AMX TXC16 Plus Manual do Utilizador descarregar pdf (Página 41)

COE305 LAB MANUAL

3.1 Background

It is often necessary to control how long certain actions last. This can be achieved

using software delays, or more accurately by the use of a timer (i.e. 8253 PIT chip). In

this experiment, you will learn how to generate time delays using both software delays

and 8253 PIT chip. Also, you will learn how to use time delays to control the operation

of some devices (e.g. LEDs and Relays), and to generate periodical waveforms of

different frequencies.

3.2 Software Delays

The easiest way to generate delays is a software delay. If we create a program which

loops around itself, and does this for a fixed number of times, for a given processor,

running at a given clock rate, this process will almost take the same time each time the

program is executed. All we have to do is to write this loop such that it takes 1 second.

Then, by calling this loop n times, we generate a delay of n seconds. Notice that the

time taken by the loop does not need to be 1 second. For example, a loop that takes

0.25 seconds to execute, can be called 4×n times (i.e. n/0.25) to generate a delay of n

seconds. The question now is how to determine the time taken by a loop. This can

be answered by the following example.

Example 3.1: Calculate the total time taken by the following loop.

MOV CX, 8000h; load CX with a fixed value 8000h (32768)

L1: DEC CX ; decrement CX, loop if not zero

JNZ L1

From the 8086 data sheets, we find that DEC CX requires 2 clock cycles and

JNZ requires 16 clock cycles. Thus, the total number of clock cycles required

by these two instructions is 18 clock cycles.

Since the FLIGHT-86 board is running at 14.7456/3 MHz, 1 clock cycle will

take 3/14.7456 microseconds, and 18 clock cycles will take 54/14.7456

microseconds. Thus, the total time taken by the loop is 32768 × (54/14.7456

× 10

-6

) = 0.12 seconds

The previous loop requires 0.12 seconds. Thus, this loop needs to be executed almost

8 times to generate a delay of 1 second. The following example shows how to use this

loop inside a program to turn ON/OFF an LED for specific amounts of time.

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AMX TXC16 Plus Manual do Utilizador Página 41