AN2577 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

AN2577

Application note Using the eTPU angle clock

STMicroelectronics 

Overview

1

Overview

AN2577

The enhanced time processor unit (eTPU) is an autonomous slave processor offered on

several STMicroelectronics microcontrollers designed for real time automotive

applications.The processor is tightly coupled to up to 32 channels, each associated with an

input and an output signal. The input/output channels each have a pair of Match and

Capture units interfaced to one of two timer/counter (TCR) registers. Logic in the channel

enabled the hardware to detect or drive pin transitions with a high degree of timing

precision.

In a typical application, at least one of the TCRs is driven by a real time clock derived from

the MCU clock frequency. The second TCR can be driven by an 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.

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