5aca

7/26/2019 5ACA

1/20

8051 timer

MODE 0

MODE 1

MODE 2

MODE 3

MODE 0

MODE 1

MODE 2

TIMER 0 TIMER 1

EC 6504 MICROPROCESSORS AND MICROCONTROLLERS

UNIT V INTERFACING MICROCONTROLLER

1. 8051 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 !eeping time and"or

calculating the amount of time between events, # $ounting the events themselves, or %

&enerating baud rates for the serial port. one of the primary uses of timers is to measure time. 'e

will discuss this use of timers first and will subsequently discuss the use of timers to count

events. 'hen 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.

KARTHIK. S SSM INSTITUTE OF ENGINEERING AND TECHNOLOGY

7/26/2019 5ACA

2/20


TIME SFR

TMOD REGISTER

8051 has two timers which each function essentially the same way. )ne timer is T*+-0 and the

other is T*+-1. The two timers share two /-s T+) and T$)2 which control the timers,

and each timer also has two /-s dedicated solely to itself T30"T40 and T31"T41.

GATE

!"e# set, timer"counter is enabled, i$ *2T pin is high and T- is set.

!"e# %&e're(, timer"counter is enabled, i$ T- bit set.

C)T*

!"e# set+1,, counter operation input from T input pin.

!"e# %&e'r+0,, timer operation input from internal cloc6.

T"e TMOD -te is #/t -it '((ress'-&e.

007 +) 0

017 +) 1

107 +) #

117 +) %


7/26/2019 5ACA

3/20


TCON REGISTER

TF 1Timer 1 /er$&/2 $&'3

TF0TR1

Timer 1 R# %/#tr/& -it

TR0IE1

I#terrt 1

IE0IT1 Timer 1 i#terrts

IT0

8051 TIMER)COUNTER


7/26/2019 5ACA

4/20


TIMER 0

MODE 0

17-it Time M/(e +m/(e 0,

Timer mode (0( is a 1%7bit timer. This is a relic that was 6ept around in the 8051 to maintain

compatibility with its predecessor, the 808. &enerally the 1%7bit timer mode is not used in new

development. 'hen the timer is in 1%7bit mode, T4 will count from 0 to %1. 'hen T4 is

incremented. *t will (reset( to 0 and increment T3. Thus, effectively, only 1% bits of the two

timer bytes are being used: bits 07 of T4 and bits 079 of T3. This also means, in essence, the

timer can only contain 81# values. *f you set a 1%7bit timer to 0, it will overflow bac6 to ;ero

81# machine cycles later.


7/26/2019 5ACA

5/20


16-it Time M/(e +m/(e 1,

Timer mode (1( is a 1

7/26/2019 5ACA

6/20


8-it Time M/(e +m/(e ,

Timer mode (#( is an 87bit auto7reload mode. 'hat is that, you may as6> imple. 'hen a timer

is in mode #, T3 holds the (reload value( and T4 is the timer itself. Thus, T4 starts counting

up. 'hen T4 reaches #55 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 T3.


7/26/2019 5ACA

7/20


8.5 S&it Timer M/(e +m/(e 7,

Timer mode (%( is a split7timer mode. 'hen Timer 0 is placed in mode %, it essentially becomes

two separate 87bit timers. That is to say, Timer 0 is T40 and Timer 1 is T30. ?oth timers count

from 0 to #55 and overflow bac6 to 0. @ll the bits that are related to Timer 1 will now be tied to

T30.

'hile Timer 0 is in split mode, the real Timer 1 i.e. T31 and T41 can be put into modes 0, 1 or

# normally77however, you may not start or stop the real timer 1 since the bits that do that are now

lin6ed to T30. The real timer 1, in this case, will be incremented every machine cycle no matter

what.

. PROGRAMS IN 8051 TIMERS

Timer +ode 1:

*n following, we all use timer 0 as an eample.

16-ittimer T30 and T40

T307T40 is incremented continuously when T-0 is set to 1. @nd the 8051 stops to

increment T307T40 when T-0 is cleared.


7/26/2019 5ACA

8/20


The timer wor6s with the internal system cloc6. *n other words, the timer counts up each

machine cycle. 'hen the timer T307T40 reaches its maimum of ////3, it rolls over to 0000, and

T/0 is raised.

Arogrammer should chec6 T/0 and stop the timer 0.

teps of +ode 1 1"%:

1. $hoose mode 1 timer 0

B MOV TMOD9:01;

#. et the original value to T30 and T40.

B MOV T;09:FF;

B MOV TL09:FC;

%. Cou had better to clear the flag to monitor: T/0D0.

B CLR TF0

. tart the timer.

B SET< TR0

teps of +ode 1 #"%

5. The 8051 starts to count up by incrementing the T307T40.

B T;0TL0= FFFC;9FFFD;9FFFE;9FFFF;90000;


7/26/2019 5ACA

9/20


teps of +ode 1 %"%

7/26/2019 5ACA

10/20


EB'm&e 1

EB'm&e

3e#er'te (e&' si#3 timer 0

DELA

SET< TR0 Jstart the timer 0

AGAIN>N< TF09AGAIN

CLR TR0 Jstop timer 0

CLR TF0 Jclear timer 0 flag

RET

S/&ti/#


7/26/2019 5ACA

11/20


*n the above program notice the following steps.

1. T+) D 0000 0001 is loaded.

#. FFF; is loaded into T30 B T40.

%. A1.5 is toggled for the high and low portions of the pulse.

. The 4@C subroutine using the timer is called.

5. *n the 4@C subroutine, timer 0 is started by the KSET< TR0L instruction.

7/26/2019 5ACA

12/20


7/26/2019 5ACA

13/20


E @synchronous serial data communication is widely used for character7oriented transmissions

E The start bit is always a 0 low and the stop bits is 1 high

D't' Tr'#s$er R'te

E The rate of data transfer in serial data communication is stated in -s -its er se%/#(.

E @nother widely used terminology for bps is -'( r'te.

B *t is modem terminology and is defined as t"e #m-er /$ si3#'& %"'#3es er se%/#(

8051 Seri'& P/rt

E ynchronous and @synchronous

E $)2 -egister is used to $ontrolE ata Transfer through TFd O -Fd pins

E ome time 7 $loc6 through TFd Ain

/our +odes of )peration:


7/26/2019 5ACA

14/20


M/(e 0 S#%"r/#/s Seri'& C/mm#i%'ti/#

M/(e 1 8

7/26/2019 5ACA

15/20


8051

Seri'& P/rt M/(e 0

The erial Aort in +ode70 has the following features:

1. erial data e#ter '#( eBits t"r/3" R?D

#. T?D outputs the %&/%

%. 8 bits are transmitted " received

. The baud rate is fied at 1"1# of the oscillator frequency

8051 Seri'& P/rt M/(e 1

The erial Aort in +ode71 has the following features:

1. erial data e#ters t"r/3" R?D

#. erial data eBits t"r/3" T?D

%. )n receive, the stop bit goes into -?8 in $)2

4. 10 -its are transmitted " received

1. Start bit (0)

2. Data bits (8)

3. Stop Bit (1)

5. ?aud rate is determined by the Timer 1 over flow rate

8051 Seri'& P/rt M/(e

The erial Aort in +ode7# has the following features:

1. erial data e#ters t"r/3" R?D


7/26/2019 5ACA

16/20


#. erial data eBits t"r/3" T?D

%. th data bit T

7/26/2019 5ACA

17/20



7/26/2019 5ACA

18/20


Pr/3r'm +Tr'#s$er,


7/26/2019 5ACA

19/20


Pr/3r'm +Re%eie,

!rite ' r/3r'm $/r t"e 8051 t/ tr'#s$er &etter HA seri'&& 't 4800 -'( r'te9 %/#ti#/s&.


7/26/2019 5ACA

20/20


Pr/3r'm t"e 8051 t/ re%eie -tes /$ ('t' seri'&&9 '#( t t"em i# P1. Set t"e -'( r'te 't

4800.


5aca

Documents