUSB0 -c stk500v1 -b 19200 -F -u -U flash:w:<YOUR FILE>.s.hex
.nolist ; We don't want to actually include defs in our listing file..include "m168def.inc" ; m168def.inc defines all the pins on the Mega168 so we can ; use them by their names rather than addresses (not fun)..list ; We DO want to include the following code in our listing ;D rjmp main ; You usually place these two lines after all yourmain: ; directives. They make sure that resets work correctly. ldi r16,0xFF ; LoaD Immediate. This sets r16 = 0xFF (255) out DDRB,r16 ; Out writes to SRAM, which is one way of accessing ; pins. DDRB controls PORTB's in/out state. ldi r16,0x00 ; r16 is where we'll store current LED state ; 0x00 means all off. This is preserved over loops.loop: ; This is a label (like main above) where we can come back to com r16 ; Flip all the bits in r16 (one's complement) ; So, originally, we go from 0x00 (off) to 0xFF (on) out PORTB,r16 ; set all B pins to current state. PORTB is where our favorite flashing pin is (pin 13)!; Waiting for a specified time:; Ok, so we want to wait one second between each LED flip.; Our Arduino should be clocked at 16Mhz, so that means we have to wait; 16 million cycles between flips. Different instructions take different ; amounts of clock cycles to complete, so we have to count the cycles ; to get an accurate wait time. We're going to use a combination of a; word and a byte to get the number of cycles right, since just a single; byte or word can't hold near enough to wait for our needed time.; Counting cycles:; Below you'll see a couple instructions with numbers in their comments.; The numbers represent how many clock cycles the instruction takes.; You can find all of these cycle amounts in the atmega168 datasheet.; 1/2 means it can take either one or two cycles depending on the flag; state. For these branch instructions (BRNE), they take one cycle if; the condition is false, and two cycles if it's true. We'll only care; about the two cycle case, since that will be happening 99% of the time.; Also, just for extra laziness we wont care about the outside loop's cycles.; The inner is the really time critical one anyway ;D; The math:; The Arduino is clocked at 16Mhz, or 16000000 cycles per second.; Our main loop takes 4 cycles to complete one loop, since SBIW; takes 2 cycles and BRNE will (usually) take 2 cycles. This means; that we need to find an X < 256 and Y < 65536 so that X*Y*4 = 16000000.; X=100 and Y=40000 fit that bill quite nicely, so we'll go with those.; The outer loop runs 100 times, and the inner runs 40000 times per outer; loop. This gives us about 1 second per flip! ldi r17,100 ; r17 is our outer loop counterouter: ; ZH and ZL are a word pseudo-register. It's actually a combination ; of r31 and r30, giving us 16 bits to use for SBIW. Handy! ; The HIGH and LOW macros just give us the high and low bits of a ; number. ldi ZH,HIGH(40000) ; 1 ldi ZL,LOW(40000) ; 1inner: ; These next two instructions SuBtract Immediate from Word and ; BRanch if Not Equal. Basically, we subtract one from the Z psuedo ; register (which begins with ZL, the low bits), and then, if we haven't ; reached 0, go back to the inner label. Otherwise we keep on going with ; the check for the outer loop! sbiw ZL,1 ;2 \_ Main Loop (4 cycles) brne inner ;1/2 / ; The following instructions DECrement and BRanch if Not Equal. BRNE ; works exactly the same as above. DEC is shorthand for subtract one ; from this register. Only takes one clock cycle, too! dec r17 ;1 brne outer ;1/2 ; Finally we've reached the end of the loop. Relative JuMP just brings ; us back to the label listed. Back to loop we go! rjmp loop
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