The document describes an 8051-based digital clock application that uses timers and interrupts. It configures Timer 0 to generate interrupts every 50ms by overflowing every 46080 cycles. Timer 1 is used to generate a 19200 baud rate for a serial port. On each Timer 0 interrupt, the clock display is updated. By generating interrupts every 50ms and counting 20 of them for 1 second, the timer configuration provides an approximate 1 second timer without exceeding the 8051's 65536 cycle limit.

1. 8051 Timers / Counters
José Borges ,Patrício Lima, Marcos Brito, Marek Mastyło, Jakub Nyk
GROUP 4
DEEC –MPIN
FEUP
April, 2010
April 10

2. Outlines
1. Introduction
2. 8051 Timer/Counter
3. Operating Modes
4. Timer Vs Counter
5. T/C and Interruptions
6. Application Examples
7. Conclusion
April 10 2

3. Introduction
 General Function:
• Calculating the amounts of time between
events
• Counting events
• Generating baud rate for serial port
April 10 3

4. Introduction
 Applications:
• Communication
̵ Generating rectangular pulses (signal
modulation)
̵ Watchdog timers
• Manufacturing Industry
- Counting objects
- Measuring intervals
• Etc
April 10 4

5. Introduction
 8051 contains two 16-bits timers
• T0
• T1
 Two different types of timer:
• Interval timer
• Counter
 Maximum value is 65536
 Initial state can be set by user
April 10 5

6. Timer/Counter
Special Function Registers
 Timers/Counters can be operated by user with
special function registers
 T0 and T1 share two SFRs: TMOD and TCON
 Each timer has also two registers dedicated to
itself: TH0/TL0 and TH1/TL1
April 10 6

7. Timer/Counter
Special Function Registers-TMOD
 TMOD (Timer Mode Register) is a non-bit-
addressable, 8-bit register:
Reference:
http://fivedots.coe.psu.ac.th/~cj/asm/slides/mcs51/timer2.pdf
April 10 7

8. Timer/Counter
Special Function Registers-TMOD
 Lower 4 bits are for Timer0
 Upper 4 bits are for Timer1
 GATE bit is used for choice of internal or
external control:
• GATE=0 is for internal control, start and stop are
controlled by software
• GATE=1 is for external control, start and stop are
controlled by software and and external source
 C/T bit decides about timer type: interval timer
or counter
April 10 8

9. Timer/Counter
Special Function Registers-TMOD
 M0 and M1 bits are used to set timer
mode (the same for Timer0 and Timer1)
 8051 delivers 4 timer modes:
M1 M0 Mode Description
0 0 Mode 0 13-bit timer
0 1 Mode 1 16-bit timer
1 0 Mode 2 8-bit auto reload
1 1 Mode 3 Split timer mode
April 10 9

10. Timer/Counter
Special Function Registers-TCON
 TCON (Timer Control Register) is a bit-
addressable, 8-bit register where 4 upper bits
are responsible for timers/counters:
Reference:
http://fivedots.coe.psu.ac.th/~cj/asm/slides/mcs51/timer2.pdf
April 10 10

11. Timer/Counter
Special Function Registers-TCON
 TR0 and TR1 are set by user to turn on (or turn
off) Timer0 or Timer1:
• TR=0 – turn off
• TR=1 – turn on
 TF0 and TF1 are Timer Flags informing about
overflow (then TF=1 and interrupt could be
activate if it’s set, should be cleaned)
April 10 11

12. Timer/Counter
Special Function Registers-TCON
 Equivalent instructions for TCON:
• Timer0:
- SETB TR0 = SETB TCON.4
- CLR TR0 = CLR TCON.4
- SETB TF0 = SETB TCON.5
- CLR TF0 = CLR TCON.5
• Timer1
- SETB TR1 = SETB TCON.6
- CLR TR1 = CLR TCON.6
- SETB TF1 = SETB TCON.7
- CLR TF1 = CLR TCON.7
April 10 12

13. Timer/Counter
Special Function Registers-TL/TH
 TH0 and TL0 are upper and lower registers of Timer0
 TH1 and TL1 are upper and lower registers of Timer1
 They help to set initial value of timer/counter
April 10 13

14. Timer Vs Counter
Differences
 Timer
• Counts machine cycles
 Counters:
• Counts events as a result of falling slope of external
input signal put on a pin
 Timer mode and counter mode are relative to
machine cycle
April 10 14

15. Timer Vs Counter
Differences
 Timer
• Input from internal system clock
 Counters:
• Show the number of events on registers
• External input from T0 input pin (P3.4) for Counter
0
• External input from T1 input pin (P3.5) for Counter
1
• External input from Tx input pin.
• We use Tx to denote T0 or T1
April 10 15

16. Timer Modes
Mode 0
 Mode 0 is identical for Timer0 and Timer1
 Timers work as 13-bit counters, an interrupt is
generated when counter overflows. It takes
8192 input pulses to generate the next
interrupt
 Timers use 8 bits of THi and 5 lower bits of TLi
 After timer overflows TFi (Timer Flag in TCON)
is set, an interrupt occurs
Where i=0,1
April 10 16

17. Timer Modes
Mode 0
 Structure of Timer1 in mode 0:
Reference:
Tomasz Starecki; Mikrokontrolery 8051 w praktyce, Twarda
oprawa, 2002. ISBN: 83-910067-4-3, Pic. 3.1
April 10 17

18. Timer Modes
Mode 1
 Similar to mode 0
 Timers use 8 bits of THi and 8 bits of TLi
 Timer is a 16-bit counter, it takes 65536 input
pulses to generate the next interrupt
 Improved capacity
April 10 18

19. Timer Modes
Mode 1
 Structure of Timer0 in mode 1:
Reference:
Tomasz Starecki; Mikrokontrolery 8051 w praktyce, Twarda
oprawa, 2002. ISBN: 83-910067-4-3, Pic. 3.1
April 10 19

20. Timer Modes
Mode 2
 Timers are 8-bit auto reload
 Timer is operated by TLi, when TLi overflows
TFi is set
 TLi is auto reloaded with Thi value when
overflows
 THi is never modified when TLi overflows
April 10 20

21. Timer Modes
Mode 2
 Structure of Timer1 in mode 2:
Reference:
Tomasz Starecki; Mikrokontrolery 8051 w praktyce, Twarda
oprawa, 2002. ISBN: 83-910067-4-3, Pic. 3.1
April 10 21

22. Timer Modes
Mode 3
 Split-timer mode
 Timer1 can be put in other modes
 Timer0 operates TL0 and TH0 as two separate 8-bit
timers/counters
 TL0 works as a 8-bit timer/counter
 TH0 is a 8-bit timer which counts machine cycles
 Timer0 is operated with TF0 and TR0, TF1 and TR1
are not used
April 10 22

23. Timer Modes
Mode 3
 Structure of Timer1 in mode 3:
Reference:
Tomasz Starecki; Mikrokontrolery 8051 w praktyce, Twarda
oprawa, 2002. ISBN: 83-910067-4-3, Pic. 3.1
April 10 23

24. Timer Modes
Example
 Choose mode 1 for Timer:
 MOV TMOD,#01H
 Set the value of TH0 and TL0:
 MOV TH0,#FFH
 MOV TL0,#FCH
 Clear Timer flag and start the timer:
 CLR TF0
 SETB TR0
April 10 24

25. Timer Modes
Example
 The 8051 starts to count up by incrementing the TH0-TL0
 TH0-TL0= FFFCH,FFFDH,FFFEH,FFFFH,0000H
Reference:
http://www.iau-neyshabur.ac.ir/nokhodchian/5-timer(part%201).ppt
April 10 25

26. How does a timer count?
 Timer/Counter counts up
 It is incremented by microcontroller
• Timer is incremented every machine cycle
• Coutner is incremented when event is detected
April 10 26

27. How does a timer count?
 A single machine cycle consists of 12
crystal pulses, thus timer will count:
11 059 000 / 12 = 921 583 per second
April 10 27

28. How does a timer count?
 Example: How many times will the timer be
incremented in 0.05 seconds?
0.05 * 921 583 = 46 079.15 times
 Accuracy is not perfect
April 10 28

29. How does a timer count?
 Timer/Counter counts up
 It is incremented by microcontroller
• Timer is incremented every machine cycle
• Coutner is incremented when event is detected
April 10 29

30. How does a timer count?
 A single machine cycle consists of 12
crystal pulses, thus timer will count:
11 059 000 / 12 = 921 583 per second
April 10 30

31. How does a timer count?
 Example: How many times will the timer be
incremented in 0.05 seconds?
0.05 * 921 583 = 46 079.15 times
 Accuracy is not perfect
April 10 31

32. Timer/ Counter
and Interruptions
 Timer/ Counter can be configured to start an
interruption routine
Reference:
Philips Semiconductors – Family 8051
April 10 32

33. How a timer
interruption occurs
 Some Special Registers are used
• ET0 and ET1 from Interruption Enable
Register
• PT0 and T1 from Interruption Priority
Register
• TF0 and TF1 from T/C Control Register
April 10 April 10 33
33

34. How a timer
interruption occurs
 Timer/counter interruption process :
• ET bit must be set
• TR bit must be set to run the timer
• The interruption is initialized when overflow
occurs
April 10 April 10 34
34

35. How a timer
interruption occurs
 In this point the Timer overflow Flag is cleared
by hardware
Reference:
Philips Semiconductors – Family 8051
April 10 35

36. Software overflow
detection
 How the overflow can be detected if
interruption is not enable?
• In some cases ,it’s not necessary to enable an
interruption
• In this case the overflow is detected by software
• A routine is necessary to check constantly the
overflow occurrence
April 10 36

37. Software overflow
detection
• The overflow occurs when the TF is high
• TF must be cleared to by software to turn ON
the timer
April 10 37

38. Software overflow
detection
 Delay routine example without enable
interruption:
cseg at 0000h
jmp main
…
main:
…
…
setb tr0
acall delay50ms
…
…
delay50ms:
mov TL0,#low(46080)
mov TH0,#high(46080)
clr TF0
jnb TF0,$
ret
April 10 38

39. Application Example
Digital Clock
 Circuit schematic
• Interface with LCD
display and serial port.
April 10 39

40. Application Example
Digital Clock
 Interruption configuration
• IE
mov IE,#10010010b
Or
setb EA
setb ES
setb ET0
April 10 40

41. Application Example
Digital Clock
 Interruption configuration
• TCON
orl TCON,#00101000b
Or
setb TR0
setb TR1
April 10 41

42. Application Example
Digital Clock
 Interruption configuration
• TMOD
mov TMOD,#00100001b
Not bit adressable
April 10 42

43. Application Example
Digital Clock
 Interruption configuration
• SCON
mov SCON,#01110000b
Or
setb SM1
setb SM2
setb REN
April 10 43

44. Application Example
Digital Clock
 Interruption configuration
• Serial port baud rate
mov TH1,#0xFD
• TH1 = TL1=253 mov TH1, TL1
11.0592𝑀𝐻𝑧
𝑏𝑎𝑢𝑑 𝑟𝑎𝑡𝑒 = 2 ∗
32 ∗ 12 ∗ 256 − 253
𝑏𝑎𝑢𝑑 𝑟𝑎𝑡𝑒 = 19200
April 10 44

45. Application Example
Digital Clock
 Interruption configuration
Problem :
• 8051 only count up to 65536
us
How we count 1 second ?
April 10 45

46. Application Example
Digital Clock
 Interruption configuration
TIMER equ 65536-46080
Solution :
mov TL0,#high(TIMER)
• Count 20*50ms mov TH0,#low(TIMER)
12
50000𝑢𝑠 = ∗ 𝐶𝑖𝑐𝑙𝑒𝑠
11.0592𝑀ℎ𝑧
𝑐𝑖𝑐𝑙𝑒𝑠 = 46080
April 10 46

47. Application Example
Digital Clock
 Interruption configuration
mov IE,#10010010b
mov TMOD,#00100001b
orl TCON,#00101000b
mov SCON,#01110000b
mov TH1,#0xFD
mov TH1, TL1
mov TL0,#high(TIMER)
mov TH0,#low(TIMER)
April 10 47

48. Application Example
Digital Clock
DIGITAL CLOCK
VDD
VSS
VEE
RW
RS
D0
D1
D2
D3
D4
D5
D6
D7
E
1
2
3
4
5
6
7
8
9
10
11
12
13
14
mov IE,#10010010b GND
VDD
mov TMOD,#00100001b
RXD
TXD
C1
33p
C2
33p
orl TCON,#00101000b
RTS
CTS
X1
mov
U1
19
XTAL1
SCON,#01110000b
AD[0..7]
11.0592MHz
mov
18
XTAL2 TH1,#0xFD
A[8..15]
30
mov TH1, TL1
ALE
31
EA
9 29
RST PSEN
mov
1
2
P1.0
TL0,#high(TIMER)
P3.0/RXD
10
11
P1.1 P3.1/TXD
3 12
mov TH0,#low(TIMER)
P1.2 P3.2/INT0
4 13
P1.3 P3.3/INT1
5 14
P1.4 P3.4/T0
6 15
P1.5 P3.5/T1
7 16
P1.6 P3.6/WR
8 17
P1.7 P3.7/RD
8051
April 10 48

49. Application Example
Counter w/ Ext. Enabler
+1
0
1
1
0
0
1
0
1
April 10 49

50. Application Example
Leds equ P1
overflow equ R0
Counter w/ Ext. Enabler
cseg at 00h
jmp main
cseg at 1bh Initialization
jmp Timer1 ;Counter/ Timer 1 Int.
cseg at 40h
Timer1:
Counter
inc overflow ;Do a Task Leds = TL1 Interrupt
TF1
reti
main:
Return
mov IE,#88H ;Enables the Timer1
mov TMOD,#0E0h ;Gate =1, Counter selected, Mode 2 Do a Task
mov TH1,#00h ;With 00h it counts 255 times
mov TL1,#00h
setb TR1 ;Begin to count
Led:
mov Leds,TL1 ; shows the value with Leds of the actual count
jmp Led
end
April 10 50

51. Application Example
Digital Clock
 Reference:
• http://www.8052.com/tuttimer.phtml
• http://www.8052.com/tutlcd2.php
• http://www.8051projects.net/lcd-
interfacing/introduction.php
• ISIS Proteus -
http://www.labcenter.co.uk/download/prodemo_d
ownload.cfm#professional
• Philips Semiconductors - 80C51 Family
April 10 51

52. Thank you!
April 10 52