8051 basics
8051 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? 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.
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.
Lets say we want to know how many times the timer will be incremented in .05 seconds. We can do simple multiplication:
.05 * 921,583 = 46,079.15.
This tells us that it will take .05 seconds (1/20th of a second) to count from 0 to 46,079. Actually, it will take it .049999837 seconds--so were off by .000000163 seconds--however, thats close enough for government work. Consider that if you were building a watch based on the 8051 and made the above assumption your watch would only gain about one second every 2 months. Again, I think thats accurate enough for most applications--I wish my watch only gained one second every two months!
Obviously, this is a little more useful. If you know it takes 1/20th of a second to count from 0 to 46,079 and you want to execute some event every second you simply wait for the timer to count from 0 to 46,079 twenty times; then you execute your event, reset the timers, and wait for the timer to count up another 20 times. In this manner you will effectively execute your event once per second, accurate to within thousandths of a second.
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.
As mentioned before, the 8051 has two timers which each function essentially the same way. One timer is TIMER0 and the other is TIMER1. The two timers share two SFRs (TMOD and TCON) which control the timers, and each timer also has two SFRs dedicated solely to itself (TH0/TL0 and TH1/TL1).
Weve given SFRs names to make it easier to refer to them, but in reality an SFR has a numeric address. It is often useful to know the numeric address that corresponds to an SFR name. The SFRs relating to timers are:
|
SFR Name |
Description |
SFR Address |
|
TH0 |
Timer 0 High Byte |
8Ch |
|
TL0 |
Timer 0 Low Byte |
8Ah |
|
TH1 |
Timer 1 High Byte |
8Dh |
|
TL1 |
Timer 1 Low Byte |
8Bh |
|
TCON |
Timer Control |
88h |
|
TMOD |
Timer Mode |
89h |
When you enter the name of an SFR into an assembler, it internally converts it to a number. For example, the command:
MOV TH0,#25h
moves the value 25h into the TH0 SFR. However, since TH0 is the same as SFR address 8Ch this command is equivalent to:
MOV 8Ch,#25h
Now, back to the timers. First, lets talk about Timer 0.
Timer 0 has two SFRs dedicated exclusively to itself: TH0 and TL0. Without making things too complicated to start off with, you may just think of this as the high and low byte of the timer. That is to say, when Timer 0 has a value of 0, both TH0 and TL0 will contain 0. When Timer 0 has the value 1000, TH0 will hold the high byte of the value (3 decimal) and TL0 will contain the low byte of the value (232 decimal). Reviewing low/high byte notation, recall that you must multiply the high byte by 256 and add the low byte to calculate the final value. That is to say:
TH0 * 256 + TL0 = 1000
3 * 256 + 232 = 1000
Timer 1 works the exact same way, but its SFRs are TH1 and TL1.
Since there are only two bytes devoted to the value of each timer it is apparent that the maximum value a timer may have is 65,535. If a timer contains the value 65,535 and is subsequently incremented, it will reset--or overflow--back to 0.
Lets first talk about our first control SFR: TMOD (Timer Mode). The TMOD SFR is used to control the mode of operation of both timers. Each bit of the SFR gives the microcontroller specific information concerning how to run a timer. The high four bits (bits 4 through 7) relate to Timer 1 whereas the low four bits (bits 0 through 3) perform the exact same functions, but for timer 0.
The individual bits of TMOD have the following functions:
TMOD (89h) SFR
|
Bit |
Name |
Explanation of Function |
Timer |
|
7 |
GATE1 |
When this bit is set the timer will only run when INT1 (P3.3) is high. When this bit is clear the timer will run regardless of the state of INT1. |
1 |
|
6 |
C/T1 |
When this bit is set the timer will count events on T1 (P3.5). When this bit is clear the timer will be incremented every machine cycle. |
1 |
|
5 |
T1M1 |
Timer mode bit (see below) |
1 |
|
4 |
T1M0 |
Timer mode bit (see below) |
1 |
|
3 |
GATE0 |
When this bit is set the timer will only run when INT0 (P3.2) is high. When this bit is clear the timer will run regardless of the state of INT0. |
0 |
|
2 |
C/T0 |
When this bit is set the timer will count events on T0 (P3.4). When this bit is clear the timer will be incremented every machine cycle. |
0 |
|
1 |
T0M1 |
Timer mode bit (see below) |
0 |
|
0 |
T0M0 |
Timer mode bit (see below) |
0 |
As you can see in the above chart, four bits (two for each timer) are used to specify a mode of operation. The modes of operation are:
|
TxM1 |
TxM0 |
Timer Mode |
Description of Mode |
|
0 |
0 |
0 |
13-bit Timer. |
|
0 |
1 |
1 |
16-bit Timer |
|
1 |
0 |
2 |
8-bit auto-reload |
|
1 |
1 |
3 |
Split timer mode |
Timer mode "0" is a 13-bit timer. This is a relic that was kept around in the 8051 to maintain compatability with its predecesor, the 8048. Generally the 13-bit timer mode is not used in new development.
When the timer is in 13-bit mode, TLx will count from 0 to 31. When TLx is incremented from 31, it will "reset" to 0 and increment THx. Thus, effectively, only 13 bits of the two timer bytes are being used: bits 0-4 of TLx and bits 0-7 of THx. This also means, in essence, the timer can only contain 8192 values. If you set a 13-bit timer to 0, it will overflow back to zero 8192 machine cycles later.
Again, there is very little reason to use this mode and it is only mentioned so you wont be surprised if you ever end up analyzing archaeic code which has been passed down through the generations (a generation in a programming shop is often on the order of about 3 or 4 months).
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.
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
|