Advanced Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 184

4.9 Subroutines for Creating Time Delay

We have learned how to create a time delay using program loops. These program loops can
be converted into subroutines so that they can be called from any place in a program.

Example 4.11
▼ Write a subroutine that can create a time delay of 100 ms.
Solution: By adding an rts instruction to the end of the instruction sequence of Example 2.25, a
subroutine that creates a delay of 100 ms can be created.
delay100ms

pshx

ldx #60000 ; 2 E cycles

iloop psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

nop ; 1 E cycle

nop ; 1 E cycle

dbne x,iloop ; 3 E cycles

pulx
rts
This subroutine will be more useful if it is made parameterized; that is, the delay time can
be set to be a multiple of 100 ms instead of a fixed value. The following subroutine can generate
a time delay that is a multiple of 100 ms with the multiple passed in Y:
delayby100ms
pshx
eloop3 ldx #60000 ; 2 E cycles

iloop3 psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles

pula ; 3 E cycles

psha ; 2 E cycles