1. Presented by :
 AKASH GUPTA 111257
 ANKIT SAHA 111340

2.  Introduction
 TMOD Register
 Modes of Operation
 TCON Register
 Counters

4.  The 8051 comes equipped with two timers, both of
which may be controlled, set, read, and configured
individually.
 The 8051 timer has three general functions:
 Keeping time and calculating the amount of time between
events.
 Counting the events .
 Generating baud rates for the serial port.

5. When a timer is in interval timer mode and correctly
configured, it will increment by „ONE‟ every machine cycle.
We have:
1,10,59,000 / 12 = 9,21,583 machine cycles / sec
 The 8051 uses a 11.059 MHz crystal.
 A machine cycle equals 12 clock (crystal) pulses.
Thus the counter increments 9,21,583 times per second.
Thus if a timer has counted from 0 to 50,000 you may calculate:
50,000 / 9,21,583 = .0542 seconds

7.  The 8051 has 2 timers/counters:
◦ Timer/Counter 0
◦ Timer/Counter 1
 Registers Used in the Timer :
◦ Timer 0 registers:
TH0, TL0
Exclusive
◦ Timer 1 registers:
TH1, TL1
◦ TMOD (Timer mode register)
Shared by both
◦ TCON (Timer control register)

8.  Registers THx & TLx
They are 16 bit wide.
 These registers store:
The time delay as a timer.
The number of events as a counter.
 Timer 0: TH0 & TL0
Timer 0 high byte , timer 0 low byte
 Timer 1: TH1 & TL1
Timer 1 high byte, timer 1 low byte

9. D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6
TH0 TL0
D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6
TH1 TL1
Timer 0
Timer 1
Timer Registers

11.  Timer mode register = TMOD
◦ An 8-bit register
 lower 4 bits : Timer 0 Mode setting (0000 : not used)
 upper 4 bits : Timer 1 Mode setting (0000 : not used)
◦ Not bit-addressable

12. GATE C/T M1 M0 GATE C/T M1 M0
Timer 1 Timer 0
(MSB) (LSB)
BIT NAME EXPLANATION OF THE 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 1
4 T1M0 Timer mode bit 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 0
0 T0M0 Timer mode bit 0

13. 0 : Timer operation
(clock : Machine cycle)
1 : Counter operation
(clock : Tx input pin)
GATE C/T M1 M0 GATE C/T M1 M0
Timer 1 Timer 0
(MSB) (LSB)

14.  Every timer has a mean of starting and stopping.
Timer is enabled only while the INT pin is high and TR
control pin(in TCON) is set.
◦ GATE = 0
 Internal control
 The start and stop of the timer are controlled by the software
(Set/clear the TR)
◦ GATE = 1
 External control
 The hardware way of starting and stopping the timer by
software and an external source.

15.  M0 and M1 select the timer mode for timers 0 & 1.
M1 M0 Mode Operating Mode
0 0 0 13-bit timer mode
8-bit THx + 5-bit TLx (x= 0 or 1)
0 1 1 16-bit timer mode
8-bit THx + 8-bit TLx
1 0 2 8-bit auto reload
8-bit auto reload timer/counter;
THx holds a value which is to be reloaded
into TLx each time it overflows.
1 1 3 Split timer mode

17. 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
4
Operating
Modes

18.  This is a relic mode.
◦ Included in 8051 to maintain compatibility with its predecessor
8048.
 The counters are counting up:
◦ TLx will count from 0 to 31.
◦ When TLx is incremented from 31, it will “reset” (overflow) to
0.
◦ Now THx will be incremented.
 Hence effectively only 13 bits are used.
◦ Bits 0-4 of TLx.
◦ Bits 0-7 of THx..

19.  This is the most commonly used mode.
 This mode operates in a fashion almost like the Mode 0, only
this time all 16 bits are used.
 The counting:
◦ TLx is incremented from 0(00h) to 255(FFh).
◦ When TLx is incremented from 255, it resets to 0 and
causes THx to be incremented by 1.
◦ Hence we have a maximum count of „65,025‟ (255*255)
machine cycles.

20. 1. Choose mode 1 timer 0
MOV TMOD,#01H
2. Load registers TL and TH with initial count values
MOV TH0,#FFH
MOV TL0,#FCH
3. Clear the flag TF0
CLR TF0
4. Start the timer.
SETB TR0

21. 5. The 8051 starts to count up by incrementing the TH0-TL0.
◦ TH0-TL0= FFFCH,FFFDH,FFFEH,FFFFH,0000H
FFFC FFFD FFFE FFFF 0000
TF = 0 TF = 0 TF = 0 TF = 0 TF = 1
TH0 TL0Start timer
Stop timer
Monitor TF until TF=1
TR0=1 TR0=0
TF

22. ÷ 12
TR
TH TL TF
Timer
overflow
flag
C/T = 0
TF goes high when FFFF 0
XTAL
oscillator

23.  When a timer is in mode 2, THx holds the "reload value" and
TLx is the timer itself.
 Thus the counting proceeds as:
◦ TLx starts counting up.
◦ TLx reaches 255 and is subsequently incremented.
◦ Now instead of resetting to 0 (as in the case of modes 0
and 1), it will be reset to the value stored in THx.

24. 1. Choose mode 2 timer 0
MOV TMOD,#02H
2. Set the original value to TH0.
MOV TH0,#38H
3. Clear the flag to TF0=0.
CLR TF0
4. After TH0 is loaded with the 8-bit value, the 8051 gives a copy of it
to TL0.
TL0=TH0=38H

25. 5. Start the timer.
SETB TR0
6. The 8051 starts to count up by incrementing the TL0.
TL0= 38H, 39H, 3AH,....
7. When TL0 rolls over from FFH to 00, the 8051 set TF0=1. Also, TL0 is
reloaded automatically with the value kept by the TH0.
TL0= FEH, FFH, 00H (Now TF0=1)
The 8051 auto reload TL0=TH0=38H.
Clr TF0
Go to Step 6 (i.e., TL0 is incrementing continuously).
 Note that we must clear TF0 when TL0 rolls over. Thus, we can monitor
TF0 in next process.
8.Clear TR0 to stop the process.
Clr TR0

26. XTAL
oscillator ÷ 12
TR1
TL1
TH1
TF1 overflow flag
reload
C/T = 0

27.  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 independently.
 What happens to timer1?
◦ 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.

28. A good possible use 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.

30.  Finally, there is one more SFR that controls the two
timers and provides valuable information about them.
 Timer control register: TCON
◦ Upper nibble : TIMER
◦ Lower nibble : INTERRUPTS

31. The Register bits represent the following values :
BIT NAME BIT ADDRESS EXPLANATION OF
THE FUNCTION
TIMER
7 TF1 8Fh
Timer 1 Overflow. This
bit is set by the
microcontroller when
Timer 1 overflows.
1
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.
1
5 TF0 8Dh
Timer 0 Overflow. This
bit is set by the
microcontroller when
Timer 0 overflows.
0
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.
0

32.  TF (timer flag, control flag)
◦ TF0 : timer 0
◦ TF1 : timer 1.
◦ TF is like a carry.
Originally, TF=0.
When TH-TL roll over to 0000
from FFFFH
Then , TF is set to 1
 TF=0:not reached
 TF=1:reached
TR (run control bit)
TR0 : Timer 0
TR1: Timer 1
Turn timer - ON or OFF
TR = 0 : OFF (stop)
TR = 1 : ON (start)
TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0
Timer 1 Timer0 for Interrupt
(MSB) (LSB)

33. This SFR is “Bit-Addressable”
What does this mean ?
It means that if you want to set the bit TF1
(which is the highest bit of TCON)
you could execute the command :
SETB TF1
Rather than executing this command :
MOV TCON , #80h
Because it is bit-addressable.
It decreases program execution time.

35.  As far as the use of a timer/counter as an event counter is
concerned ,everything that we have talked about in the last
section also applies to programming it as a counter ,except
the source of the frequency.
 When used as a timer ,the 8051‟s crystal is used as the
source of the frequency.
 However ,when used as a counter ,it is a pulse outside of the
8051 that increments the TH,TL registers.
 These timers can also be used as counters counting
events happening outside the 8051.

36. Pin Port Pin Function Description
14 P3.4 T0 Timer/Counter 0 external input
15 P3.5 T1 Timer/Counter 1 external input
GATE C/T=1 M1 M0 GATE C/T=1 M1 M0
Timer 1 Timer 0
(MSB) (LSB)

37.  16-bit counter (TH0 and TL0)
 TH0-TL0 is incremented when TR0 is set to 1 and an external
pulse (in T0) occurs.
 When the counter (TH0-TL0) reaches its maximum of FFFFH, it
rolls over to 0000, and TF0 is raised.
 Programmers should monitor TF0 continuously and stop the
counter 0.
 Programmers can set the initial value of TH0-TL0 and let
TF0=1 as an indicator to show a special condition. (ex: 100
people have come).

38. Timer 0
external
input Pin
3.4
TR0
TH0 TL0 TF0
TF0 goes high when
FFFF 0
overflow
flag
C/T = 1

39.  8-bit counter.
It allows only values of 00 to FFH to be loaded into TH0
 Auto-reloading
• TL0 is incremented if TR0=1 and external pulse occurs.

40. Thank You!