1. TIMERS
Prepared by:
Islam Samir

2. AGENDA





What is a timer? Why do we use it?
Counters.
Prescalars.
Timers, how to make digital hardware that counts time?
TMR0.

3. WHAT IS A TIMER?
In many applications, we need to do certain action every time
interval T.
 So, we need some hardware which tells the MCU that this time
interval has elapsed to do this action.
Examples
I.
Real time systems.
II.
Data acquisition systems.
III. Time division multiplexing communication systems.
IV.
Digital clocks.
V.
Operating systems.


4. WHAT IS A TIMER?

To implement such applications, we need some digital
hardware that counts time.

We do this simply by using a digital counter and controlling
the time it takes to overflow.

5. COUNTERS

A counter is essentially a register that, for every clock cycle, goes
through a predetermined sequence of binary states.(usually, the
numbers sequence 0,1,2,3…..).

6. COUNTERS
Ex. For a 2-bit counter, it will count from 0 to 4 ( 2 n-1)
and on the next clock edge it will overflow to 0 again.
So, it counted 4 counts ( 2 2 ) to overflow.
Rule#1:
2 ncounts to overflow.
 An n-bit counter will count

7. PRESCALARES

If you measured the frequency of the MSB in a n-bit counter
operates with Fosc, you’ll find it (Fosc/2 n).
Rule#2:
 The MSB bit in an n-bit counter changes with a frequency
equals the clock frequency divided by 2 n.
-
So, we can use synchronous counters to count, or to divide the
input frequency (used as a clock to this counter).

8. TIMERS
Ex.
- Suppose using a 4-bit counter, with a clock frequency =100
HZ
1
 The time of each count= 100 sec. = one clock cycle.
 The time for that counter to overflow=
1
1
 24 
 16  0.16 sec.
100
100

9. TIMERS
If we changed the frequency to 500 HZ,
the time to overflow will be:
-
1
1
4
2 
 16  0.032 sec.
500
500
-
If we used a 6-bit counter, the time to overflow will
1
1
be:
 26 
 64  0.64 sec.
100
100

10. TIMERS
So, by determining:
1.
The operating frequency of the clock
2. The number of counts.

We can determine the time for this counter to overflow, and
do some action every time this happens
(by generating an interrupt).
-
This is the idea of timers, they are counters which we
configure to count time, according to the relation:
Time to overflow =No. of counts* Time of one count

11. TIMERS

1.
-
2.
-
We determine the operating frequency by:
Choosing the clock that operates the counter.
This clock can be the bus clock that operates the MCU, it can
be obtained also from another timer’s overflow.
Dividing the frequency before operating the Timer.
This is done by using a Prescaler. In this case, the operating
frequency will be:
Fosc/ 4
prescalar

12. BASIC BLOCK DIAGRAM
Fosc/4
8-bit Prescalar
3 (selection lines)
Mux
Timer

13. HOW DO WE USE TIMERS?
Suppose that we want an n-bit counter to overflows every
seconds.
and the operating frequency of the MCU is Fosc:
1.
Determine the number of counts by the relation:

4
Time to overflow =No. of counts* Time of one count
4  prescalar
Time  No.ofcounts (
)
Fosc
change the number of counts, and the prescalar value until you get
the required time.

Time  Fosc
No.ofcounts 
4  prescalar

14. TIMERS
-
According to the required number of counts, and the timer’s
number of bits, determine the initial value:
2 n - No. of counts
Initial value=

15. TIMERS
-
Or, we can use the compare mode of the CCP module.

16. Determine time to
overflow
Set Fosc of the
MCU
Set prescalar value
Get the number of
counts
F
Counts< 2 n - 1
T
Get initial value

17. TMR0



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8-bit timer/counter.
Readable and writable.
8-bit software programmable prescaler.
Clock source selectable to be external or internal
To use TMR0, we can use the ins. clock (Fosc/4), or an external
clock.
 Edge select for external clock.
 Interrupt on overflow from FFh to 00h.

18. EXAMPLE ON TMR0
Make TMR0 overflow and generate an interrupt every 4 sec.
when using a 32.768 KHZ oscillator.
-
1.
4 sec.  32 .768 KHZ
No.ofcounts 
4  prescalar
Try first the no. of counts
2 n (256) , you’ll get: prescalar=128.

19. EXAMPLE

Generate a 1HZ square wave only using TMR0.
-
Note that this example can be used to check if the timer
operates as required or not when using it in any application.

20. 7-SEGMENT DISPLAY

21. APPLICATION: [0->99] COUNTER

22. [0->99] COUNTER
The program is divided into 2 parts:
1- Updating the counter every 1 second.
2- Switching the operating 7-segment every 1/60 second.

23. REAL TIME SYSTEMS

-
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Very important:
The time of execution of the ISR associated with the timer’s
interrupt, must be less than the time that the timer takes to
overflow.
If it takes more time, optimize the code or use higher
oscillator frequency.

24. ASSIGNMENT

-
Generate a 100KHZ square wave only using TMR0.
Choose the suitable clock frequency and prescalar value.