Motorola ONCE SC140 Montage- und Bedienungsanleitung

AN2090/D

Rev. 0, 11/2000

Using the SC140 Enhanced OnCE™

Stopwatch Timer

Application Note

Kim-Chyan Gan

and Yuval Ronen

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Abstract and Contents iii

In software application development on embedded devices, optimization is critical. Optimized code

provides not only faster processing speed but also lower power consumption and longer battery life.

Optimized code also minimizes CPU load, enabling more applications to fit into a single chip. In selecting

a processor for a system, it is important to understand and compare the speed of execution of different

processors. All of these applications may require a stopwatch as a criterion for measurement. This

application note presents the stopwatch timer application developed on the Software Development

Platform (SDP). The stopwatch timer is implemented using the Enhanced OnCE™resource, which is

non-intrusive to SC140 operation, therefore, timing can be correctly measured. Two stopwatch techniques,

with and without source code modification, are presented. This application note also explains how to port

stopwatch applications to other SC140 based devices.

1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2SC140 Enhanced OnCE Stopwatch Timer Capabilities . . . . . . . . . . . . . . . . . . . 2

2.1 Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.3 Implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3Setting Up the Stopwatch Timer Within an Application . . . . . . . . . . . . . . . . . . . 3

3.1 Initializing the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.2 Starting the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.3 Stopping the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.4 Converting Cycles to Actual Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.5 Putting it All Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.6 Adapting Stopwatch Timer Code to Other SC140 Devices . . . . . . . . . . . . . . . . . . . . . . . . 8

4Setting Up the Stopwatch Timer Within the Debugger . . . . . . . . . . . . . . . . . . . . 8

4.1 Initializing the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.2 Stopping the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5Setting Up the System Clock Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1 Setting Up the PLL in Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Setting Up the PLL in Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6Verifying Correct Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.1 Using the LED on the SDP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.2 Testing the Stopwatch Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Abstract and Contents

iv Using the SC140 Enhanced OnCE Stopwatch Timer 

7Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Appendix A

Complete Example of Profiling

Introduction 1

1Introduction

A stopwatch timer is an apparatus for measuring the exact duration of an event. Measuring time during

execution of code on a Digital Signal Processor (DSP) is useful to identify opportunities for code

optimization, to understand system loading, and to compare execution speeds of different processors.

This application note presents techniques to implement a stopwatch timer on the StarCore, SC140, using

the built-in features of the DSP’s Enhanced On-Chip Emulation (OnCE) module.

Although many devices with embedded DSPs provide application-specific timer blocks, these timers

cannot be used for debugging purposes without interference from the application itself. The ability to use

the Enhanced OnCE as a stopwatch timer allows for non-intrusive timing capabilities and is available on

any system that integrates the SC140 core.

Two techniques to set up the stopwatch timer are described in this application note:

1. Setting up the stopwatch timer using the SC140 code within an application.

2. Setting up the stopwatch timer using the Metrowerks Code Warrior debugger1.

The examples given in this application note describe the use of the stopwatch timer in the StarCore,

SC140’s Software Development Platform (SDP). Minor configuration changes are needed to apply the

techniques to other SC140-based devices. The necessary modifications are also described in this

application note.

This document is organized to present these techniques as follows: Section 2, “SC140 Enhanced OnCE

Stopwatch Timer Capabilities,”describes the specific capabilities of the stopwatch timer for the SC140

Enhanced OnCE. Section 3, “Setting Up the Stopwatch Timer Within an Application,”describes

instrumenting the SC140 application code to use the stopwatch timer. Section 4, “Setting Up the

Stopwatch Timer Within the Debugger,”presents the set up of the stopwatch timer within the Metrowerks

Code Warrior debugger.

Supporting information is included in the remainder of the document. Section 5, “Setting Up the System

Clock Speed,”describes how to set the clock rate of the SC140 in the SDP by configuring the PLL in

hardware and software applications. Section 6, “Verifying Correct Setup,”describes techniques to verify

that the system is set up correctly and that the measurements are reasonable. The Section 7, “Conclusion,”

provides a summary of this document. Section 8, “References,”lists additional documentation to help the

reader and Appendix A, “Complete Example of Profiling,”provides a complete profile using the

Enhanced OnCE stopwatch timer in the SC140.

1. At the time this application note was written, the release of Metrowerks Code Warrior is Beta v.1.0. All the figures related to screen capture

of Code Warrior tools may vary in future versions.

2Using the SC140 Enhanced OnCE Stopwatch Timer 

2SC140 Enhanced OnCE Stopwatch Timer

Capabilities

This section presents the features of the Enhanced OnCE stopwatch timer and the resources required for

implementation. The capabilities of the implementation of the stopwatch timer are also explained.

2.1 Features

The Enhanced OnCE stopwatch timer provides the following features:

•A 64-bit counter, incrementing on each DSP clockcycle. The counter is less susceptible to overflow

with 64-bit precision.

•The stopwatch timer can be used repeatedly during the execution of an application.

Conversion between clock cycles and absolute time, based on the operating clock frequency of the DSP, is

described in Section 3.4, “Converting Cycles to Actual Time,”on page -6.

2.2 Resources

The Enhanced OnCE stopwatch timer requires use of these resources:

•One Enhanced OnCE event detector (of the six available on each DSP)

•The Enhanced OnCE event counter

•Program memory of 724 bytes

Because the Enhanced OnCE supports only one event counter, the stopwatch timer cannot be used if the

event counter is required for other debugging purposes (such as; the set up of a debugger breakpoint that

requires counting occurrences of events).

2.3 Implementation

The stopwatch timer implementation allocates a variable in memory which serves as the target for memory

write operations. The Enhanced OnCE event detector is set up to detect writes to this flag variable. When

setting up of an Enhanced OnCE event detector to detect memory access operations, it is necessary to

specify which of the two data memory buses should be “snooped”. Because the selection of the bus to be

used is performed dynamically, the event detector is set up to snoop both buses (XABA or XABB). Upon

detecting the write to the flag variable, the event detector enables the Enhanced OnCE event counter,

which starts counting down.

The Enhanced OnCE event counter can be configured as either a 64-bit counter or a 32-bit counter.

Configuring the counter to use 64-bits eliminates the danger of counter overflow, at a negligible extra cost.

To stop the stopwatch timer, an appropriate value is written into the Enhanced OnCE memory-mapped

event counter control register. When this operation is completed, the cycle countdown halts. At this point it

is possible to read out the values of the Enhanced OnCE event counter registers, and translate them into

elapsed number of cycles, or elapsed absolute time.

Setting Up the Stopwatch Timer Within an Application 3

3Setting Up the Stopwatch Timer Within an

Application

This section describes the operations necessary to initialize, start, and stop the stopwatch timer within an

application. The sequence of operations is shown in Figure 1. Additionally, this section presents the

conversion of cycles to actual time and puts all the application code together. Finally, this section explains

how to adapt the stopwatch timer code to other SC140-based devices.

Figure 1. Sequence of Operations

3.1 Initializing the Stopwatch Timer

The C code to set up the stopwatch timer is shown in Code 1.

Code 1. Event Detector Setup Code

* Header file contains definitions of EOnCE memory-mapped register addresses,

* and definition of the WRITE_IOREG() macro.

#include “EOnCE_registers.h”

static volatile long EOnCE_stopwatch_timer_flag; /*Global dummby variable*/

void EOnCE_stopwatch_timer_init()

{WRITE_IOREG(EDCA1_REFA,(long)&EOnCE_stopwatch_timer_flag);

/* Address to snoop for on XABA */

WRITE_IOREG(EDCA1_REFB,(long)&EOnCE_stopwatch_timer_flag);

/* Address to snoop for on XABB */

WRITE_IOREG(EDCA1_MASK,MAX_32_BIT);

/* No masking is performed in address comparison */

WRITE_IOREG(EDCA1_CTRL,0x3f06);

/* Detect writes on both XABA and XABB */

}

The header file “EOnCE_registers.h”contains the macro definitions, such as EDCA1_MASK, which

provides each of the Enhanced OnCE memory-mapped register’s corresponding memory address.

This header file also defines the C macros: READ_IOREG() and WRITE_IOREG(). These macros

simplify the read and write operations on memory-mapped registers.

Sequence Instructions

to be Timed

Initialize Stopwatch

Enable Stopwatch

Disable Stopwatch

4Using the SC140 Enhanced OnCE Stopwatch Timer 

Initializing the stopwatch timer consists of setting up the Address Event Detection Channel (EDCA). The

role of EDCA in the stopwatch timer implementation is to trigger the commencement of the cycle

countdown. The Enhanced OnCE supports six EDCAs. The implementation presented in this application

note uses EDCA1, though this choice is arbitrary.

Set up of the EDCA requires initializing the following four registers:

•The 32-bit EDCA reference value register A (EDCA1_REFA).

•The 32-bit EDCA reference value register B (EDCA1_REFB).

•The 32-bit EDCA mask register (EDCA1_MASK).

•The 16-bit EDCA control register (EDCA1_CTRL).

3.1.1 Event Detector Control

The register, EDCA1_CTRL, controls the behavior of the EDCA. The fields of the EDCA1_CTRL register

are shown in Figure 2.

Figure 2. EDCA Control Register (EDCA1_CTRL)

Table 1 describes the settings of these fields in the stopwatch timer implementation.

The EDCA becomes enabled as soon as these values are written into the control register. The EDCA stays

enabled for the duration of the program execution to enable repeated use of the stopwatch timer.

3.1.2 Address Comparison Setup

The purpose of the address comparison in the EDCA is to detect writes to the stopwatch timer flag

variable. Because writes may take place on either of the two data memory buses, both registers,

EDCA1_REFA and EDCA1_REFB, are set up to contain the address of the stopwatch timer flag variable.

The register EDCA1_MASK allows masking of address bits when comparing the sampled address with

those in the EDCA1_REFA and EDCA1_REFB registers. In the implementation of the stopwatch timer all

Table 1. EDCA_CTRL Settings

Field Setting

(binary value) Description

EDCAEN 1111 This channel is enabled

CS 11 Trigger event if address matches either comparator A or comparator B

CBCS 00 An “address match”is detected when sampled bus value equals value in

EDCA_REFB

CACS 00 An “address match”is detected when sampled bus value equals value in

EDCA_REFA

ATS 01 Detect write access only

BS 10 The sampled buses are XABA and XABB

Setting Up the Stopwatch Timer Within an Application 5

32-bits of the flag variable’s address are used. Thus, EDCA1_MASK is set to 0xffffffff, meaning all

address bits will be compared.

3.2 Starting the Stopwatch Timer

The C code to start the stopwatch timer is shown in Code 2.

Code 2. C Code to Start the Stopwatch Timer

#include “EOnCE_registers.h”

void EOnCE_stopwatch_timer_start()

{

WRITE_IOREG(ECNT_VAL,MAX_32_BIT); /* Countdown will start at (2**32)-1 */

WRITE_IOREG(ECNT_EXT,0); /* Extension will count up from zero */

WRITE_IOREG(ECNT_CTRL,0x12c);

/* Counting will be triggered by detection on EDCA1 */

EOnCE_stopwatch_timer_flag = 0;

/* This write to the flag triggers the counter */

}

Before triggering the stopwatch timer, it is necessary to initialize the counter registers. Initializing the

event counter requires set up of the following three 32-bit registers:

•Event counter value register (ECNT_VAL)

•Extension counter value register (ECNT_EXT)

•Event counter control register (ECNT_CTRL)

Once these initializations are complete, the C code triggers the stopwatch timer and cycle counting

commences.

3.2.1 Event Counter Control

This section describes the initialization of the event counter registers. The register, ECNT_CTRL, controls

the behavior of the event counter. The fields of the ECNT_CTRL register are shown in Figure 3.

Figure 3. Event Counter Control Register (ECNT_CTRL)

Table 2 describes the settings of these fields in the stopwatch timer implementation.

Table 2. ECNT_CTRL Settings

Field Setting

(binary value) Description

EXT 1 Event counter operates as a 64-bit counter

ECNTEN 0010 The event counter is disabled, and will be enabled when an event is detected

by EDCA1

ECNTWHAT 1100 The counter will advance on each core clock cycle

6Using the SC140 Enhanced OnCE Stopwatch Timer 

3.2.2 Counter Registers

ECNT_VAL is a countdown counter, and ECNT_EXT is a countup counter. ECNT_VAL is decremented

on each occurrence of an event, as specified in the control register. ECNT_EXT is incremented each time

there is an underflow in ECNT_VAL.

For maximum cycle counting capacity, the stopwatch timer implementation initializes ECNT_VAL to the

largest possible value which is 4294967295, or 0xffffffff.

ECNT_EXT is initialized to zero.

3.3 Stopping the Stopwatch Timer

The C code to stop the stopwatch timer is shown in Code 3.

Code 3. C Code to Stop the Stopwatch Timer

#include “EOnCE_registers.h”

void EOnCE_stopwatch_timer_stop(unsigned long *clock_ext, unsigned long *clock_val)

{

WRITE_IOREG(ECNT_CTRL,0); /* Disable event counter */

READ_IOREG(ECNT_VAL,*clock_val); /* Save ECNT_VAL in program variable */

READ_IOREG(ECNT_EXT,*clock_ext); /* Save ECNT_EXT in program variable */

*clock_val = (MAX_32_BIT-*clock_val); /* Adjust for countdown */

}

To stop the stopwatch timer, the ECNT_CTRL register is set to zero. After stopping the stopwatch timer,

the routine copies the values of ECNT_VAL and ECNT_EXT registers into program variables. This

allows putting the stopwatch timer into use again without losing the result of the previous measurement.

Because ECNT_VAL contains the result of a countdown process, the routine converts that result into the

actual number of cycles elapsed by subtracting the ECNT_VAL from the value to which it was initialized,

specifically, 4294967295.

3.4 Converting Cycles to Actual Time

The stopwatch timer measures durations in units of core clock cycles. Most often the units of interest are

units of absolute time, such as milliseconds or microseconds.

Conversion from core clock cycles, as measured by the event counter registers, to milliseconds is

computed in Equation 1.

Eqn. 1

In this equation, EXT is the value in ECNT_EXT, VAL is the value in ECNT_VAL, and Clock Speed is

measured in hertz.

Code 4 on page -7 shows the C code for clock-cycle-to-time conversion.

The C code depends on setting the value of the constant CLOCK_SPEED in EOnCE_stopwatch.c to match

the clock speed as set in the PLL.

The conversion routine distinguishes between three different output units: seconds, milliseconds, and

microseconds. Handling each unit separately allows the computations to be performed using integer

arithmetic without loss of accuracy.

Time ms() EXT 0xffffffff×VAL+()

1000

ClockSpeed

--------------------------------

×=

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