AN1979

Sensors

5

Freescale Semiconductor

Multiplexing of the microcontroller output pins allows

communication of the LCD to be accomplished with three pins

instead of eight or 11 I/O pins usually required. With an eight-

bit shift register, we are able to manually clock in eight bits of

data. The enable line, EN, is manually enabled when eight

bytes have been shifted in, telling the LCD the data on the data

bus is available to execute. The LCD will only be written to and

the contrast can be held at a constant brightness, allowing the

read/write and the VEE bits to be held low, also minimizing

additional I/O lines.

OTHER

This system is designed to run on a 9.0 V battery. It

contains a 5.0 V Regulator to provide a 5.0 V supply to the

pressure sensor, microcontroller, and LCD. The battery is

mounted on the back of the board using a space saving spring

battery clip.

ALTIMETER/BAROMETER SOFTWARE

This application note describes the software version that

was available during publication. However updated software

versions may be available with further functionality and menu

selections. Check our website update for updates to Sensor

Products Reference designs.

Software User Instructions

When the system is turned on or reset, the microcontroller

will flash the select LED and display the program title on the

LCD for five seconds or until the select (SEL) button is

pushed. Then the menu screen is displayed. Using the select

(SEL) push button, the user can scroll through the menu

options for a software program. To run the altimeter program,

use the (SEL) select button to high-light the “Alti/Barometer”

option, then press the enter (ENT) push button. The Altimeter

program will display current barometric pressure reading, the

calculated altitude in feet, a message displaying a simple

weather prediction such as “sunny”, “rainy”, “steady” without a

pressure change, and “history” before enough history is

collected to make a prediction. In the top right corner of the

display, a scrolling graphical history displays data points

representing the past forty pressure readings.

Calibration and Calibration Software

There are two forms of calibration for this system. The first

calibration is used for the barometer part of the system. This

calibration was already done before you received the

reference design and only needs to be done once per system.

To calibrate the barometer module, a two-point calibration is

performed using a highly accurate pressure generator. The

system takes a calibration point at 64 kPa and another at 105

kPa. Holding down both the SEL and ENT buttons on system

power-up will put the system into calibration mode. At this

point, the calibration menu will be displayed with the

previously sampled offset voltage. To recalibrate the system,

apply a pressure of 64 kPa and press the SEL button (PB1).

This A/D value is then saved to a location in the

microcontroller memory. To obtain the second calibration

point, using the accurate pressure generator apply a pressure

of 105 kPa directly to the sensor. Then press the ENT button

(PB2). This signal is similarly sampled, averaged and saved to

Table 3. Parts List

Ref

Qty.

Description

Value

Vendor

Part No.

U3

1

Pressure Sensor

1.0

Freescale

MPXM2102A

C1

1

0.1 µF

Generic

C2

1

Op-Amp Cap

0.1 µF

Generic

C3

1

Shift Register Cap

0.1 µF

Generic

D1

1

Red LED

—

Generic

D2

1

Green LED

—

Generic

S2, S3

1

Push buttons

—

Generic

U1

1

Microcontroller

8-Pin

Freescale

MC68HC908QT4

U2

1

16x2 B&W LCD

16x2

Seiko

L168200J000

U4

1

Shift Register

—

Texas

74HC164

U5

1

Voltage Regulator

5.0 V

Fairchild

LM78L05ACH

U6

1

Quad Op-Amp

—

ADI

AD8544

R1, R4

1

1/4 W Resistor

10 K

Generic

R2, R3

2

1/4 W Resistor

1.0 K

Generic

R5, R6

2

1/4 W Resistor

3.65 K

Generic

R7, R8

2

1/4 W Resistor

10 K

Generic

R9, R11

2

1/4 W Resistor

1.0 K

Generic

R10, R12

2

1/4 W Resistor

200 K

Generic