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;********************************************************
;Chip Model: MEGA328P
;Assembler header file
.INCLUDE "m328pdef.inc" ;tells the AVR assembler to add the contents of a file to our program
;********************************************************
.ORG 0 ;indicate the beginning of the address
;initialize the SP to point to the last location of RAM (RAMEND)
LDI R20, HIGH(RAMEND) ;load R20 with the high byte of RAMEND
OUT SPH, R20 ;initialize SPH
LDI R20, LOW(RAMEND) ;load R20 with the low byte of RAMEND
OUT SPL, R20 ;initialize SPL
LDI R20, 0xFF ;load R20 with the value 0xFF in hex
OUT DDRD, R20 ;make PORTD an output port
BACK:
SBI PORTD,0 ;turn on PD0
CALL DELAY ;time delay, call the DELAY subroutine
CBI PORTD,0 ;turn off PD0
CALL DELAY ;time delay, call the DELAY subroutine
JMP BACK ;jump to BACK, keep doing it forever (an infinite loop)
DELAY:
LDI R20, 100 ;load R20 with the value 100 in dec
L0: LDI R21, 250 ;load R21 with the value 250 in dec
L1: LDI R22, 250 ;load R22 with the value 250 in dec
L2:
NOP ;do nothing (1 Instruction cycle)
NOP ;do nothing (1 Instruction cycle)
DEC R22 ;decrement R22 by one, assign Z:=1 if R22==0 (1 Instruction cycle)
BRNE L2 ;test the zero flag (Z), branch to L2 if Z==0 (2 Instruction cycles)
DEC R21 ;decrement R21 by one, assign Z:=1 if R21==0
BRNE L1 ;test the zero flag (Z), branch to L1 if Z==0
DEC R20 ;decrement R20 by one, assign Z:=1 if R20==0
BRNE L0 ;test the zero flag (Z), branch to L0 if Z==0
RET ;return to caller (the end of the DELAY subroutine)
;Let's calculate the time delay of the DELAY subroutine. You can find the crystal frequency of atmega328 by right-clicking the component atmega328 on Form and clicking Properties. Usually it's 16 Mhz. So instruction cycle = 1/16 MHz = 0.0625 μs = 62.5 ns (nanosecond)
;Delay=100*250*250*(1+1+1+2)*62.5 ns=1,953,125,000 ns=1,953,125 µs≈2 s.
;The time delay is not precise. In this calculation, we have not included the overhead associated with the two outer loops.