microcontrollers work

7/28/2019 Microcontrollers Work

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Microcontrollers are hidden inside a

urprising number of products these days. If

our microwave oven has

nLEDorLCDscreen and a keypad, it

ontains a microcontroller. All modern

utomobiles contain at least one

microcontroller, and can have as many as

x or seven: Theengineis controlled by a

microcontroller, as are theanti-lock brakes,

hecruise controland so on. Any device that

as a remote control almost certainly

ontains a microcontroller:TVs,VCRsand

igh-end stereo systems all fall into this

ategory. NiceSLRanddigital cameras,cell

hones,camcorders,answering

machines,laser printers,telephones(the

nes with caller ID, 20-number memory,

tc.), pagers, and feature-

adenrefrigerators,

ishwashers,washersanddryers(the ones

with displays and keypads)... You get the

dea. Basically, any product or device that

nteracts with its user has a microcontroller

uried inside.

n this article, we will look at microcontrollers

o that you can understand what they are

nd how they work. Then we will go onetep further and discuss how you can start

working with microcontrollers yourself -- we

will create a digital clock with a

microcontroller! We will also build a digital

hermometer. In the process, you will learn

n awful lot about how microcontrollers are

sed in commercial products.

What is a Microcontroller?

A microcontroller is a computer. All

computers -- whether we are talking abo

personaldesktop computeror a

largemainframe computeror a

microcontroller -- have several things in

common:

All computers have aCPU(central

processing unit) that executes programs

you are sitting at a desktop computer rig

now reading this article, the CPU in that

machine is executing a program that

implements the Web browser that is

displaying this page.

The CPU loads the program from

somewhere. On your desktop machine,

browser program is loaded from thehard

disk.

The computer has someRAM(random-access memory) where it can store

"variables."

And the computer has some input and

output devices so it can talk to people. O

your desktop machine,

thekeyboardandmouseare input devic

and themonitorandprinterare outputdevices. A hard disk is an I/O device -- it

handles both input and output.

The desktop computer you are using is a

"general purpose computer" that can run

any of thousands of programs.

Microcontrollers are "special purpose
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omputers." Microcontrollers do one thing

well. There are a number of other common

haracteristics that define microcontrollers.

a computer matches a majority of these

haracteristics, then you can call it a

microcontroller":

Microcontrollers are "embedded" inside

ome other device (often a consumer

roduct) so that they can control the

eatures or actions of the product. Another

ame for a microcontroller, therefore, is

embedded controller."

Microcontrollers are dedicated to one tasknd run one specific program. The program

s stored inROM(read-only memory) and

enerally does not change.

Microcontrollers are often low-power

evices. A desktop computer is almost

lways plugged into a wall socket and might

onsume 50 watts of electricity. A battery-perated microcontroller might consume 50

milliwatts.

A microcontroller has a dedicated input

evice and often (but not always) has a

mall LED or LCD display for output. A

microcontroller also takes input from the

evice it is controlling and controls the

evice by sending signals to different

omponents in the device. For example, the

microcontroller inside a TV takes input from

heremote controland displays output on

he TV screen. The controller controls the

hannel selector, thespeakersystem and

ertain adjustments on the picture tube

electronics such as tint and brightness.

Theengine controllerin a car takes inpu

from sensors such as the oxygen and kn

sensors and controls things like fuel mix

spark plug timing. Amicrowave

ovencontroller takes input from a keypa

displays output on an LCD display and

controls arelaythat turns the microwave

generator on and off.

A microcontroller is often small and low

cost. The components are chosen to

minimize size and to be as inexpensive

possible. A microcontroller is often, but not

always, ruggedized in some way. The

microcontroller controlling a car's engine

example, has to work in temperature

extremes that a normal computer genera

cannot handle. A car's microcontroller in

Alaska has to work fine in -30 degree F C) weather, while the same microcontro

in Nevada might be operating at 120

degrees F (49 C). When you add the he

naturally generated by theengine, the

temperature can go as high as 150 or 18

degrees F (65-80 C) in the engine

compartment. On the other hand, amicrocontroller embedded inside a VCR

hasn't been ruggedized at all.

The actual processorused to implemen

microcontroller can vary widely. For

example, the cell phone shown onInside

Digital Cell Phonecontains aZ-80
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rocessor. The Z-80 is an 8-

itmicroprocessordeveloped in the 1970s

nd originally used in home computers of

he time. The Garmin GPS shown inHow

GPS ReceiversWork contains a low-power

ersion of the Intel 80386, I am told. The

0386 was originally used in desktop

omputers.

n many products, such as microwave

vens, the demand on the CPU is fairly low

nd price is an important consideration. In

hese cases, manufacturers turn

o dedicated microcontroller chips --

hips that were originally designed to be

ow-cost, small, low-power, embedded

CPUs. The Motorola 6811 andIntel 8051are

oth good examples of such chips. There is

lso a line of popular controllers called "PIC

microcontrollers" created by a company

alledMicrochip. By today's standards,hese CPUs are incredibly minimalistic; but

hey are extremely inexpensive when

urchased in large quantities and can often

meet the needs of a device's designer with

ust one chip.

A typical low-end microcontroller chip mightave 1,000bytesof ROM and 20 bytes of

RAM on the chip, along with eight I/0 pins.

n large quantities, the cost of these chips

an sometimes be just pennies. You

ertainly are never going to run Microsoft

Word on such a chip -- Microsoft Word

equires perhaps 30 megabytes of RAM and

a processor that can run millions of

instructions per second. But then, you do

need Microsoft Word to control a microw

oven, either. With a microcontroller, you

have one specific task you are trying to

accomplish, and low-cost, low-power

performance is what is important.

Using Microcontrollers

InHow Electronic Gates Work, you learn

about 7400-series TTL devices, as well

where to buy them and how to assemble

them. What you found is that it can often

take many gates to implement simple

devices. For example, in thedigital clock

article, the clock we designed might con

15 or 20 chips. One of the big advantage

a microcontroller is that software -- a sm

program you write and execute on the

controller -- can take the place of many

gates. In this article, therefore, we will us

microcontroller to create a digital clock. T

is going to be a rather expensive digital

clock (almost $200!), but in the process

will accumulate everything you need to p

with microcontrollers for years to come.

Even if you don't actually create this digi

clock, you will learn a great deal by read

about it.

The microcontroller we will use here is a

special-purpose device designed to mak

life as simple as possible. The device is

called a "BASIC Stamp" and is created b
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ompany calledParallax. A BASIC Stamp is

PIC microcontroller that has been

ustomized to understand the BASIC

rogramming language. The use of the

ASIC language makes it extremely easy to

reate software for the controller. The

microcontroller chip can be purchased on a

mall carrier board that accepts a 9-

oltbattery, and you can program it by

lugging it into one of the ports on your

esktop computer. It is unlikely that any

manufacturer would use a BASIC Stamp in

n actual production device -- Stamps are

xpensive and slow (relatively speaking).

owever, it is quite common to use Stamps

or prototyping or for one-off demo products

ecause they are so incredibly easy to set

p and use.

hey are called "Stamps," by the way,

ecause they are about as big as a postagetamp.

he specific BASIC Stamp we will be using

n this article is called the "BASIC Stamp

Revision D".

he BASIC Stamp Revision D is a BS-1

mounted on carrier board with a 9-volt

attery holder, a power regulator, a

onnection for a programming cable, header

ins for the I/O lines and a small prototyping

rea. You could buy a BS-1 chip and wire

he other components in on a breadboard.

he Revision D simply makes life easier.

You can see from the previous table tha

you aren't going to be doing anything ex

with a BASIC stamp. The 75-line limit (th

256 bytes ofEEPROMcan hold a BASIC

program about 75 lines long) for the BS-

fairly constraining. However, you can cre

some pretty neat stuff, and the fact that

Stamp is so small and battery operated

means that it can go almost anywhere.

Programming the BASIC Stamp

You program a BASIC Stamp using

the BASIC programming language. If y

already know BASIC, then you will find t

the BASIC used in a Stamp is

straightforward but a little stripped-down

you don't know BASIC, but you do know

another language likeC, Pascal orJava

then picking up BASIC will be trivial. If yo

have never programmed before, you

probably want to golearn programming

desktop machine first. Here is a quick

rundown on the instructions available in

Stamp BASIC. (For complete

documentation, go toParallax: BASIC St

Documentation.)

Standard BASIC instructions:

for...next - normal looping statement

gosub - go to a subroutine

goto - goto a label in the program (e.g. -

"label:")

if...then - normal if/then decision
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et - assignment (optional)

eturn - return from a subroutine

nd - end the program and sleep

nstructions having to do with I/O pins:

utton - read a button on an input pin, withebounce and auto-repeat

igh - set an I/O pin high

nput - set the direction of an I/O pin to input

ow - set an I/O pin low

utput - set the direction of an I/O pin to

utput

ot - read a potentiometer on an I/O pin

ulsin - read the duration of a pulse coming

n on an input pin

ulsout - send a pulse of a specific duration

ut on an output pin

wm - perform pulse width modulation on anutput pin

everse - reverse the direction of an I/O pin

erin - read serial data on an input pin

erout - write serial data on an output pin

ound - send a sound of a specific

equency to an output pinoggle - toggle the bit on an output pin

nstructions specific to the BASIC

tamp:

ranch - read a branching table

ebug - send a debugging string to the

onsole on the desktop computer

eeprom - download a program to EEPRO

lookdown - return the index of a value in

list

lookup - array lookup using an index

nap - sleep for a short time

pause - delay for the specified time

random - pick a random number

read - read a value from EEPROM

sleep - power down for the specified tim

write - write data to EEPROM

Operations:

+ - addition

- - subtraction

* - multiplication (low-word)

** - multiplication (high-word)

/ - division

// - mod

max - return maximum of 2 values

min - return minimum of 2 values

& - AND

| - OR

^ - XOR &/ - NAND

|/ - NOR

^/ - XNOR

If statement logic:

=


6/17

>

=

=

AND

OR

VariablesAll variables in the BS-1 have pre-defined

ames (which you can substitute with

ames of your own). Remember that there

re only 14 bytes of RAM available, so

ariables are precious. Here are the

tandard names:

w0, w1, w2...w6 - 16-bit word variables0, b1, b2...b13 - 8-bit byte variables

it0, bit1, bit2...bit15 - 1-bit bit variables

ecause there are only 14 bytes of memory,

w0 and b0/b1 are the same locations in

RAM, and w1 and b2/b3 are the same, and

o on. Also, bit0 through bit15 reside in w0and therefore b0/b1 as well).

O pins

You can see that 14 of the instructions in

he BS-1 have to do with the I/O pins. The

eason for this emphasis is the fact that the

O pins are the only way for the BASIC

Stamp to talk to the world. There are eig

pins on the BS-1 (numbered 0 to 7) and

pins on the BS-2 (numbered 0 to 15).

The pins are bi-directional, meaning th

you can read input values on them or se

output values to them. The easiest way

send a value to a pin is to use

the HIGH orLOW functions. The statem

high 3 sends a 1 (+5 volts) out on pin 3.

LOW sends a 0 (Ground). Pin 3 was cho

arbitrarily here -- you can send bits out o

any pin from 0 to 7.

There are a number of interesting I/O pin

instructions. For example, POT reads th

setting on a potentiometer (variable resis

if you wire it up with acapacitoras the P

instruction expects. The PWM instructio

sends out pulse-width modulated signals

Instructions like these can make it a loteasier to attach controls and motors to t

Stamp. See thedocumentationfor the

language for details. Also, a book like Sc

Edward's Programming and Customizing

the BASIC Stamp Computercan be

extremely helpful because of the examp

projects it contains.
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Playing with a BASIC Stamp

you would like to play with a BASIC

tamp, it's very easy to get started. What

ou need is a desktop computer and

BASIC Stamp starter kit. The starter kit

ncludes the Stamp, a programming cable

nd an application that you run on your

esktop computer to download BASIC

rograms into the Stamp.

You can get a starter kit either

omParallax(the manufacturer) or from a

upplier likeJameco(who should be familiaro you from theelectronic gatesanddigital

lockarticles). From Parallax, you can order

he BASIC Stamp D Starter Kit (part number

7202), or from Jameco you can order part

umber 140089. You will receive the Stamp

pictured below), a programming cable,

oftware and instructions. The kit is $79om both suppliers. Occasionally, Parallax

uns a special called "We've Bagged the

asics" that also includes Scott

dward's Programming and Customizing

he BASIC Stamp Computer].

ooking up the Stamp is easy. You connect

into theparallel portof your PC. Then you

un a DOS application to edit your BASIC

rogram and download it to the Stamp.

o run the program in this editor, you hit

ALT-R. The editor application checks the

ASIC program and then sends it down the

wire to the EEPROM on the Stamp. The

Stamp then executes the program. In th

case, the program produces a square w

on I/O pin 3. If you hook up a logic probe

LED to pin 3 (see theelectronic gates

articlefor details), you will see the LED f

on and off twice per second (it changes

state every 250 milliseconds because of

PAUSE commands). This program woul

run for several weeks off of a 9-volt batte

You could save power by shortening the

time that the LED is on (perhaps it is on

50 milliseconds and off for 450

milliseconds), and also by using the NAP

instruction instead of PAUSE.
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8/17

Creating a Really Expensive

Digital Clock

pending $79 to flash anLEDmay seem

xtravagant to you. What you would

robably like to do is create something

seful with your BASIC stamp. By spending

bout $100 more you can create a really

ice digital clock! This may

eemextremelyextravagant, until you

ealize that the parts are reusable in a

ariety of other projects that you may want

o build later.

et's say that we would like to use the I/O

ins on the BASIC Stamp to display numeric

alues. In thedigital clock article, we saw

ow to interface to a 7-segment LED display

sing a 7447 chip. 7447s would work just as

well with the BASIC Stamp. You could wire

our of the I/O pins straight into a 7447 and

asily display a number between 0 and 9.

ince the BS-1 Stamp has eight I/O pins, it

s easy to drive two 7447s directly like this.

For a clock, we need a minimum of four

digits. To drive four 7447s with eight I/O

pins, we have to be slightly more creativ

The following diagram shows you one

approach:

In this diagram, the eight I/O lines from t

Stamp enter from the left. This approach

uses four lines that run to all four 7447s

Then the other four lines from the Stamp

activate the 7447s in sequence ("E" on t

chips means "Enable" -- on a 7447, that

would be the blanking input on pin 5). To

make this arrangement work, the BASIC

program in the Stamp would output the f

digit on the four data lines and activate t

first 7447 by toggling its E pin with the fi

control line. Then it would send out the

value for the second digit and activate th

second 7447, sequencing through all fou

the 7447s like this repeatedly. By wiringthings slightly differently, you could actu

do this with only one 7447. By using a

74154 demultiplexer chip and some driv

you could drive up to 16 digits using this

approach.

This is, in fact, a standard way to controLED displays. For example, if you have a

old LED calculator, turn it on and shake

while watching the display. You will actu

be able to see that only one digit is ever

illuminated at once. The approach is

called multiplexing the display.
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9/17

While this approach works fine for clocks

nd calculators, it has two important

roblems:

EDs consume a lot of power.

-segment LEDs can only display numeric

alues.

An alternative approach is to use anLCD

creen. As it turns out, LCDs are widely

vailable and can be easily hooked to a

tamp. For example, the two-line by 16-

haracter alphanumeric display shown

elow is available from bothJameco(partumber 150990) andParallax(part number

7910). A typical display is shown here,

mounted on a breadboard for easier

nterfacing:

his sort of LCD has several advantages:

he display can be driven by a single I/O

in. The display contains logic that lets a

tamp communicate with it serially, so only

ne I/O pin is needed. In addition, the

EROUT command in Stamp BASIC

andles serial communication easily, so

alking to the display is simple.

The LCD can display alphanumeric text:

letters, numbers and even custom

characters.

The LCD consumes very little power -- o

3 milliamps.

The only problem is that one of these

displays costs $59. Obviously, you woul

not embed one of these in atoaster oven

you were designing a toaster oven,

however, you would likely prototype with

one of these displays and then create

custom chips and software to drive muc

cheaper LCDs in the final product.

To drive a display like this, you simply

supply it with +5 volts and ground (the

Stamp supplies both from the 9-volt batt

and then hook one of the I/O pins from t

Stamp to the display's input line. The

easiest way I have found to connect theStamp's I/O pins to a device like an LCD

to use a wire-wrap tool (Jamecopart

number 34577) and 30-gauge wire wrap

wire (Jameco part number 22541 is typic

That way, no soldering is involved and th

connections are compact and reliable.

The following BASIC program will cause

BASIC Stamp to behave like a clock and

output the time on the LCD (assuming th

LCD is connected to I/O pin 0 on the

Stamp):

pause 1000 'wa

for LCD display to boot
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10/17

erout 0, n2400, (254, 1)clear the display

erout 0, n2400, ("time:")Paint "time:" on the display

'preset beforeoading program

0 = 0seconds

1 = 27minutes

2 = 6hours

gain:

0 = b0 + 1increment seconds

f b0 < 60 then minutes

b0 = 0 'ifeconds=60

b1 = b1 + 1 'hen increment minutes

inutes:

f b1 < 60 then hours

b1 = 0 'ifinutes=60

b2 = b2 + 1 'hen increment hours

ours:

f b2 < 13 then show

b2 = 1 'ifhours=13 reset to 1

show:

serout 0, n2400, (254, 135)

'position cursor on display,

'then display time

serout 0, n2400, (#b2, ":", #b":", #b0, " ")

pause 950

'pause 950 milliseconds

goto again'repeat

In this program, the SEROUT command

send data to the LCD. The sequence (25

1) clears the LCD (254 is the escape

character and 1 is the command to clear

screen). The sequence (254, 135) positi

the cursor. The other two SEROUT

commands simply send text strings to th

display.

This approach will create a reasonably

accurate clock. By tweaking the PAUSE

statement you can get the accuracy to

within a few seconds a day. Obviously, i

real clock you would like to wire up a pu

button or two to make setting it easier --

this program, you preset the time before

download the program to the Stamp.


11/17

While this approach is simple and works, it

s not incredibly accurate. If you want better

ccuracy, one good approach would be to

wire a real-time clock chip up to your

tamp. Then, every second or so, you can

ead the time from the chip and display it. A

eal-time clock chip uses aquartz crystalto

ive it excellent accuracy. Clock chips also

sually contain date information and

andleleap yearcorrection automatically.

One easy way to interface a real-time clock

o a stamp is to use a component called

he Pocket Watch B.

he Pocket Watch B is available from

othJameco(part number 145630)ndParallax(part number 27962). This part

s about as big as a quarter and contains the

lock chip, crystal and a serial interface so

hat only one I/O pin is necessary to

ommunicate with it. This component costs

bout $30 -- again, not something you want

to embed in a toaster oven, but easy to p

with when constructing prototypes

Building a Digital Thermomete

Now that you understand a little bit abou

your Stamp and the LCD, we can add

another component and create a digital

thermometer. To create a thermometer,

will use a chip called the DS1620. This c

contains: A temperature-sensing device

An analog-to-digital (A/D) converterfo

the temperature-sensing device

A shift registerto read the data out of t

A/D converter

A little EEPROM (electrically erasable

programmable read-only memory) to

remember settings

The DS1620 has two modes: In one mod

it acts as a stand-alone thermostat chip,

in the other mode you hook it up to a

computer and use it as athermometer. T

EEPROM remembers the current mode

well as the set temperatures for the

thermostat.

Hooking up the DS1620 to the Stamp is

very easy. The DS1620 comes in an 8-p

chip. Supply +5 volts from the Stamp to

8 of the DS1620. Supply ground to pin 4
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12/17

he DS1620. You then use three I/O pins

om the Stamp to drive three pins on the

DS1620:

in 1 on the DS1620 is the data pin. You

ead and write data bits on this pin.

in 2 on the DS1620 is the clock pin. Youock data in and out of the shift register

with this pin.

in 3 on the DS1620 is the reset/select pin.

You set pin 3 high to select the chip and

ommunicate with it.

or this example code, it is assumed that:

he data pin goes to I/O pin 2 on the Stamp.

he clock pin goes to I/O pin 1 on the

tamp.

he reset/select pin goes to I/O pin 0 on the

tamp.

he completed wiring looks like this:

You can get a DS1620 either

omJameco(part number 146456)

rParallax(part number 27917) in an

application kit" that includes the chip, the

apacitor, some good documentation and

ample code. Or you can buy the chip on itswn fromJameco(part number 114382). I

would suggest getting the application kit the

rst time you try using the DS1620 because

he documentation is very useful.

You can assemble the DS1620 in the

rototype area of the Stamp carrier board or

on a separate breadboard. Once you ha

assembled it, hook your LCD display up

I/O pin 3 of the Stamp, and then load an

run the following program:

symbol RST = 0 ' select/reset

line on 1620

symbol CLK = 1 ' clock line foshift registers on 1620

symbol DQ = 2 ' data line on1620

symbol DQ_PIN = pin2 ' pinrepresentation for DQ

symbol LCD = 3 ' data line forLCD

begin:

low RST ' deselect the 162unless talking to it

high CLK ' clock pin on 162should default high

pause 1000 ' wait for thethermometer and LCD to boot

setup:

high RST ' select the1620

b0 = $0C ' $0c is the1620 command byte

' saying"Write Config"
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13/17

osub shift_out ' send it to the620

0 = %10 ' %10 is the620 command byte

' to sethermometer mode


ow RST ' deselect the620

ause 50 ' delay 50ms forEPROM

tart_convert:

0 = $EE ' $EE is the620 command byte

' to startonversions

igh RST ' select the620


ow RST ' deselect the620

This is the main loop

- reads and displaysemperature every second

ain_loop:

high RST ' select the1620

b0 = $AA ' $AA is the1620 command byte

' forreading temperature

gosub shift_out ' send it tothe 1620

gosub shift_in ' read thetemperature

' from the1620

low RST ' deselect tDS1620.

gosub display ' display thtemp in degrees C

pause 1000 ' wait asecond

goto main_loop

' The shift_out subroutine senwhatever is in

' the b0 byte to the 1620

shift_out:

output DQ ' set the pin to

' outputmode

for b2 = 1 to 8


14/17

low CLK ' prepare tolock the bit

' into620

DQ_PIN = bit0 ' Send theata bit

high CLK ' latch datait into 1620

b0 = b0/2 ' shift allits right

' towardit 0

ext

eturn

The shift_in subroutine gets a-bit

temperature from the 1620

hift_in:

nput DQ ' set the DQin to

' inputode

0 = 0 ' clear w0

or b5 = 1 to 9

w0 = w0/2 ' shiftnput right.

low CLK ' ask 1620or next bit

bit8 = DQ_PIN ' read thebit

high CLK ' toggleclock pin

next

return

' Displays the temperature indegrees C

display:

if bit8 = 0 then pos ' ifbit8=1

'then temp is negative

b0 = b0 &/ b0 'invert b0 by NANDing it

'

with itself

b0 = b0 + 1

pos:

serout LCD, n2400, (254, 1) clear the LCD

serout LCD, n2400, ("Temp = ")display "Temp="

on the display

bit9 = bit0 save the half degree

b0 = b0 / 2

convert to degrees


15/17

f bit8 = 1 then neg 'ee if temp is negative

serout LCD, n2400, (#b0) 'isplay positive temp

goto half

eg:

serout LCD, n2400, ("-", #b0)'isplay negative temp

alf:

if bit9 = 0 then even

serout LCD, n2400, (".5 C")display the half degree

goto done

ven:

serout LCD, n2400, (".0 C")display the half degree

one:

eturn

you run this program, you will find that it

isplays the centigrade temperature with an

ccuracy of one-half degree.

he DS1620 measures temperatures inentigrade half-degrees. It returns the

emperature in a 9-bit 2s-complement

umber with a range of -110 to 250 F (-55 to

25 C). You divide the number you receive

y 2 to get the actual temperature. 2s-

omplement binary numbers are a

onvenient way to represent negative

values. The following list shows the valu

for a 4-bit 2s-complement number:

0111 : 7

0110 : 6

0101 : 5

0100 : 4

0011 : 3

0010 : 2

0001 : 1

0000 : 0

1111 : -1

1110 : -2

1101 : -3

1100 : -4

1011 : -5

1010 : -6

1001 : -7

1000 : -8

You can see that instead of the 4 bitsrepresenting values from 0 to 15, the 4 b

in a 2s-complement number represent th

values -8 to 7. You can look at the left-m

bit to determine if the number is negative

positive. If the number is negative, you c

invert the bits and add 1 to get the positi

representation of the number.


16/17

ere's what goes on with the digital

hermometer program shown here:

uses the symbol keyword to set up

everal constants that make the program

ightly easier to read (and also make it

asy for you to move the chip to different I/Oins on the Stamp).

sets the CLK and RST pins on the

DS1620 to their expected values.

writes a command byte to the EEPROM

n the DS1620 to tell the chip to operate in

hermometer mode." Because the mode is

tored in EEPROM, you only have to do it

nce, so you could technically take this

ection of the code out of the program after

ou run the program once (to save program

pace).

he program sends the command $EE ("$"

means "hexadecimal number" -- $EE is 238

n decimal) to tell the thermometer to start

p its conversion process.

he program then enters a loop. Every

econd, it sends a command to the DS1620

elling the DS1620 to return the current

emperature, and then it reads the 9-bit

alue that the DS1620 returns into the w0ariable. The Stamp sends and receives

ata 1 bit at a time by toggling the CLK line

n the DS1620. Remember that the w0 (16-

it) variable overlays the b0/b1 (8-bit)

ariables, which overlay the bit0/bit1/.../bit15

1-bit) variables, so when you insert a bit

om the DS1620 into bit 8 and divide w0 by

2, what you are doing is shifting each bit

the right to store the 9-bit temperature fr

the DS1620 into w0. Once the temperat

has been saved in w0, the display

subroutine determines whether the num

is positive or negative and displays it

appropriately on the LCD as a centigrad

temperature. The conversion from degre

C to degrees F is:

dF = dC * 9/5 + 32

At this point, we have succeeded in crea

an extremely expensive thermometer. W

might you do with it? Here's one idea. Le

say you work for a drug company and yo

are shipping expensive drugs across the

country that MUST remain at a certain

temperature the entire way or the drugs

spoil. What you can do with a Stamp is

create a data logging thermometer.

BothJameco(part number 143811)andParallax(part number 27960) sell a

device called the "RAM Pack module." It

contains a low-power 8-kilobyte (or

optionally 32-kilobyte) RAM chip with a

serial interface. You could add this

component (or something similar) to you

Stamp and write code that savestemperature readings to the RAM every

minute. You could then slip your Stamp

the drug shipment, and at the other end

the trip retrieve the Stamp. The RAM

module would contain the temperature

history of the entire trip and you would k

whether or not the drugs ever thawed ou
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17/17

here are all kinds of neat, useful devices

ke this that you can build with a Stamp now

hat you know how microcontrollers work!

or more information on microcontrollers

nd related topics, check out the links on

he next page.

microcontrollers work

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