Timers
Tutorial Timers
The 8051 comes equipped with two timers, both of which may be controlled, set, read, and configured individually. The 8051 timers have three general functions: 1) Keeping time and/or calculating the amount of time between events, 2) Counting the events themselves, or 3) Generating baud rates for the serial port.
The three timer uses are distinct so we will talk about each of them separately. The first two uses will be discussed in this chapter while the use of timers for baud rate generation will be discussed in the chapter relating to serial ports.
How does a timer count?
How does a timer count? The answer to this question is very simple: A timer always counts up. It doesnt matter whether the timer is being used as a timer, a counter, or a baud rate generator: A timer is always incremented by the microcontroller.
Programming Tip: Some derivative chips actually allow the program to configure whether the timers count up or down. However, since this option only exists on some derivatives it is beyond the scope of this tutorial which is aimed at the standard 8051. It is only mentioned here in the event that you absolutely need a timer to count backwards, you will know that you may be able to find an 8051-compatible microcontroller that does it.
USING TIMERS TO MEASURE TIME
Obviously, one of the primary uses of timers is to measure time. We will discuss this use of timers first and will subsequently discuss the use of timers to count events. When a timer is used to measure time it is also called an “interval timer” since it is measuring the time of the interval between two events.
How long does a timer take to count?
First, its worth mentioning that when a timer is in interval timer mode (as opposed to event counter mode) and correctly configured, it will increment by 1 every machine cycle. As you will recall from the previous chapter, a single machine cycle consists of 12 crystal pulses. Thus a running timer will be incremented:
11,059,000 / 12 = 921,583
921,583 times per second. Unlike instructions–some of which require 1 machine cycle, others 2, and others 4–the timers are consistent: They will always be incremented once per machine cycle. Thus if a timer has counted from 0 to 50,000 you may calculate:
50,000 / 921,583 = .0542
.0542 seconds have passed. In plain English, about half of a tenth of a second, or one-twentieth of a second.
Obviously its not very useful to know .0542 seconds have passed. If you want to execute an event once per second youd have to wait for the timer to count from 0 to 50,000 18.45 times. How can you wait “half of a time?” You cant. So we come to another important calculation
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