Lecture12 Timers

7/31/2019 Lecture12 Timers

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CpE 213Digital Systems Design

Timers and Counters for 8051

Lectures 12

Tuesday, 3/4/2008


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Announcements

Homework #4 is due Today! Test #2 is on March 20, 2008. Same

format as Test #1.

Test # 2 topics: 8051 HardwareSummary, 8051 Instruction Set andAssembly Language.

Seminar Today

Simon Board Schematic is onblackboard.


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Todays Class

Overview/Objectives

Timers and Counters for 8051 (Chapter 9)


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Timers and Counters for the8051


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What is a Timer/Counter?

A counter is a device that generates binarynumbers in a specified count sequencewhen triggered by an incoming clock

pulse. Timers are counters that count pulses. If

the pulses are clock pulses, then the

timers count time.


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8051 Block Diagram

Interrupt Control

External Interrupts

CPU

OSC

Timer 1

Timer 0

Serial

PortI/O PortsBus

Control

Counter

Inputs

P0 P3P1

4k byte

ROM128 byte

RAM

P2

(Address/Data)

TXD RXD


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Timers/Counter on the 8051

The 8051 comes with two 16-bit timers/counters,both of which may be controlled, set, read, andconfigured individually.

The 8051 timers have three general functions: Keeping time and/or calculating the amount of time

between events (Timer Mode).

Counting the events themselves (Event CounterMode)

Generating baud rates for the serial port. Always count UP irrespective of function as a

timer or a counter.


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Difference between a Timer

and a Counter When the incoming clock frequency is known, we cangenerate a fixed period of time known to the designerby setting a preloaded value. This is called a timer.

It is also called an interval timer since it is measuring

the time of the interval between two events. When the incoming clock is irregular and we are only

interested in the number of occurrences of the pulse,this is called a counter. Since we are countingevents, is also known as an event counter.

In 8051, timer gets its clock from the oscillator (crystalthat connected to the CPU) frequency (1/12 of it).

Counter gets the clock from an external pin: P3.4 for timer 0, and

P3.5 for timer 1.


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Difference between a Timerand Counter (Cont.)

Timer mode (if the timer control bit is set totimer, the timer uses the system clock)

Increments by 1 every machine cycle.

A single machine cycle consists of 12 crystalpulses.

Event mode (if the timer control bit is set to

event, the timer/counter uses a port bit) Increments by 1 for each pulse on P3.4 for

Timer 0, and P3.5 for Timer 1, respectively.


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Applications ofTimers/Counters

Generating time delays. Measuring pulse duration or timing

intervals.

Counting pulses or events.

Generating baud rate clock for the internal8051 serial I/O port.

Generating interrupts.

C C


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Timer/Counter ClockingSource

C/T

On-chiposcillator

Crystal

0 = Up (interval timing)

1 = Down (event counting)

Timerclock

Clocking Source

T0 (P3.4) orT1 (P3.5) pin


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Group Exercise

Find the timers clock frequency andperiod for various 8051-based systemswith the following crystal frequencies:

12 MHz 16 MHz


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Group Exercise

Find the timers clock frequency andperiod for various 8051-based systemswith the following crystal frequencies:

12 MHz 1/12 x 12 MHz = 1 MHz => T = 1 us

16 MHz

1/12 x 16 MHz = 1.333 MHz => T = .75 us

Ti S i l F i


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Timer Special FunctionRegisters (SFRs)

As mentioned before, the 8051 has two timers thatessentially function the same way.

One timer is called TIMER0 and the other is TIMER1.

The two timers share two SFRs (TMOD and TCON) that

control the timers. Each timer also has two SFRs dedicated solely to itself

(TH0/TL0 and TH1/TL1).

It is common practice to use given SFR names to refer to

them, but in reality an SFR has a numeric address. When you enter the name of an SFR into an assembler,

it internally converts it to the specified SFR address.


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Location of Timer SFRs

SFR Name Description Address

TH0 Timer 0 High Byte 8CH

TL0 Timer 0 Low Byte 8AHTH1 Timer 1 High Byte 8DH

TL1 Timer 1 Low Byte 8BH

TCON Timer Control 88HTMOD Timer Mode 89H

Ti C t l R i t


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Timer Control Register(TCON)

TCON has the following functions: Turns Timer 0 and Timer 1 on or off.

Sets trigger type (edge or level) for interrupt 0

and interrupt 1. Sets flags when timer 0 or timer 1 overflow.


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Summary of TCON

Only the upper 4 bits are related to timers; the lower 4 bitshave nothing to do with timers - they have to do withinterrupts.

TCON is a bit-addressable register.

Bit SymbolBit

Address DescriptionTCON.7 TF1 8FH Timer 1 overflow flag. Set by hardware upon

overflow; cleared by software, or byhardware when processor vectors to interruptservice routine

TCON.6 TR1 8EH Timer 1 run-control bit. Set/cleared bysoftware to turn timer on/off

TCON.5 TF0 8DH Timer 0 overflow flagTCON.4 TR0 8CH Timer 0 run-control bitTCON.3 IE1 8BH External interrupt 1 edge flag. Set byhardware when a falling edge is detected on

INT1; cleared by software, or by hardwarewhen CPU vectors to interrupt service routine

TCON.2 IT1 8AH External interrupt 1 type flag. Set/cleared bysoftware for falling edge/low-level activatedexternal interrupt

TCON.1 IE0 89H External interrupt 0 edge flagTCON.0

IT0

88H

External interrupt 0 type flag

Ti M d C t l (TMOD)


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Timer Mode Control (TMOD)Register

TMOD controls timer 0 and timer 1 functions. The registers upper nibble controls Timer 1 and

the lower nibble controls timer 0. The four bits for each timer control a gate

function, selection of counter or timer, and timermode. The timer or counter function is selected by

control bits C/T; these two timers/counters have4 operating modes, selected by appropriate M0

and M1 bits in the TMOD register. TMOD is not a bit-addressable register. It has to

be manipulated as a byte.

Ti M d (TMOD)


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Timer Mode (TMOD)Register Summary

(MSB) (LSB)

GATE TC/ M1 M0 GATE TC/ M1 M0

GATE

TC/

If GATE = 1, timer x will run

only when TRx = 1 and INTx= 1. If GATE = 0, timer x will

run whenever TRx = 1.

Timer mode select. If

TC/ = 1, timer x runs in

counter mode taking its input

from Tx pin. If TC/ = 0 ,

timer x runs in timer modetaking its input from thesystem clock.

M1

0

01

1

1

M0

0

10

1

1

Operating Mode

13-bit timer mode (8048mode).

16-bit timer mode.8-bit auto-reload mode.

THx holds a value

which is to be reloaded

into TLx each time it

overflows.

(Timer 0) TL0 is an 8-bitTimer/Counter controlled

by the standard Timer 0

control bits. TH0 is an8-bit timer only controlled

by Timer 1 control bits.

(Timer 1) Timer/Counter

1 is stopped.

Timer 1 Timer 0


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Thursdays Class (March 6, 2008)

Review lecture notes. Read Chapter 9.

Lecture12 Timers

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