1. Introduction to Micro Controllers & Embedded System Design Timer Operation
Department of Electrical & Computer Engineering Missouri University of Science & Technology
A.R. Hurson

2. We begin with a general view of timers as they are common in microprocessor/microcontroller. Then we will focus on 8051 on-chip timers.
A.R. Hurson

3. A timer is a series of divide-by-two flip flops that receive an input signal as a clock source. The clock is applied to the first flip flop which divides the clock frequency by two.
The output of the first flip flop clocks the second one, which also divides the input by two, and so on.
A.R. Hurson

4. LSB MSB Clock Q0 Q1 Q2 Flag Q0 D Clock Q1 Q2 Q + 5V
Clock Q1 Q2 Q
LSB
MSB
+ 5V
Clock Q0 Q1 Q2
Flag
Flag is set on 7-to-0
timer overflow
A.R. Hurson

5. So a timer with n stages divides the input clock frequency by 2n.
It is clear that the 1st stage (Q0) toggles at ½ the clock frequency, the 2nd stage at ¼ the clock frequency, and so on.
So a timer with n stages divides the input clock frequency by 2n.
The output of the last stage clocks a timer overflow flip flop, or flag, which is tested by software or generates an interrupt.
A.R. Hurson

6. The binary value in the timer flip flops can be thought of as a “counter” of the number of clock pulses since the timer was started.
A 16-bit timer would count from 0000H to FFFFH. The overflow flag is set on the FFFFH-to-0000H of the count.
A.R. Hurson

7. In 8051, there are 2 16-bit timers with four modes of operations.
A third 16-bit timer with three modes of operations is added in 8052.
Timers are used for:
Internal timing,
Event counting, or
Baud rate generation for the built-in serial port.
A.R. Hurson

8. In internal timing applications, a timer is programmed to overflow at a regular interval and set the timing overflow flag.
The flag is then used to synchronize the program to perform an action such as checking the state of inputs or setting data to outputs.
Other applications can use the regular clocking of timer to measure the elapse time between two conditions.
A.R. Hurson

9. Event counting is used to determine the number of occurrences of an event rather than to measure the elapse time between events.
An event is any external stimulus that provide a 1-to-0 transition on a pin on the 8051 IC.
The timer can also provide the baud rate clock for 8051’s internal serial port.
A.R. Hurson

10. The 8051 timers are accessed using six special function registers:
Timer SFR
Purpose
Address
Bit addressable
TCON
Control
88H
Yes
TMOD
Mode
89H No
TL0
Timer 0 low-byte
8AH
TL1
Timer 1 low-byte
8BH
TH0
Timer 0 high-byte
8CH
TH1
Timer 1 high-byte
8DH
A.R. Hurson

11. Byte address Byte address 90 8D 8C 8B 8A 89 88 87 83 82 81 80 P1 TH1
TL1
TL0
TMOD
TCON
PCON
DPH
DDL SP P0 FF F0 E0 D0 B8 B0 A8 A0 99 98
Byte address B
ACC
PSW IP P3 IE P2
SBUF
SCON
A.R. Hurson

12. Timer MODe register:
It contains two groups of four bits that set the operation mode for Timer 0 and Timer 1.
Bit
Name
Timer
Semantic 7
Gate 1 6
Counter/timer select bit:
1 = event counter
0 = interval timer 5 M1
Mode bit 1 4 M0
Mode bit 0 3
Timer 0 gate bit 2
Timer0 Counter/timer select bit
Timer 0 mode bit 1
Timer 0 mode bit 0
A.R. Hurson

13. Timer MODe register: Timer modes M1 M0 Mode Semantic
13-bit timer mode (8048 model) 1
16-bit timer mode 2
8-bit auto-reload mode 3
Split timer mode:
Timer 0: TL0 is an 8-bit timer controlled by timer mode bits; TH0, the same except controlled by timer 1 mode bits
Timer 1: stopped
A.R. Hurson

14. Timer CONtrol register:
It contains the status and control bits for timer 0 and timer 1. The upper four bits in TCON (i.e., TCON.4- TCON.7) are used to turn the timers on and off (TR0, TR1), or to signal a timer overflow (TF0,TF1). The lower 4 bits (i.e., TCON.0-TCON.3) have nothing to do with the timers. They are used to detect and initiate external interrupts.
A.R. Hurson

15. Timer CONtrol register:
Bit
Symbol
Address
Semantic
TCON.7
TF1
8FH
Timer1 overflow flag. Set by hardware upon overflow; cleared by software or hardware when processor vectors to interrupt service routine
TCON.6
TR1
8EH
Timer1 run-control bit. Set/cleared by software to turn timer on/off
TCON.5
TF0
8DH
Timer0 overflow flag
TCON.4
8CH
Timer0 run-control bit
A.R. Hurson

16. Timer CONtrol register:
Bit
Symbol
Address
Semantic
TCON.3
IE1
8BH
TCON.2
IT1
8AH
External interrupt1 type flag. Set/cleared by software
TCON.1
IE0
89H
External interrupt0 flag.
TCON.0
IT0
88H
External interrupt0 type flag
A.R. Hurson

17. Timer Modes and overflow flag
13-bit Timer mode (Mode 0): This is a 13-bit timer mode that provides compatibility with predecessor (i.e., 8048). The upper three bits of TLx are not used.
Timer clock
TLx (5 bits)
THx (8 bits)
Overflow flag
TFx
A.R. Hurson

18. Timer Modes and overflow flag
16-bit Timer mode (Mode 1): This is a 16-bit timer mode and similar to mode0. As the clock pulses are received, the timer counts up: i. e., 0000H,0001H, 0002H, …. An overflow occurs on the transition from FFFFH-0000H counts that sets the timer overflow flag.
The timer continues to count.
The overflow flag is the TFx bit in TCON that is read or written by software.
A.R. Hurson

19. Timer Modes and overflow flag: Mode 1
The MSB of the value in the timer register is THx bit 7 and LSB is TLx bit 0. The LSB toggles at the input clock frequency divided by 2 and the MSB toggles at the input clock frequency divided by 216 = 65,536.
The timer registers (TLx/THx) may be read or written at any time by the software.
Timer clock
TLx (8 bits)
THx (8 bits)
Overflow flag
TFx
A.R. Hurson

20. Timer Modes and overflow flag
8-bit Auto-reload mode (Mode 2): In this mode, the timer low- byte (TLx) operates as an 8-bit timer while the timer high-byte (THx) holds a reload value.
When the counts overflows from FFH, not only the timer flag is set, but also the value in THx is loaded into TLx. If THx contains 4FH, the timer counts continuously from 4FH to FFH.
A.R. Hurson

21. Timer Modes and overflow flag: Mode2
Timer clock
TLx (8 bits)
Overflow flag
THx (8 bits)
TFx
Reload
A.R. Hurson

22. Timer Modes and overflow flag
Split timer mode (Mode 3): This mode is different for each timer:
Timer 0 in mode 3 is split into two 8-bit timers. TL0 and TH0 acting as separate timers with overflows setting TF0 and TF1, respectively.
Timer 1 stopped in mode 3, but can be started by switching it into one of the other modes.
A.R. Hurson

23. Timer Modes and overflow flag: Mode 3
Mode 3 essentially provides an extra 8-bit timer: The 8o51 appears to have a third timer. When timer 0 is in mode 3, Timer 1 can be turned on and off by switching it out of and into its own mode 3. It can be used by the serial port as a baud rate generator, or it can be used in any way not requiring interrupts (since it is no longer connected to TF1).
A.R. Hurson

24. Timer Modes and overflow flag: Mode 3
Timer clock
TLx (8 bits)
THx (8 bits)
Timer clock
TL0 (8 bits)
Overflow flag
TF0
Fosc/12
TH0 (8 bits)
Overflow flag
TF1
A.R. Hurson

25. A.R. Hurson

26. A.R. Hurson

27. A.R. Hurson

28. Bit number Function Pin Number Remarks P3.0 RxD 10
Serial communication
P3.1
TxD 11
P3.2 12
External Interrupt
P3.3 13
External interrupt
P3.4 T0 14
Timer 0
P3.5 T1 15
Timer 1
P3.6 16
External Memory
P3.7 17
A.R. Hurson

29. Clocking Sources  12 Oscillator  T0 orT1 0 = interval Timing
T0 orT1
0 = interval Timing
1 = Event Counting
A.R. Hurson

30. Starting/Stopping and Controlling the Timers
The simplest way to staring and stopping the timers is with the run control bit (i.e., TRx) in TCON register.
TRx is cleared after a system reset, therefore the timers are disabled (stopped) by default. TRx is set by the program to start the timers.
A.R. Hurson

31. Starting/Stopping and Controlling the Timers
SETB TR0 ; Starts Timer 0
CLR TR0 ; Stops Timer 0 
0 = Timer stopped
1 = Timer started
Timer Clock
A.R. Hurson

32. A.R. Hurson

33. A.R. Hurson

34.  Starting/Stopping and Controlling the Timers )
Use the following diagram, tabulate the bit and byte addresses for timer registers and control bits.
Oscillator
 12 
T1 (P3.5)
TL1
TH1
TF1
TR1
GATE   )
A.R. Hurson

35. A.R. Hurson

36. Initializing and Accessing Timer Registers
Timers are usually initialized once at the beginning of a program to set the correct operating mode. During the course of the program, the timers are started, stopped, flag bits tested, cleared, and so on, as required by the application.
A.R. Hurson

37. Initializing and Accessing Timer Registers
TMOD is the first register to be initialized, since it sets the mode of operation.
Of course Timer 1 is not activated until its run control bit (TR1) is set.
If an initial count is necessary, the timer register TL1/TH1 must also be initialized.
Note: The counters count up and set the carry flag on an FFFFH-to- 0000H transition.
A.R. Hurson

38. MOV TL1, #9CH ; These two instructions initializes
MOV TH1, #0FFH ; the timer by _100H
SETB TR1 ; Starts the timer and as the result
; the overflow flag (TF1) is set 100
; sec later. During this time the
; program remain in a “wait loop”
WAIT: JNB TF1, WAIT
; when the timer overflows, we need
; to stop the timer and clear the
; overflow flag
CLR TR1
CLR TF1
A.R. Hurson

39. Example: Write a program that creates a periodic waveform on P1.0
Duration of this loop is 4 machine cycle. A 12 MHz crystal results in a 1 sec clock. So the duration of this loop is 4 sec. Adding NOPs in the loop allows us to extend this.
ORG 8100
8100 D290 LOOP: SETB P1.0 ; one machine cycle
C290 CLR P1.0 ; one machine cycle
80FA SJMP LOOP ; two machine cycle
END
A.R. Hurson

40. Short intervals and Long intervals
A 12 MHz crystal results in an on-chip clock of 1 MHz. The short interval is limited by the software not the timer clock and this is the duration of instructions. The shortest instruction in requires one machine cycle or 1 micro second.
A.R. Hurson

41. Maximum interval (sec)
Short intervals and Long intervals
The following table summarizes the techniques for creating intervals of various length.
Maximum interval (sec)
Technique
 10
software
256
8-bit timer with auto-reload
65536
16-bit timer
No limit
16-bit timer plus software loops
A.R. Hurson

42. Example: Write a program using Timer 0 to create a 10KHz square wave on P1.0
ORG 8100
MOV TMOD, #02H ; 8-bit auto-reload mode
CCE MOV TH0, #-50 ; reload value to TH0
8106 D28C SETB TR0 ; Start timer
DFD LOOP: JNB TF0, LOOP ; Wait for overflow
810B C28D CLR TF0
810D B290 CPL P1.0
810F 80F7 SJMP LOOP
END
A.R. Hurson