Timers, Timer SFRs - Microcontroller Interfacing - Lab Handouts, Lecture notes of Microcontrollers

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Microcontroller Interfacing Lab

Chapter 7 - 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 doesn’t 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, it’s 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, 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:

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, let’s 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 it’s 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.

The TMOD SFR

Let’s 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 GATE

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.

6 C/T

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.

5 T1M1 Timer mode bit (see below) 1 4 T1M0 Timer mode bit (see below) 1

3 GATE

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.

2 C/T

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.

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

13-bit Time Mode (mode 0)

Timer mode "0" is a 13-bit timer. This is a relic that was kept around in the 8051 to maintain compatibility with it’s predecessor, the 8048. Ge nerally 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.

What’s 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, you’d 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 you’ve configured a timer in mode 2 you don’t 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.

Split Timer Mode (mode 3)

Timer mode "3" is a split-timer mode. When Timer 0 is placed in mode 3, it essentially becomes two separate 8-bit timers. That is to say, Timer 0 is TL0 and Timer 1 is TH0. Both timers count from 0 to 255 and overflow back to 0. All the bits that are related to Timer 1 will now be tied to TH0.

While Timer 0 is in split mode, the real Timer 1 (i.e. TH1 and TL1) can be put into modes 0, 1 or 2 normally--however, you may not start or stop the real timer 1 since the bits that do that are now linked to TH0. The real timer 1, in this case, will be incremented every machine cycle no matter what.

The only real use I can see of using split timer mode is if you need to have two separate timers and, additionally, a baud rate generator. In such case you can use the real Timer 1 as a baud rate generator and use TH0/TL0 as two separate timers.

The TCON SFR

Finally, there’s one more SFR that controls the two timers and provides valuable information about them. The TCON SFR has the following structure:

TCON (88h) SFR

Bit Name Bit Address Explanation of Function Timer

7 TF1 8Fh Timer 1 Overflow. This bit is set by the microcontroller when Timer 1 overflows.

6 TR1 8Eh

Timer 1 Run. When this bit is set Timer 1 is turned on. When this bit is clear Timer 1 is off.

5 TF0 8Dh Timer 0 Overflow. This bit is set by the microcontroller when Timer 0 overflows.

4 TR0 8Ch

Timer 0 Run. When this bit is set Timer 0 is turned on. When this bit is clear Timer 0 is off.

As you may notice, we’ve only defined 4 of the 8 bits. That’s because the other 4 bits of the SFR don’t have anything to do with timers--they have to do with Interrupts and they will be discussed in the chapter that addresses interrupts.

A new piece of information in this chart is the column "bit address." This is because this SFR is "bit-addressable." What does this mean? It means if you want to set the bit TF1--which is the highest bit of TCON--you could execute the command:

MOV TCON, #80h ... or, since the SFR is bit-addressable, you could just execute the command: SETB TF This has the benefit of setting the high bit of TCON without changing the value of any of the other bits of the SFR. Usually when you start or stop a timer you don’t want to modify the other values in TCON, so you take advantage of the fact that the SFR is bit-addressable.

Initializing a Timer

Now that we’ve discussed the timer-related SFRs we are ready to write code that will initialize the timer and start it running.

As you’ll recall, we first must decide what mode we wa nt the timer to be in. In this case we want a 16-bit timer that runs continuously; that is to say, it is not dependent on any external pins.

REPEAT: MOV A,TH

MOV R0,TL

CJNE A,TH0,REPEAT

In this case, we load the accumulator with the high byte of Timer 0. We then load R0 with the low byte of Timer 0. Finally, we check to see if the high byte we read out of Timer 0--which is now stored in the Accumulator--is the same as the current Timer 0 high byte. If it isn’t it means we’ve just "rolled over" and must reread the timer’s value --which we do by going back to REPEAT. When the loop exits we will have the low byte of the timer in R0 and the high byte in the Accumulator.

Another much simpler alternative is to simply turn off the timer run bit (i.e. CLR TR0), read the timer value, and then turn on the timer run bit (i.e. SETB TR0). In that case, the timer isn’t running so no special tricks are necessary. Of course, this implies that your timer will be stopped for a few machine cycles. Whether or not this is tolerable depends on your specific application.

Detecting Timer Overflow

Often it is necessary to just know that the timer has reset to 0. That is to say, you are not particularly interest in the value of the timer but rather you are interested in knowing when the timer has overflowed back to 0.

Whenever a timer overflows from it’s highest value back to 0, the microcontroller automatically sets the TFx bit in the TCON register. This is useful since rather than checking the exact value of the timer you can just check if the TFx bit is set. If TF0 is set it means that timer 0 has overflowed; if TF1 is set it means that timer 1 has overflowed.

We can use this approach to cause the program to execute a fixed delay. As you’ll recall, we calculated earlier that it takes the 8051 1/20th of a second to count from 0 to 46,079. However, the TFx flag is set when the timer overflows back to 0. Thus, if we want to use the TFx flag to indicate when 1/20th of a second has passed we must set the timer initially to 65536 less 46079, or 19,457. If we set the timer to 19,457, 1/20th of a second later the timer will overflow. Thus we come up with the following code to execute a pause of 1/20th of a second:

MOV TH0,#76 ;High byte of 19,457 (76 * 256 = 19,456) MOV TL0,#01 ;Low byte of 19,457 (19,456 + 1 = 19,457) MOV TMOD,#01 ;Put Timer 0 in 16-bit mode SETB TR0 ;Make Timer 0 start counting JNB TF0,$ ;If TF0 is not set, jump back to this same instruction

In the above code the first two lines initialize the Timer 0 starting value to 19,457. The next two instructions configure timer 0 and turn it on. Finally, the last instruction JNB TF0,$ , reads "Jump, if TF0 is not set, back to this same instruction." The "$" operand means, in most assemblers, the address of the current instruction. Thus as long as the timer has not overflowed and the TF0 bit has not been set the program will keep executi ng this same instruction. After 1/20th of a second timer 0 will overflow, set the TF0 bit, and program execution will then break out of the loop.

Timing the length of events

The 8051 provides another cool toy that can be used to time the length of events.

For example, let's say we're trying to save electricity in the office and we're interested in how long a light is turned on each day. When the light is turned on, we want to measure time. When the light is turned off we don't. One option would be to connect the light switch to one of the pins, constantly read the pin, and turn the timer on or off based on the state of that pin. While this would work fine, the 8051 provides us with an easier method of accomplishing this.

Looking again at the TMOD SFR, there is a bit called GATE0. So far we've always cleared this bit because we wanted the timer to run regardless of the state of the external pins. However, now it would be nice if an external pin could control whether the timer was running or not. It can. All we need to do is connect the light switch to pin INT0 (P3.2) on the 8051 and set the bit GATE0. When GATE0 is set Timer 0 will only run if P3.2 is high. When P3.2 is low (i.e., the light switch is off) the timer will automatically be stopped.

Thus, with no control code whatsoever, the external pin P3.2 can control whether or not our timer is running or not.

USING TIMERS AS EVENT COUNTERS

We've discussed how a timer can be used for the obvious purpose of keeping track of time. However, the 8051 also allows us to use the timers to count events.

How can this be useful? Let's say you had a sensor placed across a road that would send a pulse every time a car passed over it. This could be used to determine the volume of traffic on the road. We could attach this sensor to one of the 8051's I/O lines and constantly monitor it, detecting when it pulsed high and then incrementing our counter when it went back to a low state. This is not terribly difficult, but requires some code. Let's say we hooked the sensor to P1.0; the code to count cars passing would look something like this: