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AN2618: DescriptioneTPU host interface; File Size273KB，23 Pages; ManufacturerSTMicroelectronics; Websitehttp://www.st.com/

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eTPU host interface

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AN2618

Application note

eTPU host interface

Introduction

The eTPU is the new generation of Time Processing Unit (TPU). Besides the hardware

enhancement, significant improvements have been made to the accompanying software

development tools; these tools make the enhanced Time Processing Unit (eTPU) easy to

use. A high level language (C) compiler has been developed to allow the user to program

the eTPU by using C language instead of microcode.

To program the eTPU effectively, the programmer still needs to have a clear understanding

of how the eTPU hardware works. Coding in C, the programmer can focus more on the

application logic and leave the mechanics of the eTPU programming to the compiler (i.e.,

help of the eTPU simulator and debugger, eTPU software can be developed much like the

software for the host CPU. Productivity of software development can be significantly

improved.

The introduction of the eTPU C compiler also changes the way the host interfaces to the

eTPU functions. With the help of the compiler, the same symbol can be referenced by the

both eTPU and host software. The host software can interface with eTPU functions via API

functions, instead of accessing physical memory locations and registers. For each eTPU

function, a host interface API function can now be developed as a part of the eTPU C

program. The host application can call these API functions to interface with the eTPU. The

references to these API functions and symbols for parameters are resolved at compile time.

The implementation details of the eTPU functions are hidden from the host application. This

design improves the flexibility of the eTPU functions’ implementation and the portability of

the host application code. This application note discusses how to build the host interface for

eTPU functions.

This application note shows how to build the host interface to access eTPU functions. The

eTPU PWM driver is used as an example to illustrate what the host needs to do to configure

eTPU module, channel and initialize PWM function. The appRev 2lication note also

describes the details of how to export eTPU software information to the host compiler. The

working code example is presented in the Appendix. The user can compile both host and

eTPU code, then download to actual hardware for testing.

September 2013

Rev 2

1/23

www.st.com

AN2618

Overview

Host interface software adds another layer of abstraction between the host CPU and eTPU.

The host interface API functions hide the complexity of the interaction between the host

CPU and eTPU, providing a simple interface for host applications. Ideally, every eTPU

function shall have one or more host interface API functions.

The interface software between host and eTPU facilitates three major tasks:

eTPU hardware initialization – configure eTPU peripheral hardware

eTPU function initialization – pass initial parameters and initiate function execution

eTPU function run time interactive control – update function parameters and handle

handshaking

Once the eTPU peripheral and eTPU functions are initialized, each eTPU function can start

to execute with initial function parameters. The host interface API functions need to be

provided for interactive control (i.e. parameters updating and control mode changing etc.).

To update the parameter or change control mode, the host is responsible for passing

updated parameters to the eTPU functions, and then informing the eTPU that the function

parameters have changed. If a coherent change of function parameters is required, the logic

has to be built in the eTPU functions to ensure the coherency. For some eTPU functions, the

interaction between host and eTPU is an essential part of the operation. In both host and

eTPU software, the logic is needed to handle the handshaking between host and eTPU. The

interaction between the eTPU and host can be encapsulated in the host interface API

functions.

The host code and eTPU code are compiled by different compilers. The host compiler is

normally used to build a single code image for both host and eTPU. In order to build eTPU

code and host code together, the eTPU software building information (i.e. eTPU code

image) has to be exported to the host compiler. The symbol information has to be exported

from the eTPU compiler to the host compiler as well. For the Byte Craft eTPU compiler, the

mechanism is implemented as a set of host interface macros. In the eTPU code, these

macros are inserted to generate proper executable and symbol information for the host

compiler.

The host interface design process is illustrated in the eTPU PWM driver examples in the

Appendices.

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eTPU and host interface hardware

AN2618

eTPU and host interface hardware

The host interface hardware for the eTPU is shown in

Figure 1.

The host and eTPU can

communicate to each other via event or by data.

The host has access to all eTPU host interface registers. When the host wants the eTPU’s

services, it can issue the host service request (HSR) by writing the eTPU channel control

registers. Once the host service request is acknowledged, a thread of eTPU code

associated with this HSR is activated for execution. The eTPU code in the thread

implements the functions requested by the host. When eTPU needs the host service, it can

issue the interrupt request or DMA data transfer request, or generate a global exception.

The events handling logic is needed in the host software to provide services to the eTPU

corresponding to these requests.

The eTPU code memory (RAM) and data memory (RAM) are accessible by both host and

eTPU. eTPU code memory stores the eTPU executable binary image. At the power up

initialization, following the defined sequences, the host transfers the eTPU code stored in

the flash memory to the eTPU code memory. During the execution, the eTPU micro-engine

fetches the micro-instructions from the code memory.

Figure 1.

eTPU host interface hardware

The eTPU data memory is implemented by using tri-port RAM. It provides for data sharing

between the host and eTPU engines. Both host and eTPU can read and write the eTPU

data memory in any data size (8, 16, 24, and 32 bit). Since the eTPU data bus is 24 bits

wide, it is best to pack together a data word with an 8 bit value and a 24 bit value in order to

reduce data memory utilization. To reduce the overhead of packing and unpacking the data,

a virtual mirror memory space of the normal eTPU data memory has been created. The

virtual memory space is called Parameter Sign Extension (PSE) memory space. When the

host reads the 32-bit data from the PSE memory space, it will return the sign extended 24-

bit data. The conversion is needed if the 24-bit data is unsigned. Similarly, when the host

writes the 32-bit data to the eTPU PSE memory, only the three least significant bytes will be

written to the eTPU data memory. The most significant byte is untouched.

4/23

AN2618

Host interface software

The function partition between the application software and low-level driver is defined during

the software architecture design. To make the software portable, the application software is

normally designed to interface with the low-level driver via abstracted interface APIs. The

host interface of the eTPU functions should be designed such that the implementation of the

low-level driver is hidden from the application software.

Once the application software interface to the low-level driver is defined, the functionality of

the low-level driver can be partitioned between host CPU and eTPU. The eTPU interface

software running on the host CPU is an integral part of the low-level driver. It handles the

details of the interaction between host and eTPU.

The eTPU code and host interface code are compiled by two different compilers. In order to

resolve the eTPU code symbol reference in the host code, it is necessary to export the

symbolic information from the eTPU compiler to the host compiler. Similarly, to build a single

executable image, it is necessary to export eTPU code image to the host compiler. Several

types of information are exported from eTPU_C through host interface files, generated from

#pragma write

statements during compilation. For detailed information on

#pragma

write,

see the eTPU_C documentation.

An example of application software and low-level driver partition is shown in

Appendix A on

page 11.

3.1

Initialization overview

The eTPU initialization is an important part of the host interface. At the power up, the host

has to configure the eTPU peripheral properly before the eTPU function can be executed.

The eTPU initialization process is accomplished by both host CPU and eTPU. During the

initialization, the functional partition between host CPU and eTPU is as follows:

Host responsibility:

●

eTPU module initialization

eTPU channel configuration

Providing initial eTPU function parameters

Initiating the eTPU function execution

Responding to the HSR

Transitioning into the initialization state

eTPU responsibility:

●

The eTPU initialization can be broken down into three steps: eTPU module initialization,

eTPU channel initialization and eTPU function initialization. The following sections discuss

the interface design and the data exchange between host CPU and eTPU for each step of

initialization.

3.2

eTPU module initialization

At the power up initialization, the eTPU peripheral hardware is configured by the host. The

eTPU module initialization includes the following steps:

5/23

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