TIMERS AND COUNTERS

INTRODUCTION:-

• In 8051,  2 timers are there  i.e  T0, T1. These  timer are 16 bit registers
• The timer puls run at 1/12th clock cycle.
• The clock frequency for serial communication is 11.0592 M Hz 
•  So a single machine cycle run "1" instruction the timing is =1.0892 us i.e  {12/11.0592 M Hz} 

APPLICATION:-

• Generate a delay 
• Event Counter
• Baud rate for serial communication
• Timers can be used as a timer as well as counter but not at a time
• each timer is a 16 bit wide
• Each timer can split two equal  parts  i.e TL{0/1} and  TH{0/1}

FOR T0:

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

<----------------TH0------------------------------------> <-------------------------TL0------------------------>

FOR T1:

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

 <----------------TH0------------------------------------> <-------------------------TL0------------------------>

                                                                                   

Timer registers:-

• In 8051, six timer registers are there 

1. TMOD Register
2. TCON Register
3. TH0
4. TL0
5. TH1
6. TL1

TMOD REGISTER:-

• TMOD Register is a 8 bit register. 
• Which is a Byte addressable register.
• This register is used for timer mode section , timer  start by what  and finally  timer or counter selection either timer 0 or timer 1

              i.e   We should n't set bit by bit in this register

7 G

6 C/T

5 M1

4 M0

3 G

2 C/T

1 M1

0 M0

                <--------------TIMER1---------------------------> <------------TIMER0---------------------------->

        

GATE (G);-

•    If  G=0, Then Timer start by  software  / Program
•    If   G=1, Then Timer start by Hardware /  External   

COUNTER / TIMER  (C/T);-

• If  (C/T)=0 , i.e  Timer will be selected
• If (C/T)=1, i.e  Counter will be selected

MODE SELECTION BITS (M1,M0):-

M1	M0	MODE	SELECTION
0	0	0	13 BIT TIMER
0	1	1	FULL 16 BIT TIMER
1	0	2	8 BIT AUTO RELOAD TIMER
1	1	3	SPLIT TIMER

TCON:-

• Timer Control register is 8 bit register.
• It is bit addressable register.  i.e  we can change bit by bit
• Timer control register is used to  control the timer i.e we can start and stop by using this register

• TIMER OVER FLOW FLAG [TF]:-

                  This  TIMER OVER FLOW  SET by  hardware  i.e TF=1   When the timer gets overflow.

               

                   FOR TIMER0:-

                       

                                        TF0=1

                    

                    FOR TIMER1:-

           

                                        TF1=1

                 This TIMER OVERFLOW  FLAG is CLEAR by software, when the timer  is overflow.

                              FOR TIMER0:-

                       

                                        TF0=0

                    

                              FOR TIMER1:-

           

                                        TF1=0

• TIMER CONTROL  BIT [ TR  ]:-

                                If  TR bit is SET then the timer will start.

                                           

                              FOR TIMER0:-

                       

                                        TR0=1

                    

                              FOR TIMER1:-

           

                                        TR1=1

     

                              If TR bit  is CLEAR then the timer will stop.

                                  

                              FOR TIMER0:-

                       

                                        TR0=0

                    

                              FOR TIMER1:-

           

                                        TR1=0

• EXTERNAL INTERRUPT  [ IE ]:-

                           External interrupt edge flag is  set / clear by hardware  when interrupt is processed.

• TIMER INTERRUPT [ IT ]:-

                            These are timer interrupt flags. These bare  SET for negative  edge trigger/  CLEAR  for level trigger