1. 8051 Timers Timers are the peripherals of the 8051 Microcontroller.
Timers are peripherals in any of the controllers.
This peripheral is mainly used in time based applications.
A timer is a specialized type of clock. It can be used to control the sequence of an event or process.
The 8051 comes equipped with two timers (Timer 0 and Timer 1).
These timers are 16 bit registers.

2. General Functions of the Timer
Keeping time and/or calculating the amount of time between the events.
Counting the Events themselves, or
Generating baud rate for the serial port.
Note:
While generating delay this circuitry worked as Timer.
While counting the number of events occurred at the particular time this circuitry worked as counter.

3. Baud Rate
A number related to the speed of data transmission in a system.
The rate indicates the number of electrical oscillations per second that occurs within a data transmission.
The higher the baud rate, the more bits per second that are transferred.

4. For Timer: (Internal Pulses Working)
Crystal
Oscillator
/ 12
Circuitry
TIMER “X”
MHz
f = KHz
Timer will get clock frequency from the crystal oscillator that is MHz.
This frequency is given as input the circuitry that is called /12 (Divide by 12) circuitry. Because one machine cycle is nothing but 12 clock cycles.

5. For Counter : (External Pulses Working)
Timer 0 T1
Timer 1
T0 and T1 is the external input. So counter will get external input from the T0 and T1

6. SFRs relating to Timers
SFR Name
Description
SFR Address
TH0
TIMER 0 Higher byte
8Ch
TL0
TIMER 0 Lower byte
8Ah
TH1
TIMER 1 Higher byte
8Dh
TL1
TIMER 1 Lower byte
8Bh
TCON
TIMER Control
88h
TMOD
TIMER Mode
89h

7. TMOD Register If we program lower nibble, Timer 0 will get affected.
Gate
C/T M1 M0
Timer 1
Timer 0
If we program lower nibble, Timer 0 will get affected.
If we program higher nibble, Timer 1 will get affected.

8. Gate:
0 – Software Control
1 – H/W Control (Instruction with Interrupt Signal)
C / T:
0 – Timer
1- Counter

9. Mode Bits Information M0 and M1
Description
13 bit Timer 1
16 bit Timer
8 bit Auto Load
Split Timer

10. TCON Register LSB MSB Interrupts Timers TF1 TR1 TF0 TR0 IE1 IT1 IE0

11. External Interrupt 1 Edge Flag IT1 External Interrupt 1 Type Flag IE0
Bit Names
Description
TF1
Timer 1 Overflow Flag
TR1
Timer Run Control bit
TF0
Timer 0 Overflow Flag
TR0
Timer 0 Run Control Bit
IE1
External Interrupt 1 Edge Flag
IT1
External Interrupt 1 Type Flag
IE0
External Interrupt 0 Edge Flag
IT0
External Interrupt 0 Type Flag

12. Hardware Configuration of Timers

13. Timer Mode Explanations
Mode 0 : TLX = 5 bit, THX = 8 bit
Timer Address : 0000H to 1FFFH
TLX
THX
TFX
Interrupt

14. Timer Address : 0000H to FFFFH
Mode 1 : TLX = 8 bit, THX = 8 bit
Timer Address : 0000H to FFFFH
TLX
THX
TFX
Interrupt

15. TLx = 8 bits THx = for storing Value, 00H to FFH
Mode 2 : Auto Reload (It is 8 – bit Mode), Because we can supposed to use only TLX 8 – bit only.
TLx = 8 bits THx = for storing Value, 00H to FFH
We have to initialize the TL register with some value.
TH register is initialized with auto reload value.
Interrupt
TLX
TFX
Reload TLx

16. Mode 3 : TL0 = Timer 0 and TH0 = Timer 1 Timer 1 = 16 bit Timer
Interrupt
TL0
TF0
Interrupt
TH0
TF1

17. Time Delay Calculation
If we want to generate a time delay of 10 microseconds (or) 1 milliseconds.
Step 1: Divide the required time delay by
1.085 Microseconds.
Why Microseconds?
MHz / 12 = 9.216KHz
So, T = 1/f = 1/9.216KHz
= 1.085Microseconds.

18. Time Delay Calculation
Step 2:
(Maximum value of Timer + 1)10 – (n)10= (Z)10.
Maximum value of timer is depending on the
mode of a Timer.
In case of Mode 0 :
Maximum value is = 1FFF.
In case of Mode 1 :
Maximum Value is = FFFF.
Note :
FFFF + 1 =

19. Time Delay Calculation
Step 3: Convert (Z)10 to (YYXX)10
It will be loaded into TH and TL with YY & XX
Respectively.
TH = YY.
TL = XX.

20. Example:
Generate a time a time delay of 250microsecods. Assume Timer mode is 1.
1) (n)10 = 250microsecods/1.085microseconds
= 230microseconds.
2) (z)10 = (FFFF+1) – (n)10
= – 230 =
3) Hexadecimal Value of is FF1AH.
YY = FF
XX = 1A
TH = FFH and TL = 1AH