Page 162 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes
Figure 11.6: Offset and Non-Offset Branching

11.3 Time Delays

A special class of electromechanical relays called time-delay relays provide delayed action, either
upon power-up or power-down, and are commonly denoted in ladder logic diagrams by “TD”
or “TR” designations near the coil symbols and arrows on the contact symbols. Here is an example
of a time-delay relay contact used in a motor control circuit:

Figure 11.7: Time Delays

• Knowing the combinations of cycles, one can calculate how long such an instruction would
require to complete.
• Knowing how many T-States an instruction requires, and keeping in mind that a T-State is
one clock cycle long, we can calculate the time using the following formula:
Delay = No. of T-States / Frequency
For example a “MVI” instruction uses 7 T-States.

Therefore, if the Microprocessor is running at 2 MHz, the instruction would require 3.5 mSeconds
to complete.

• We can use a loop to produce a certain amount of time delay in a program.
• The following is an example of a delay loop:
MVI C, FFH 7 T-States

LOOP DCR C 4 T-States
JNZ LOOP 10 T-States

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