e021028.pdf

MICRO PROCESSOR

ISAC (4)

part 4: basic applications

By Prof. B. vom Berg, P. Groppe and M. Müller-Aulmann

In this fourth instalment of the series we will look at our ﬁrst applications

of the ISAC cube and the simple motherboard, which will give a good

introduction to the possibilities of the device. Complete example pro-

grams developed for these applications, along with extra information, are

available for download from the Elektor Electronics website.

Before we present the individual example

applications, a few initial remarks are in order

regarding the example programs and their

use.

– The applications and demonstration pro-

grams are written in C51 code in such a

way that (with one exception) they can be

used with the restricted version of µVision2

(maximum code size of 2 Kbytes, no float-

ing-point arithmetic). For larger application

programs the full version of µVision2, or

another 8051 tool, must be resorted to.

– In order that something ‘visible’ is pro-

duced, all the programs guide the user via,

and produce output on, the PC’s monitor.

That means that you will need to run a ter-

minal program on your PC. We have

assumed that you will be using HyperTer-

minal, since this program is included with

Windows. You can, however, use any other

terminal program: the required communi-

cations parameters are 9600 baud, 8 data

bits, no parity, 1 stop bit.

– Before starting a program on the ISAC cube,

you should start up your chosen terminal

program and then press the reset button on

the cube.

– Ve ry important: if you have been using

HyperTerminal and then want to download

a new program into the ISAC cube using

the serial downloader, you must ﬁrst clear

the existing connection with HyperTermi-

nal (using ‘Disconnect’ on HyperTerminal).

Otherwise HyperTerminal will keep control

of the COM port and the serial downloader

will not be able to access it. An apparently

meaningless error message will appear if

the serial downloader is unable to

ﬁnd the ADuC812. In this case you

will have to cancel the error mes-

sage, release the connection in

HyperTerminal, and then relaunch

the downloader.

– Communication between the

microcontroller core and the on-

chip peripherals uses the 8051-

style Special Function Register

(SFR) ( Figure 1 ). Further informa-

tion on the meaning of the individ-

ual bits within the SFR can be

found in the ADuC812 data sheet

or in specialist 8051 books.

D/A converter application

Outputting different signals:

With the aid of program dac_1.c two

different signals (a sawtooth and a

squarewave) can be output using

the D/A converter at two different

resolutions (8 bits and 12 bits). Con-

Elektor Electronics

1/2002

MICRO PROCESSOR

nect an oscilloscope to the relevant

outputs to verify the results.

The reference voltage used here is

the digital power supply voltage V DD

(=+5 V), and so the output voltages

produced lie in the range 0 to +5 V.

1. Using the internal 2.5 V reference

voltage. In this case 1 LSB corre-

sponds to a voltage step of

610 µV.

2. Using an external reference volt-

age in the range +2.3 V to +5.0 V,

connected to the V Ref pin. In view

of the 12 bit resolution, this volt-

age should be specially filtered

and regulated. If, for example,

V Ref is connected to AV DD supply

(+5 V), 1 LSB will correspond to a

voltage step of 1.2 mV.

A/D converter application

Reading the voltages on the input

channels:

On starting up program adc_1.c you

will be asked to supply the number

of the channel to be read, where you

have the following possibilities:

– number 0-7: external measurement

channels ADC 0 to ADC 7.

υ 010048 - 4 - 12

Figure 2. The linear relationship between volt-

age U

The input voltage to be measured

must always lie between 0 V and

the reference voltage used (whether

internal or external), i.e. in the range

and temperature

temperature dependence is –3 mV/°C. If the

temperature rises by 1°C the temperature-

dependent voltage will drop by 3 mV and

vice versa. But there is a very important

point to note here: these ﬁgures are only typ-

ical values and vary from device to device. If

you compare ADuC812s from different pro-

duction batches, you will ﬁnd rather different

values. Analog Devices do guarantee, how-

ever, that the temperature-dependent voltage

is highly linear in die temperature.

This relationship is characterised by a (lin-

ear) equation U

ADuC8x

External

Program Memory

(up to 64K bytes)

SFRs

8K bytes

Internal Flash / EE

Program Memory

640 byte

Internal Flash / EE

Data Memory

multichannel

ADC

)) ( Fig-

ure 2 ). For the user, this means that two cali-

bration readings have to be taken to estab-

lish the parameters of the linear equation

before meaningful temperature values can

then be obtained. If you then have an

ADuC812 from the same production batch

(which is usually the case if they come from

the same tube), nothing needs to be changed.

If, however, you buy ADuC812s from different

batches, this calibration will have to be

repeated several times over. In the worst case

a different set of parameters for the linear

relationship will have to be obtained for each

=f1(

) (or

=f2(U

8052

compatible

core

DAC (s)

Watchdog Timer

External RAM

Data Memory

(up to 16 M bytes)

256 bytes

Internal RAM

Power Supply

Digital I / O

010048 - 4 - 11

Figure 1. The SFR register.

– number 8: channel connected to

the on-chip temperature sensor

(see next example).

0 to +2.5 V when using the internal

reference voltage or in the range 0 to

+5 V if an external 5 V reference is

used.

+5V

(= AV DD )

When you have entered a number,

the voltage present at the selected

input channel is converted once per

second and the result displayed as a

decimal and as a hexadecimal value

on the PC’s monitor. Readings are

stopped by pressing the reset but-

ton, and a new input channel can

then be chosen.

Note: The A/D converter has a

resolution of 12 bits and can operate

with two different reference volt-

ages:

Measuring the internal die temper-

ature and the ambient tempera-

ture:

The ADuC812 includes an on-chip

temperature sensor that produces a

temperature dependent voltage that

is connected to the ninth channel

(i.e., channel number 8) of the A/D

converter. The datasheet gives the

following information about this sen-

sor: the output voltage is 600 mV at

a temperature

Analogue input

(A / D converter)

U in

ADuC812

010048- 4 - 13

Figure 3. Voltage divider at the input to the A/D

converter switched by push-buttons.

= 25°C, and the

1/2002

Elektor Electronics

MICRO PROCESSOR

+5V

(= AV DD )

ADuC 812

ISAC-Cube

motherboard,

type 1

optoelectronic

encoder

MRTC 25

10k

ADC7

1k1

channel A

P2.0

P2.1

channel B

switch

P2.2

1k3

ADuC 812

ISAC Cube

motherboard,

Type 1

10K Ω

1k8

+5V

GND

+5V

GND

2k4

010048 - 4 - 15

3k3

all resistors

+ / – 2% tolerance!

Figure 5. Connecting an opto-electronic shaft encoder.

5k1

3k2

Table 1

Characteristics of the MRTC25 opto-electronic shaft encoder

Reference voltage:

V REF = AV DD = +5V

connected to V REF by wire link

16k

Electrical characteristics:

Resolution (pulses/360°)

51k

010048 - 4 - 14

Channels

2 (A and B; 90° phase offset)

Supply voltage

5 V DC ±0.5 V

Figure 4. Series of push-buttons connected to

an analogue input pin.

Current consumption

20 mA

Output signals

TTL-level squarewave; internal 10 k Ω

pull-up

Push-button

5 V, 10 mA (non-inductive load)

ADuC812. The program temp812.c has been

developed to help in the calculation. When

the program is downloaded you will also ﬁnd

instructions on how to determine the para-

meters of the linear relationship, on measur-

ing the ambient temperature , and on deriv-

ing the settings programmed into temp812.c.

Observe the instructions given on how to use

the program.

Operating life

> 1,000,000 cycles

channel A

trigger instants

channel B

Push-buttons connected to an analogue

input:

The basis for this method of connecting push-

buttons, which allows an entire keyboard to

be connected to one analogue input pin of the

A/D converter, is a ‘multiply switched voltage

divider’. We start with a simple voltage

divider ( Figure 3 ).

With this circuit we have:

No button pressed:

direction: CCW

channel A

trigger instants

channel B

U in =5 V

010048 - 4 - 16

direction: CW

Button 0 pressed:

U in =0 V

Button 1 pressed:

U in =5 [R1/(R0+R1)] V

Figure 6. Timing diagram for the two directions of rotation.

If the two resistors R1 and R0 have the same

value, for example 1 k

, we have U in =2.5 V.

For each button pressed, a different and

distinguishable voltage appears on the ana-

logue input and is converted by the A/D con-

verter. In this way, for each button that might

be pressed we obtain a different digital value

and hence we can identify uniquely the button

pressed. There is a further advantage to this

circuit: if more than one button is pressed

simultaneously, only the button nearest the

analogue input will be recognised. In other

words, we can build a keyboard in

which the individual keys are priori-

tised. If, for example, button 1 is

already pressed and subsequently

button 0 is pressed, U in will immedi-

ately be pulled to the voltage corre-

sponding to button 0 (0 V). Thus but-

ton 1 is overridden and will no

longer be recognised, while button 0

has the highest priority and will

always be recognised. When but-

ton 0 is subsequently released, the

other buttons can then be read.

In order to use a large number of

buttons close-tolerance resistors are

required, as well as a well-regulated

power supply to the voltage divider.

Figure 4 shows a practical circuit for

a keyboard with ten push buttons.

A small disadvantage of this idea

Elektor Electronics

1/2002

MICRO PROCESSOR

Figure 6 . Electrical characteristics of the

shaft encoder are given in Table 1 . Program

drehge_1.c shows how to interface to this

interesting input device.

servo no.

Control of eight servos:

Using the eight digital outputs of port 2 and

a little software, in this case written in

assembler, it is easy to control eight servos.

For each servo a control pulse of between

0.9 ms and 2.1 ms must be generated every

20 ms ( Figure 7 ). The simplest approach is to

divide the 20 ms period into eight subinter-

vals and then drive each servo in turn from

the appropriate output on port 2 to the

desired position. Program servo.asm gener-

ates the 2.5 ms intervals using Timer 2 in

auto-reload mode and generates the required

servo pulse durations using Timer 0 as a

16 bit timer.

t / ms

010048 - 4 - 17

Figure 7. Timing diagram for controlling a servo.

Digital I/O port application

Use of an opto-electronic shaft

encoder:

An opto-electronic shaft encoder

makes a simple and elegant input

device. Here we use a type MRTC25

from Megatron, which offers two out-

put channels (A and B) for count and

direction indication (clockwise/coun-

terclockwise). It also provides a con-

tact that closes when the shaft is

pressed down.

Connection to the ISAC cube or to

the simple (type 1) motherboard pro-

ceeds as shown in Figure 5 . The tim-

ing diagram for clockwise and coun-

terclockwise rotation is shown in

should not be ignored: the buttons

can of course only be read while they

are pressed: there is no automatic

keyboard buffer. This omission can,

however, be rectified by dedicated

software. A suitable program for

reading a 10-key keyboard con-

nected to analogue input pin ADC7

of the ADuC812 is provided in pro-

gram ana_tast.c . When downloading

this program you will also find a

table showing the relationship

between the button pressed, the

input voltage U in and the converted

digital value, safe push button read-

ing interval and button priority level

(0=highest priority).

Interval: 20 ms / 8 = 2.5 ms (Timer 2)

Servo pulse width: 0.9 ms to 2.1 ms (Timer 0)

In the Timer 2 interrupt service routine

Timer 0 is started with the pulse width

required for the servo, and the appropriate

pin of port 2 is set. In the Timer 0 interrupt

service routine all port 2 pins are cleared and

the timer stopped.

In order to achieve exact timing in the main

program, an extra software delay is employed

in the Timer 2 interrupt service routine. This

short program is readily tested using the

ADuC812 simulator ( Figure 8 ).

Of course, applications for the ADuC812 are

limited only by your imagination. With a little

effort on the software side up to 24 servos

could be controlled using the 24 digital out-

puts. When used in combination with the I 2 C

interface we can even construct a replace-

ment ‘I 2 C Servo Interface’ ( Elektor Electron-

ics , September 2001) — and our replacement

offers a resolution of more than 1000 steps.

The servos could also be controlled by sen-

sors via the eight analogue inputs — which

is exactly what this series (ISAC — Intelli-

gent Sensor/Actuator Controller) is about.

(010048-4)

Component source:

MEGATRON Bauelemente,

Hermann-Oberth-Strasse 7,

D-85640 Putzbrunn, Munich, Germany.

Tel.: +49 89 46094 146

Figure 8. servo.hex in the ADuC812 simulator (pulses output for servo 5 on

pin P2.5).

1/2002

Elektor Electronics

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: