BCI Capnocheck Plus - Timing and Sensor Circuits

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Chapter

3:

Capnocheck

Plus

Circuit

Description

Real

Time

Clock.

The

Real

Time

Clock

U11

is

a

system

“watch”.

It

is

powered

from

either

the

main

+5VDC

or

the

Lithium

Battery

BT1

when

the

Monitor

is

turned

off.

Switching

between

main

and

battery

power

is

performed

by

the

watch

dog

timer

U19.

Crystal

X2

is

directly

connected

to

U11

to

provide

a

very

accurate

time

reference.

The

Real

Time

Clock

IC

can

be

set

and

read

by

the

microprocessor

through

serial

communications.

The

data

line

coming

to

U11

is

by-directional,

and

is

used

for

both

data

in

and

out.

Transistor

Q6

and

resistor

R36

convert

two

separate

input

and

output

serial

data

signals

into

one

bi-

directional

data

signal

using

open

collector

with

passive

pull

up

approach.

Diode

D11

is

used

for

ESD

protection.

Watch

Dog

Timer

Watch

Dog

Timer

IC

U19

is

used

to

perform

the

following

three

functions:

«

Its

internal

comparator

is

used

in

the

ON/OFF

circuitry

(see earlier);

*

Itswitches

V-RAM

from

the

main

+5VDC

power

to

3VDC

Lithium

Battery

BT1,

providing

power

for

both

RAM

and

the

Real

Time

Clock;

*

It

resets

the

microprocessor

at

power

up;

+

Itresets

the

microprocessor

if

signal

WDI

is

not

generated

for

a

long

time,

which

indicates

that

microprocessor

was

upset

by

ESD

discharge

and

is

not

running

properly.

Analog

Section

CO,

Sensor

The

analog

section

is

used

to

acquire

data

from

sensors

and

read

battery

voltage.

Analog

-

to

-

Digital

Converter

(ADC)

Ul4

can

convert

up

to

eight

input

voltages

in

the

range

of

0

to

VREF.

Signal

VREF

is

generated

by

the

voltage

reference

U16

and

equals

2.50VDC.

ADC

communicates

to

the

microprocessor

serially.

The

CO;

Sensor

(or

CO2

Bench)

generates

two

output

voltages.

One

indicates

CO,

concentration,

and

the

other

is

proportional

to

the

CO2

detector’s

temperature.

The

light

source

inside

the

Bench

is

precisely

controlled

by

the

current

sink

circuitry

built

around

U15B

and

the

0.1%

accurate

resistor,

R45.

The

voltage

across

R45

is

maintained

equal

to

VREF,

hence

the

current

through

resistor

is

very

stable

and

accurate.

Capnocheck

Plus

Service

Manual

-

3-7

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