Uploaded October 2012 | Updated September 2026, 2 weeks ago
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.
Thus, we now have a system with which to measure time. All we need to review is how to control the timers and initialize them to provide us with the information we need.
16-bit Time Mode (mode 1)
Timer mode "1" is a 16-bit timer. This is a very commonly used mode. It functions just like 13-bit mode except that all 16 bits are used.
TLx is incremented from 0 to 255. When TLx is incremented from 255, it resets to 0 and causes THx to be incremented by 1. Since this is a full 16-bit timer, the timer may contain up to 65536 distinct values. If you set a 16-bit timer to 0, it will overflow back to 0 after 65,536 machine cycles.
8-bit Time Mode (mode 2)
Timer mode "2" is an 8-bit auto-reload mode. What is that, you may ask? Simple. When a timer is in mode 2, THx holds the "reload value" and TLx is the timer itself. Thus, TLx starts counting up. When TLx reaches 255 and is subsequently incremented, instead of resetting to 0 (as in the case of modes 0 and 1), it will be reset to the value stored in THx.
For example, lets say TH0 holds the value FDh and TL0 holds the value FEh. If we were to watch the values of TH0 and TL0 for a few machine cycles this is what wed see:
Machine Cycle TH0 Value TL0 Value
1 FDh FEh
2 FDh FFh
3 FDh FDh
4 FDh FEh
5 FDh FFh
6 FDh FDh
7 FDh FEh
As you can see, the value of TH0 never changed. In fact, when you use mode 2 you almost always set THx to a known value and TLx is the SFR that is constantly incremented.
Whats the benefit of auto-reload mode? Perhaps you want the timer to always have a value from 200 to 255. If you use mode 0 or 1, youd have to check in code to see if the timer had overflowed and, if so, reset the timer to 200. This takes precious instructions of execution time to check the value and/or to reload it. When you use mode 2 the microcontroller takes care of this for you. Once youve configured a timer in mode 2 you dont have to worry about checking to see if the timer has overflowed nor do you have to worry about resetting the value--the microcontroller hardware will do it all for you.
The auto-reload mode is very commonly used for establishing a baud rate which we will talk more about in the Serial Communications chapter.
-~-~~-~~~-~~-~-
Please watch: "Sci Hub Not working 2018 August"
youtube.com/watch?v=uV3EntVBxTY
-~-~~-~~~-~~-~-
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.
Thus, we now have a system with which to measure time. All we need to review is how to control the timers and initialize them to provide us with the information we need.
16-bit Time Mode (mode 1)
Timer mode "1" is a 16-bit timer. This is a very commonly used mode. It functions just like 13-bit mode except that all 16 bits are used.
TLx is incremented from 0 to 255. When TLx is incremented from 255, it resets to 0 and causes THx to be incremented by 1. Since this is a full 16-bit timer, the timer may contain up to 65536 distinct values. If you set a 16-bit timer to 0, it will overflow back to 0 after 65,536 machine cycles.
8-bit Time Mode (mode 2)
Timer mode "2" is an 8-bit auto-reload mode. What is that, you may ask? Simple. When a timer is in mode 2, THx holds the "reload value" and TLx is the timer itself. Thus, TLx starts counting up. When TLx reaches 255 and is subsequently incremented, instead of resetting to 0 (as in the case of modes 0 and 1), it will be reset to the value stored in THx.
For example, lets say TH0 holds the value FDh and TL0 holds the value FEh. If we were to watch the values of TH0 and TL0 for a few machine cycles this is what wed see:
Machine Cycle TH0 Value TL0 Value
1 FDh FEh
2 FDh FFh
3 FDh FDh
4 FDh FEh
5 FDh FFh
6 FDh FDh
7 FDh FEh
As you can see, the value of TH0 never changed. In fact, when you use mode 2 you almost always set THx to a known value and TLx is the SFR that is constantly incremented.
Whats the benefit of auto-reload mode? Perhaps you want the timer to always have a value from 200 to 255. If you use mode 0 or 1, youd have to check in code to see if the timer had overflowed and, if so, reset the timer to 200. This takes precious instructions of execution time to check the value and/or to reload it. When you use mode 2 the microcontroller takes care of this for you. Once youve configured a timer in mode 2 you dont have to worry about checking to see if the timer has overflowed nor do you have to worry about resetting the value--the microcontroller hardware will do it all for you.
The auto-reload mode is very commonly used for establishing a baud rate which we will talk more about in the Serial Communications chapter.
-~-~~-~~~-~~-~-
Please watch: "Sci Hub Not working 2018 August"
youtube.com/watch?v=uV3EntVBxTY
-~-~~-~~~-~~-~-
