Delay Calulation

A register is loaded with a number , depending on the time delay required ,and then the register is decremented until it reaches zero by setting up a loop with a conditional jump instruction.The loop causes the delay, depending on the clock period of the system.

Code for looping:

mvi c,count 7T

loop: dcr c 4T

jnz loop 10/7 T

if the clock frequency of the system f = 2MHZ

clock period (T) = 1/f = 1/2*10 power-6 = 0.5 microsecond

here, we have to calculate three time variable (t0, tL , tLA)

time to execute MVI C ,count
    t0 = 7T

  = 7 x 0.5 microsecond

 t0  = 3.5 microsecond

time delay in loop (tL) = T x loop T states x count

here T = 0.5 microsecond

loop T states = 4T + 10T = 14T

tL = 0.5 microsecond x 14 x count

tL = (7 x count) microsecond

The T states for JNZ is 10/7

so we also have to consider the 7T states of JNZ . when the loop executes for specified count times , and at last when the condition is false , the T states it takes will be only 7T so we have to subtract 3T from the tL that we have calculated above.

tLA = 3T

= 3 x 0.5 microsecond

 = 1.5 microsecond

now total time taken tt is given by

tt= t0 + tL - tLA

 = 3.5 + (7 x count ) -1.5 microsecond

 tt = 2 + 7 x count

now if you need delay for 1 milli second then

1ms = 2 + 7 x count microsecond

count = (1000-2)/7

count = 140

the maximum delay can be 1.8ms

ie count is 255 in decimal

if you need more time delay you can use second method

Time delay using a register pair

The time delay can be considerably increased by setting a loop and using a register pair with a 16 bit number( maximum FFFFh)

.The 16 bit number is decremented by using the instruction DCX .However the instruction DCX doesnot set the zero flag and without the test flags ,jump instructios cannot check desired data conditions .Additional techniques ,therefore must be used tto set the Zero flag.now we use OR instruction [ Continue reading]