MPXM2102AS datasheet(128/670 Pages) MOTOROLA

128 / 670 page

AN1635

2–92

Motorola Sensor Device Data

www.motorola.com/semiconductors

The bulk of the circuit hardware is contained in a display box

mounted on the top front side of the cage. Since the accel-

erometers are physically located far away from the mother

board (about 10 feet of wiring), op–amps were used to buffer

the accelerometers’ output and drive the transmission line.

The four accelerometer signals are then simultaneously fed

into a comparator network and four of the ADC inputs on an

MC68HC11 microcontroller. The MC68HC11 was selected

because it has the capability of converting four A/D channels

in one conversion sequence and operates at a higher clock

speed. These two features reduce the overall time interval

between digitizations of the analog signal (that result from the

minimum required time for proper A/D conversion and from

software latency) thus allowing a more accurate representa-

tion of the acceleration waveform to be captured. The

comparator network serves a similar purpose by eliminating

the additional software algorithm and execution time that

would be required to continually monitor the outputs of all four

accelerometers and determine whether impact has occurred

or not. By minimizing this delay (some is still present since the

output signal must exceed a threshold, and a finite amount of

time is required for this) more of the initial and more significant

part of the signal is captured.

The comparator network employs four LM311’s configured

to provide an OR function, and a single output is fed into an

input capture pin on the MCU. A potentiometer and filter

capacitor are used to provide a stable reference threshold

voltage to the comparator network. The threshold voltage is

set as close as possible to the accelerometers’ offset voltage

to minimize the delay between ball impact and the triggering

of the conversion sequence, but enough clearance must be

provided to prevent false triggering due to noise. Because the

comparator network is wired such that any one of the accel-

erometer outputs can trigger it, the threshold voltage must be

higher than the highest accelerometer offset voltage. Hystere-

sis is not necessary for the comparator network, because

once the MCU goes into the conversion sequence it ignores

the input capture pin.

The system is powered using a commercially available 9 V

supply. A Motorola MC7805 voltage regulator is used to pro-

vide a steady 5 Volt supply for the operation of the MCU, the

accelerometers, the comparator network, and the op–amp

buffers. The 9 V supply is directly connected to the common

anode 8–segment LED displays. Each segment can draw as

much as 30 mA of current. Therefore, to ensure proper opera-

tion, the power supply selected to build this circuit should be

capable of supplying at least 600 mA. Ports B and C on the

MCU are used to drive the LED displays. Each port output pin

is connected via a resistor to the base of a BJT, which has the

emitter tied to ground. A current limiting resistor is connected

between the collector of each BJT and the cathode of the

corresponding segment on the display. To minimize the

amount of board space consumed by the output driving cir-

cuitry, MPQ3904s (quad packaged 2N3904s) were selected

instead of the standard discrete 2N3904s. The zero bit on Port

C is connected to a combination BJT and MOSFET circuit that

drives the “Your Speed” and “Best Speed” LED’s. The circuit

is wired so that the LED’s toggle, and only one can be ON at

a time.

Figure 4 shows a schematic of the circuit used. Part (a)

shows the accelerometers, the op–amps used to buffer the

outputs and drive the transmission lines, the comparator net-

work and the potentiometer used to set the detection thresh-

old. Part (b) shows the MCU, with its minimal required

supporting circuitry. Part (c) shows the voltage regulator, a

mapping of the cathodes to the corresponding segments on

the LED displays, the BJT switch circuitry used to drive the

seven segment display LEDs (although not shown on the

schematic, this circuit block is actually repeated 15 times), and

finally, the circuitry used to drive the “Your Speed”/“Best

Speed” LEDs.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 128 Page - Motorola, Inc

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