Programming 8051 Timers: Using Timers to Measure Time
One of the primary uses of timers is to measure time.
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. A single machine cycle consists of 12 crystal pulses. Thus a running timer will be incremented:11,059,000 / 12 = 921,583 times per second.
Unlike instructions 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. To execute an event once per second you’d have to wait for the timer to count from 0 to 50,000 18.45times.
To calculate how many times the timer will be incremented in .05 seconds, a simple multiplication can be done: 0 .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 46.0. To work with timers is to control the timers and initialize them.
The TMOD 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 1whereas the low four bits (bits 0 through 3) perform the exact same functions, but for timer 0. The modes of operation are:
Timer mode "0" is a 13-bit timer. 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. 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.
16-bit Time Mode (mode 1)
Timer mode "1" is a 16-bit timer. 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.
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, if TH0 holds the value FDh and TL0 holds the value FEh values of TH0 and TL0 for a few machine cycles:
The value of TH0 never changed. When we use mode 2 you almost always set THx to a
known value and TLxis the SFR that is constantly incremented. The benefit of auto-reload mode is the timer 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. Auto-reload mode is very commonly used for establishing a baud rate in Serial Communications.
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. 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,e, will be incremented every machine cycle always. The only real use in split timer mode is if you need to have two separate timers and, additionally, a baud rate generator you can use the real Timer 1 as a baud rate generator and use TH0/TL0 as two separate timers.
Reading the Timer
There are two common ways of reading the value of a 16-bit timer; which you use depends on your specific application. You may either read the actual value of the timer as a 16-bit number, or you may simply detect when the timer has overflowed.
Reading the value of a Timer
If timer is in an 8-bit mode either 8-bit Auto Reload mode or in split timer mode, you simply read the 1-byte value of the timer. With a 13-bit or16-bit timer the timer value was 14/255 (High byte 14, low byte 255) but you read 15/255.Because you read the low byte as 255. But when you executed the next instruction a small amount of time passed--but enough
for the timer to increment again at which time the value rolled over from 14/255 to 15/0. But in the process you’ve read the timer as being 15/255.
You read the high byte of the timer, then read the low byte, then read the high byte again. If the high byte read the second time is not the same as the high byte read the first time you repeat the cycle. In code, this would appear as:
REPEAT: MOV A, TH0 MOV R0, TL0
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. 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).
Detecting Timer Overflow
Whenever a timer overflows from its highest value back to 0, the microcontroller automatically sets the TFx bit in the TCON register. 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. it takes the 8051 1/20thof 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.
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 executing 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.
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