AN780

DS00780A-page 1

© 2002 Microchip Technology, Inc.

15-Kilogram Scale Using the TC500A and the TC520

FIGURE 1: Functional block diagram.

INTRODUCTION

A 15kg weighing scale was designed using Microchip's TC500A

Analog Processor and the TC520 16-bit Controller. The scale is

required to resolve down to 1/8 gram and correct to within 61/2

gram. This project takes into account all aspects of a functional scale:

•

Dynamic Range

•

Strain Gauge Compensation

•

Zeroing

•

Oversampling

•

Units Conversion (kilograms to pounds)

The TC500A is an analog processor device which performs a dual-

slope analog-to-digital conversion function. All of the counting and

timing for the conversion must be controlled by an external source.

In nearly all applications, this control source is a microprocessor.

The microprocessor is programmed to monitor the status and to

control the timing of the TC500A. It must also be programmed to

count the conversion results.

Author:

Ted Dabney,

Microchip Technology, Inc.

The TC500A has no agenda of its own so it can be used to generate

slow, high resolution conversions or fast, low resolution conver-

sions. The trade-off for accuracy is about 1000 counts per millisec-

to 60mS. Typically, the total conversion time is about 4 times the

integration time but, with the TC500A, this is quite flexible.

eq1

The TC520 is a digital interface device which can be used to

replace all of the TC500A timing and counting functions performed

by a microprocessor. The TC520 can use either a crystal or

external clock as a time-base to control the operation of either a

TC500 or a TC500A.

Level

Shift

Control Logic

Analog

Switch

Control

Signals

2

16

Analog

Common

+

+

–

BUF

Buffer

Integrator

–

Comp 1

Comp 2

Comp

Output

14

GND

15

11

5

10

1

3

4

6

8

9

7

12

13

B

Control Logic

TC500/TC500A

+

+

–

+

–

+

+

–

–

+

Phase

Decoding

Logic

Polarity

Detection

–

+

A

B Converter State

0

0 Zero Integrator Output

0

1 Auto-zero

1

0 Signal Integrate

1

1 Deintegrate

+

A