A Performance Analysis System for the Sport of Bowling

2.6 HARDWARE/SOFTWARE INTERACTION DESIGN CONSIDERATIONS
There are a number of areas where the design decisions for the hardware and the
embedded software are interdependent. These issues are presented next.
2.6.1 Microprocessor Clock Frequency
The clock frequency directly affects the current drawn by the microprocessor: the lower
the clock frequency, the less current the 87C752 draws from the battery. The lowest
standard oscillator frequency that supports the sample rate, the serial communications
baud rates, and the processing demands of the application is 3.6864 MHz. The internal
clock frequency for the 87C752 is 1 / 12 of the oscillator frequency, resulting in an
instruction time of 3.26 µsec (307,200 instructions per second) for a 3.6864 MHz crystal.
Quartz crystals and ceramic resonators are available in standard frequencies for
microprocessors. Crystals easily can maintain an accuracy (over their temperature range)
of 200 ppm (.03%), while ceramic resonators (cheaper and smaller) typically run in the 1-
2% range. During the feasibility phase, a crystal has been used to enhance the accuracy
of the sample clock, with the hope of switching to a ceramic resonator, in combination
with some clock calibration software, as part of the development phase.
2.6.2 Sampling Rate
The 87C752 microprocessor contains an 8-bit ADC with a conversion time of 130 µsec
(~7600 Hz) at the clock frequency of 3.6864 MHz. It is important to minimize the
sample rate, since the amount of data collected directly affects the amount of external
data memory required. Complicating matters, EEPROM technology has a fairly slow
write speed (5-10 msec per page write), thus presenting the possibility that the rate of
writing data to memory might not keep pace with the production of data to be written.
Also, it is desirable to run the microprocessor in IDLE mode as much as possible, since it
cuts the current draw in half, and the light sensor (TSL251) and the '752's on-board ADC
can be turned off between sample times. The higher the sample rate, the more time the
microprocessor must spend collecting and processing data, which shortens the life of the
battery.
Data must also be collected from the impact sensor, but this is a different process. An
impact can occur asynchronous to the time of a light sample, with a much shorter
duration than the sample period. Therefore, impact detection is interrupt driven. By
limiting the stored value of the light samples to the range 0-127 (0x00-0x7F), the impact
data for a sample can be stored in the high-order bit of the sample byte, thus associating
each impact with the light sample during which the impact occurred.
The primary factors in determining an appropriate sampling rate are the maximum
angular velocity (rpms), the minimum and maximum linear velocities (ball speed), and
the amount of memory available for the storage of captured data.
• Maximum RPMs: According to tests conducted by the ABC (American Bowling
Congress), the best bowlers, those that average greater than 210, generate an
average rotational rate of about 300 rpms (5 Hz) at release. In fact, the ABC
found a direct correlation between average score and angular velocity [12].
There is an upper bound on the number of revolutions the ball can make. If the
ball did not skid at all, it would travel 27 inches per revolution, while revolving
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