SE PAI Unit 5_Timer Programming in 8051 microcontroller_Part 1

1
Unit 5: 8051 Timers/Counters Programming
(Part 1)
Subject: Processor Architecture & Interfacing
(SPPU, Pune 2015 Course of Information Technology)
Class: SEIT Semester: II
Prepared By,
Ms. K. D. Patil, Assistant Professor
Department of Information Technology
(NBA accredited)
Sanjivani College of Engineering, Kopargaon-423603.
Maharashtra, India.
(An Autonomous Institute, Affiliated with SPPU, Pune.)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 certified

Contents
Prepared By: Ms. K. D. Patil, Dept. of Information
Technology, Sanjivani COE, Kopargaon2
 Part 1
 Introduction
 Special Function Registers
 Timer 0 Register
 TMOD Register
 TCON Register
 How to UseTimer
 Operating Modes ofTimer
 Part 2
 Timer Programming
 Mode 1 Programming
 Mode 2 Programming
 Introduction to Counters
 Counter Programming

Learning Outcomes
 Student will be able to
 IntroduceTimers/Counters of 8051 microcontroller
 Operating modes of 8051Timers
 Special Function Registers and configurations of 8051
Timers/Counters
 Apply knowledge of 8051 assembly language to write a code for
8051Timer

Introduction to Timers
 The 8051 has two timers/counters.
 Timer 0
 Timer 1
 They can be used
 as Timers to generate a time delay or
 as Event counters to count events happening outside the
microcontroller
 BothTimer 0 andTimer 1 are 16 bits wide
 Since 8051 has an 8-bit architecture, each 16-bits timer is
accessed as two separate registers of low byte and high byte

Special Function Registers of Timers
 TMOD Register
 TCON Register

Timer 0 Register
6
 The 16 bit register ofTimer 0 is accessed as low byte and
high byte.
 Low byte register is called asTL0 (Timer 0 low Byte)
 High byte register is called asTH0 (Timer 0 High Byte)
 Can be assessed and read as any register
 Example:
MOVTL0,#4FH ; moves value 4F intoTL0
MOV R5,TH0 ; savesTH0 into R5
TH0 TL0
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Timer 1 Register
 The 16 bit register ofTimer 1 is accessed as low byte and
high byte.
 Low byte register is called asTL1 (Timer 1 low Byte)
 High byte register is called asTH1 (Timer 1 High Byte)
 Can be assessed and read as any register
TH1 TL1
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

TMOD (Timer Mode) Register
 Byte addressable
 Both timers 0 and 1 use the same register, called
TMOD (timer mode) to set the various timer
operation modes
 It is a 8-bit register
 The lower 4 bits are forTimer 0
 The upper 4 bits are forTimer 1
 In each case,The lower 2 bits are used to set the timer mode
 The upper 2 bits to specify the operation

GATE C/T M1 M0 GATE C/T M1 M0
Timer 1 Timer 0
M1 M0 Mode Operating Mode
0 0 0 13-bitTimer mode
8-bit timer/counterTHx withTLx as 5-bit prescaler
16-bit timer/counterTHx andTLx cascaded. No
prescaler
1 0 2 8-bit auto reload
8-bit auto reload timer/counterTHx holds the value
which is to be reloadedTLx each time it overflows
1 1 3 SplitTimer mode
MSB LSB

 GATE
 EveryTimer has means of starting and stopping
 Some do this by Hardware or Software
 Some do this by both
 8051 timer have both controls
 When, GATE = 1
 Hardware way of starting and stopping timer
 Timer/counter is enable only while the INTx pin is high and theTRx control pin is
set
 enables and disablesTimer by means of a signal brought to the INT1 pin (P3.3) /
INT0 pin (P3.2)
 When, GATE = 0
 Software way of starting and stopping timer
 Operates regardless of the logic state of INTx bit
 enabled whenever theTRx control bit is set
 To start timer use instruction “SETBTRx”
 To clear timer use instruction “CLRTRx”

 C/T (Counter/Timer)
OSC 1/12
PinTx (P3.4/P3.5)
f f/12
C/T=0
C/T=1
To timer /counter

 C/T (Counter/Timer)
 Decides whetherTimer is used as Delay Generator (Timer)
or An Event Counter (Counter)
 When, C/T = 0
 Time delay generator
 input from internal system clock
 When, C/T = 1
 Event counter
 Timer counts pulses brought to theT0 (P3.4) pin orT1 (P3.5)
pin

TCON (Timer Control) Register
 Bit addressable
 8 - bit register
 Upper 4 bits are used to storeTF andTR bits of both timersTimer
0 andTimer 1
 The lower 4 bits are reserved for controlling the interrupts bits
TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0
T1 T0 INT 1 INT 0
MSB LSB

TCON (Timer Control) Register
14
TCON Bit Function
TF1 TCON.7 Timer 1 Overflow Flag. set by hardware whenT/C 1 overflows.
Cleared by hardware
TR1 TCON.6 Timer 1 run control Bit. Set or cleared by software to turn
timer on/off
TF0 TCON.5 Timer 0 Overflow Flag. set by hardware whenT/C 0 overflows.
Cleared by hardware
TR0 TCON.4 Timer 0 run control Bit. Set or cleared by software to turn
timer on/off
IE1
IE0
TCON.3
TCON.1
Keep track of edge-triggered interrupt only
External Interrupt 1 edge flag bit (High to Low)
External Interrupt 0 edge flag bit (High to Low)
IT1
IT0
TCON.2
TCON.0
Set/cleared by software to specify falling edge/low level triggered
interrupt
Interrupt 1 type control bit (Low to High)
Interrupt 0 type control bit (Low to High)

Equivalent Instructions for TCON
15
ForTimer 0
SETB TR0 SETB TCON.4
CLR TR0 CLR TCON.4
SETB TF0 SETB TCON.5
CLR TF0 CLR TCON.5
ForTimer 1
SETB TR1 SETB TCON.6
CLR TR1 CLR TCON.6
SETB TF1 SETB TCON.7
CLR TF1 CLR TCON.7

How to Use Timer 0?

Operating Modes of Timer
M1 M0 Mode Operating Mode
8-bit timer/counterTHx withTLx as 5-bit prescaler
16-bit timer/counterTHx andTLx cascaded. No prescaler
1 0 2 8-bit auto reload
8-bit auto reload timer/counterTHx holds the value
which is to be reloadedTLx each time it overflows
1 1 3 SplitTimer mode

Mode 1
 It is a 16-bit timer
 Hence, it allows value of 0000 to FFFFH to be loaded into the
timer’s registerTL andTH
 AfterTH andTL are loaded with a 16-bit initial value, the
timer must be started.
 This is done by SETB TR0 for timer 0 and SETBTR1 for timer 1
 After the timer is started, it starts to count up. It counts up
until it reaches its limit of FFFFH

Mode 1
 When it rolls over from FFFFH to 0000, it sets high a flag bit calledTF
(timer flag)
 Each timer has its own timer flag:TF0 for timer 0, andTF1 for timer 1
 This timer flag can be monitored by using the instruction JNBTFx, target
(polling)
 When this timer flag is raised, one option would be to stop the timer with the
instructions CLRTR0 or CLRTR1, for timer 0 and timer 1, respectively
 After the timer reaches its limit and rolls over, in order to repeat the
process
 TH andTL must be reloaded with the original value, andTF must be
reloaded to 0

Mode 0
 Same like mode 1
 It is 13 bitTimer instead of 16-bit
 Can hold the values between 0000 H to 1FFF H inTH-TL
 Timer reaches its maximum of 1FFF, it rolls over to 0000
andTF raised

Mode 2 (Auto reload Timer)
 It is an 8-bit timer
 It allows only values of 00 to FFH to be loaded into the timer’s
registerTH
 AfterTH is loaded with the 8-bit value, the 8051 gives a copy of
it toTL
 Then the timer must be started
 This is done by the instruction SETB TR0 for timer 0 and SETB TR1
for timer 1
 After the timer is started, it starts to count up by incrementing
theTL register
 It counts up until it reaches its limit of FFH
 When it rolls over from FFH to 00, it sets high theTF (timer flag)
 ForTimer 0,TF0 goes high and forTimer 1,TF1 goes high

Mode 2 (Auto reload Timer)
 When theTL register rolls from FFH to 00H andTF is set to
1,TL is reloaded automatically with the original value kept
by theTH register
 To repeat the process, we must simply clearTF and let it go
without any need by the programmer to reload the original
value
 This makes mode 2 an auto-reload, in contrast with
mode 1 in which the programmer has to reloadTH andTL
TF goes high
when FF -> 00

Reference
 “The 8051 Microcontroller and Embedded Systems using
Assembly and C” , Muhammad Ali Mazidi, Janice Gillispie
Mazidi, Rolin D. McKinlay, Second Edition, Pearson
publication

SE PAI Unit 5_Timer Programming in 8051 microcontroller_Part 1

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