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AN2577: DescriptionUsing the eTPU angle clock; File Size237KB，34 Pages; ManufacturerSTMicroelectronics; Websitehttp://www.st.com/

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AN2577 Overview

Using the eTPU angle clock

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AN2577

Application note

Using the eTPU angle clock

Introduction

This document describes the angle clock feature of the enhanced time processor unit and

offers practical advise on the application and use of this feature. The note assumes a

familiarity with the eTPU and the eTPU_C language used in the byte craft compiler.

The eTPU is particularly suited to applications where input and output actions are

synchronized to the position of a rotating wheel. The eTPU angle clock provides hardware

which, when combined with the proper software, can track the wheel angle through

accelerations, decelerations, and a wide range of speeds.

If a wheel is tracked by the eTPU angle clock, many output matches and input captures can

operate on an angle count as well as a time count, and even on combinations of the two.

Compensation for speed changes are made in the angle clock only. An angle triggered

event will occur on the best estimate of the match angle.

The angle clock is complex to set up, but when running it offers an excellent integration of

fast operating hardware with the complex control of a software program. A reference design

has been built to demonstrate the operation of a periodic angle clock with other engine

control functions. See your STMicroelectronics representative for details.

September 2013

Rev 2

1/34

www.st.com

Contents

AN2577

Contents

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

eTPU angle clock hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1

2.2

2.3

2.4

2.5

Tooth wheel input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Tick counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Missing teeth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Zero count synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Error recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

System design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1

Steady state system design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.1

3.1.2

3.1.3

3.1.4

3.1.5

3.1.6

Periodic angle clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Free running angle clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Angle resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Reference point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Missing teeth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Tick rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2

Acceleration and deceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.1

3.2.2

Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Deceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3

Startup system design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.3.1

3.3.2

3.3.3

Initial parameter values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Acquisition and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Starting the angle clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4

Anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4.1

3.4.2

Error prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Error recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Inserting a tooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Deleting a tooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

3.5

Tooth circuit failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

eTPU software design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.1

Host setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2/34

AN2577

Contents

4.2

State machine description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.2.1

4.2.2

4.2.3

4.2.4

4.2.5

4.2.6

4.2.7

4.2.8

4.2.9

4.2.10

4.2.11

4.2.12

Initialize crank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

First edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

First period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Testing for a possible gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Apparent gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Possible gap verifying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Gap verified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Gapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

New revolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Adjust angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Stall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Angle clock application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.1

5.2

5.3

Free running angle counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Periodic angle clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Angle math . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3/34

Overview

AN2577

Overview

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.

4/34

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