AN1979

Sensors

Freescale Semiconductor

4

Table 2 shows the theoretical maximum resolution that this

reference design can achieve. However, factors such as noise

within the circuit, sensitivity of the sensor, and voltage offsets

in the amplification scheme should be taken into

consideration. Accommodating for these factors in the

software can filter out some of these factors.

Further testing is required to determine the accuracy of the

reference design without the limiting A/D converter.

DISPLAY

The display of the barometric pressure, barometric

pressure history, current calculated altitude, and a simple

weather prediction is displayed on a 16x2 LCD.

Figure 4. Barometric Display

Due to the limited number of bi-directional data pins on the

microcontroller, a system was designed to serially buffer the

display data. Using a shift register to hold display data, the

LCD is driven with only three lines of output from the

microcontroller: an enable line, a data line, and a clock signal

while the two LEDs are multiplexed with the data line and

clock signal.

Figure 5. Multiplexed LCD Circuit

Table 2. Microcontroller Accuracy Comparisons

Z (ft)

P (kPa)

V (mV)

Amp scheme

Vamp (mV)

Vamp – 1 bit

P0

Px

∆Z (m)

∆Z (ft)

Micro

-1000

105

20.265

(Vx–12.8)*650

4852.3

4832.6

20.265

20.235

12.66

41.54

8 bits

12000

64.259

12.852

33.8

14.2

12.852

12.822

19.97

65.53

8 bits

-1000

105

20.265

(Vx–12.8)*650

4852.3

4847.4

20.265

20.257

3.15

10.35

10 bits

12000

64.259

12.852

33.8

28.9

12.852

12.844

4.97

16.32

10 bits

-1000

105

20.265

(Vx–12.8)*650

4852.3

4851

20.265

20.263

0.79

2.59

12 bits

12000

64.259

12.852

33.8

32.6

12.852

12.85

1.24

4.08

12 bits

-1000

105

20.265

(Vx–12.8)*650

4852.3

4852.2

20.265

20.265

0.05

0.16

16 bits

12000

64.259

12.852

33.8

33.7

12.852

12.852

0.08

0.25

16 bits

101.3kPam

+1170 ft

SEL

ENT

FREESCALE

Select Button

Enter Button

steady

_-------

-----_----- -----_

Barometric Pressure

History

Simple Weather

Prediction

Altitude

(must calibrate)

Barometric Pressure

EN

RS

RW

D80

D81

D82

D83

D84

D85

D86

D87

VEE

LCD

A

B

CLK

HC164

R3

1 K

R2

1 K

PTA3

PTA4

PTA5

HC908QT4