AN2897 Freescale Semiconductor / Motorola, AN2897 Datasheet - Page 2

• Manufacturer Part Number: AN2897
• Description: Using the eTPU Angle Clock
• Manufacturer: Freescale Semiconductor / Motorola

Overview
asynchronous external signal, by a real time clock with a different time base, or by special Angle Clock
circuitry on the eTPU. The purpose of the Angle Clock is to synchronize with the angle of a spinning shaft
and provide a counter representation of the instantaneous position of the shaft. If the shaft provides an
input signal related to teeth spaced along the circumference, the Angle Clock extrapolates the number of
ticks between the adjacent pairs of teeth, thereby providing a finer resolution of angle than the shaft signal
alone can provide. The Angle Clock was designed for automotive engine control, but finds application in
a variety of rotational devices.
An Angle Clock can be implemented in hardware or in software, but either implementation has limitations.
A software angle clock cannot divide an input tooth signal into very fine ticks without overwhelming the
processor. Even before the tick rate reaches its limit, latency on the processor will begin to distort the ideal
count pattern. A hardware angle clock can provide very fine resolution of the ticks, but the hardware
becomes complex and large when the logic is required to handle errors and exceptional input signals.
The eTPU Angle Clock has been implemented as a software supported hardware subsystem, effectively
eliminating both of these significant limitations. The hardware subsystem has provisions for fast tick
counting plus compensation for missing teeth, which are often used to provide an angle key for the shaft.
The hardware also provides automatic compensation for acceleration and deceleration, and assists in error
recovery. The software handles synchronization and exception handling.
Toothed Wheel ‘36 - 1’
Sensor
35 Teeth Spaced 10º
Figure 1. Toothed Wheel
Using the eTPU Angle Clock, Rev. 0
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Freescale Semiconductor