Module-03 Timers and serial port communication

Module-03
Timers and Counters in 8051

Introduction
The 8051 microcontroller has two 16-bit timer/counter registers, Timer 0
(P3.4) and Timer 1(P3.5). They can be used either as Timers to generate a time
delay or as Event counters to count events happening outside the
microcontroller.
Application of Timers & Counters
•Timers:
• Generating accurate delays
• PWM signal generation
• Event scheduling
•Counters:
• Counting external pulses (e.g., sensor triggers)
• Measuring the frequency of external signals
• Counting events in automation systems

Real-Time Application of Timer (7 Seg Display)

Real-Time Application of Counters (Heart
rate monitoring)

Timer 0 and Timer 1
Timer 0 and Timer 1 are 16-bits wide. Since 8051 has an 8-bit architecture, each
16-bit timer is accessed as two separate registers of low byte and high byte.
The low-byte register is called TL0/TL1 and The high-byte register is called
TH0/TH1. They can be accessed like any other register.

Differences b/w Timers and counters

Timer/Counter Registers
Timer/Counter Registers
•TMOD (Timer Mode Register): Configures the mode and operation of Timer 0
and Timer 1.
•TCON (Timer Control Register): Contains control flags for starting, stopping,
and checking overflow.
•TH & TL (Timer High and Timer Low Registers): Store the timer/counter
values.

Module-03 Timers and serial port communication

Timer Control (TCON) Register
• TCON register is also one of the registers whose bits are directly in
control of timer operation.
• Only 4 bits of this register are used for this purpose, while rest of them is
used for interrupt control to be discussed later.

TF1 bit is automatically set on the Timer 1 overflow.
TR1 bit enables the Timer 1.
• 1 – Timer 1 is enabled.
• 0 – Timer 1 is disabled.
TF0 bit is automatically set on the Timer 0 overflow.
TR0 bit enables the timer 0.
• 1 – Timer 0 is enabled.
• 0 – Timer 0 is disabled.
Timer Control (TCON) Register

TMOD Register (Timer Mode)
• The TMOD register selects the operational mode of the timers
T0 and T1.
• The low 4 bits (bit0 – bit3) refer to the timer 0, while the high
4 bits (bit4 – bit7) refer to the timer 1.

• GATE1 enables and disables Timer 1 by means of a signal
brought to the INT1 pin (P3.3):
• 1 – Timer 1 operates only if the INT1 bit is set.
• 0 – Timer 1 operates regardless of the logic state of the
INT1 bit.
• C/T1 selects pulses to be counted up by the timer/counter 1:
• 1 – Timer counts pulses brought to the T1 pin (P3.5).
• 0 – Timer counts pulses from internal oscillator.
• T1M1, T1M0 These two bits select the operational mode of the
Timer 1.

GATE0 enables and disables Timer 0 using a signal brought to
the INT0 pin (P3.2):
• 1 – Timer 0 operates only if the INT0 bit is set.
• 0 – Timer 0 operates regardless of the logic state of the
INT0 bit.
• C/T0 selects pulses to be counted up by the timer/counter 0:
• 1 – Timer counts pulses brought to the T0 pin (P3.4).
• 0 – Timer counts pulses from internal oscillator.
• T0M1, T0M0 These two bits select the operational mode of the
Timer 0.

Summary of Modes in the Timers

How to load the Timer registers with the required time delay
Mode 1 (16 bit timer/counter)
Step:1
Value to be loaded in the Timer = FFFF – (delay Value) + 1
Step:2
load into THx and TLx register
Mode 0 (13 bit timer/counter)
Step:1
Value to be loaded in Timer = 1FFF – (Delay Value) +1
Step:2
load into THx and TLx register

Steps for programming timers in 8051
Load the TMOD value register indicating which timer (0 or 1) is to be used
and which timer mode is selected.
Load registers TL and TH with initial count values.
Start the timer by the instruction “SETB TR0” for timer 0 and “SETB TR1
for timer 1.
Keep monitoring the timer flag (TF) with the “JNB TFx, target” instruction to
see if it is raised.
Stop the timer with the instructions “CLR TR0” or “CLR TR1”, for timer 0 and
timer 1, respectively.
Clear the TF flag for the next round with the instruction “CLR TF0” or “CLR
TF1”, for timer 0 and timer 1, respectively.
Go back to step 2 to load TH and TL again.

Data Types and Time Delay
in the 8051 using C

Data Types and Time Delay in the 8051 using C
Why Program the 8051 using C Language???
Compilers produce hex files that is downloaded to ROM of
microcontroller
The size of hex file is the main concern
Microcontrollers have limited on-chip ROM
Code space for 8051 is limited to 64K bytes
C programming is less time consuming, but has larger hex file size
 The reasons for writing programs in C
 It is easier and less time consuming to write in C than Assembly
 C is easier to modify and update
 We can use code available in function libraries
 C code is portable to other microcontroller with little modification

 A good understanding of C data types for 8051 can help programmers
to create smaller hex files
 Unsigned char
 Signed char
 Unsigned int
 Signed int
 Sbit (single bit)
 Bit and sfr

Module-03 Timers and serial port communication

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