1. Find the time delay for the delay subroutine shown below if the system has an

Question

1.      Find the time delay for the delay subroutine shown below if the system has an AVR with a clock frequency of 20 MHz. Do not consider the time delay required to call the delay subroutine.  In other words, start the delay calculation after the first LDI instruction in the subroutine has been fetched.  Do include the time required to fetch and execute the RET instruction.  Instruction cycle times are provided in the handout. SHOW YOUR WORK including the number of clock cycles per instruction and how you add these up to get the total delay.

DELAY:    LDI  R20, 200

BACK:     LDI  R25, 100

          NOP

          NOP

          NOP

HERE:     DEC  R25

          BRNE HERE

          DEC  R20

          BRNE BACK

          RET

Explanation / Answer

cycles = 1+ (4 + (300-1)+1+2)*200-1

cycles = 306*200

delay = 306*200/(20*10^6) = 3.06msec

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