MPXM2102AS datasheet(103/670 Pages) MOTOROLA

103 / 670 page

AN1611

2–67

Motorola Sensor Device Data

www.motorola.com/semiconductors

HARDWARE DESCRIPTION AND OPERATION

Since MMA2200W is fully signal–conditioned by its internal

op–amp and temperature compensation, the output of the

accelerometer can be directly interfaced with an analog–to–

digital (A/D) converter for digitization. A filter consists of one

RC network should be added if the connection between the

output of the accelerometer and the A/D converter is a long

track or cable. This stray capacitance may change the position

of the internal pole which would drive the output amplifier of the

accelerometer into oscillation or unstability. In this design, the

cut–off frequency is chosen to be 15.9 kHz which also acts as

an anti–alias filter for the A/D converter. The 3dB frequency

can be approximated by the following equation.

–3dB +

πRC

Referring to the schematic, Figure 3, the MMA2200W

accelerometer is connected to PORT D bit 5 and the output of

the amplifier is connected to PORT D bit 6 of the micro-

controller. This port is an input to the on–chip 8–bit analog–to–

digital (A/D) converter. Typically, the accelerometer provides

a signal output to the microprocessor of approximately 0.3 Vdc

at –55g to 4.7 Vdc at +55g of acceleration. However, Motorola

only guarantees the accuracy within

±40g range. Using the

same reference voltage for the A/D converter and accelerom-

eter minimizes the number of additional components, but does

sacrifice resolution. The resolution is defined by the following:

count

Vout

255

The count at 0g = [2.5/5]

255

∝ 128

The count at +25g = [3.5/5]

255

∝ 179

The count at –25g = [1.5/5]

255

∝ 77

Therefore the resolution 0.5g/count

The output of the accelerometer is ratiometric to the voltage

applied to it. The accelerometer and the reference voltages

are connected to a common supply; this yields a system that

is ratiometric. By nature of this ratiometric system, variations

in the voltage of the power supplied to the system will have no

effect on the system accuracy.

The liquid crystal display (LCD) is directly driven from I/O

ports A, B, and C on the microcontroller. The operation of a

LCD requires that the data and backplane (BP) pins must be

driven by an alternating signal. This function is provided by a

software routine that toggles the data and backplane at

approximately a 30 Hz rate. Other than the LCD, one light

emitting diode (LED) are connected to the pulse length

converter (PLM) of the microcontroller. This LED will lights up

for 3 seconds when an impact greater or equal to 7g is

detected.

The microcontroller section of the system requires certain

support hardware to allow it to function. The MC34064P–5

provides an undervoltage sense function which is used to

reset the microprocessor at system power–up. The 4 MHz

crystal provides the external portion of the oscillator function

for clocking the microcontroller and provides a stable base for

time bases functions, for instance calculation of pulse rate.

SOFTWARE DESCRIPTION

Upon power–up the system, the LCD will display CAL for

approximately 4 seconds. During this period, the output of the

accelerometer are sampled and averaged to obtain the zero

offset voltage or zero acceleration. This value will be saved in

the RAM which is used by the equation below to calculate the

impact in term of g–force. One point to note is that the

accelerometer should remain stationary during the zero

calibration.

Impact

+ [count * count

offset ]

resolution

In this software program, the output of the accelerometer is

calculated every 650

µs. During an impact, the peak decelera-

tion is measured and displayed on the LCD for 3 seconds

before resetting it to zero. In the mean time, if a higher impact

is detected, the value on the LCD will be updated accordingly.

However, when a low g is detected (e.g. 1.0g), the value will

not be displayed. Instead, more samples will be taken for

further averaging to eliminate the random noise and high

frequency component. Due to the fact that tilting is a low g and

low frequency signal, large number of sampling is preferred to

avoid unstable display. Moreover, the display value is not hold

for 3 seconds as in the case of an impact.

Figure 4 is a flowchart for the program that controls the

system.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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