TC820CLW Просмотр технического описания (PDF) - Microchip Technology

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производитель

TC820CLW

3-3/4 digit A/D converter with frequency counter and logic probe

Microchip Technology 

TC820

FIGURE 5-4:

5pF

SUGGESTED CRYSTAL

OSCILLATOR CIRCUIT

TC820

10pF

37

40kHz

38

39

470kΩ

22MΩ

Where low cost is important, the RC circuit of Figure 5-5

can be used. The frequency of this circuit will be

approximately:

EQUATION 5-1:

TOSC =

0.3

RC

FIGURE 5-5:

5pF

RC OSCILLATOR CIRCUIT

TC820

10pF

37

38

39

110kΩ

75pF

Typical values are R = 10kΩ and C = 68pF. The resis-

tor value should be ≥ 100kΩ. For accurate frequency

measurement, an RC oscillator frequency of 40kHz is

required.

5.5 System Timing

All system timing is derived from the clock oscillator.

The clock oscillator is divided by 2 prior to clocking the

A/D counters. The clock is also divided by 8 to drive the

buzzer, by 240 to generate the LCD backplane fre-

quency, and by 40,000 for the frequency counter time-

base. A simplified diagram of the system clock is

shown in Figure 5-6.

FIGURE 5-6:

RC

Oscillator

Components

SYSTEM CLOCK

GENERATION

TC820

XTAL

Oscillator

Components

OSC1

OSC2

OSC3

A/D

Counter

÷2

Buzzer

LCD

Backplane

Driver

Counter

Time-Base

÷8

÷ 240

÷ 40,000

5.6 Component Value Selection

5.6.1

AUTO-ZERO CAPACITOR - CAZ

The value of the auto-zero capacitor (CAZ) has some

influence on system noise. A 0.47µF capacitor is rec-

ommended; a low dielectric absorption capacitor

(Mylar) is required.

5.6.2

REFERENCE VOLTAGE

CAPACITOR - CREF

The reference voltage capacitor used to ramp the inte-

grator output voltage back to zero during the reference

integrate cycle is stored on CREF. A 0.1µF capacitor is

typical. A good quality, low leakage capacitor (such as

Mylar) should be used.

DS21476B-page 18

© 2002 Microchip Technology Inc.

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