ST AN2400 Application note

AN2400

Application note

Audio player evaluation board

based on ST7Lite

Introduction

This application note demonstrates how to add audio playback to any application using a general-purpose ST7 microcontroller. To demonstrate this feature, an evaluation board, source code in C, schematics, and layout are available.

The Audio player evaluation board reconstructs audio signals through the PWM of a ST7FLITES2 microcontroller. Serial Flash is used to store the audio data in a binary file format. To store this audio data in the Flash, the sound files need to be in .wav format so that can be converted into .bin file format with the help of the PC GUI available with this package. Once the data is stored, the microcontroller reads it through an SPI interface and generates the sound using the PWM feature. A sixth order low pass filter removes any unwanted high frequency components from the signal before passing it through a speaker.

The key features of the Audio player evaluation board are as follows:

■ 1% calibrated on-chip RC oscillator. The used microcontroller, ST7FLITES2, contains an

internal RC oscillator with an accuracy of 1% for a given device. It can be calibrated to obtain a frequency required for the application. There is therefore no need for any external oscillator.

■ LVD (brown-out) on chip. The purpose of the Low Voltage Detector (LVD) is to ensure that

the ST7 always functions in its safe area. No external reset circuit is required.

■ Small footprint solution.

■ Low power operation. Reduces battery/power supply cost (through extended battery life).

■ Supports lower density serial Flash if fewer messages needed.

■ Order of filter can be reduced to cut cost even more.

November 2006 Rev 1 1/23

www.st.com

Contents AN2400

Contents

1 Audio player evaluation board overview . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Instructions for use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Audio player evaluation board concept . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Audio recording . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Audio reproduction flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Filter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 WAV file conversion process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Appendix A Files description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

A.1 Function description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

A.2 Interrupt routine description: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Appendix B Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Appendix C Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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AN2400 Audio player evaluation board overview

1 Audio player evaluation board overview

This evaluation board includes the following main components:

● 8-bit microcontroller: ST7FLITES2. However, the ST7Flite02 can also be used.

● An external 8 MBit Serial Flash, M25P80 family

● External components to build a low cost passive filter.

● An audio amplifier (TDA7233)

● 3.3V low-cost voltage regulator

Figure 1. Audio player evaluation board block diagram

ST7Lite02

DIP16

File Index

6th order RC filter

Vol ume Control

Powe r

Amplifier

TDA 7233

Play

Reset

Microcontroller

SPI

Serial Flash

Volt age

External Input

Regulator

Powe r Supply Circuit

ZIF

Socket

Binary LED display

The main features of the evaluation board are as follows:

● Plays pre-recorded audio clips stored on an ST 8-Mbit serial Flash

● Can play up to 15 different audio files

● Easy to use Index and Play push-buttons

● Power input 6V DC

● Phone-quality sound: 8 kHz/8 bit

External Speaker

Connection

1.1 Instructions for use

Assuming the external Flash is already programmed with sounds, you need to:

1. Power-on the board

2. Connect the board to the speaker

3. Select the desired sound by pressing the Index button. The File Index indicates the sound to be played with the help of 4 LEDs.

4. Push the Play Sound button.

5. Adjust the volume potentiometer to your convenience.

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Audio player evaluation board overview AN2400

Figure 2. Labeled diagram of Audio player evaluation board

ICC Connector

External Speaker Jack

ZIF Socket for Flash

Power LED

File Index

Power Jack

LEDs

File Index Switch

Volume Control

Play Switch

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AN2400 Audio player evaluation board concept

2 Audio player evaluation board concept

2.1 Audio recording

A PC is used to prepare audio to be programmed onto the external serial Flash. The file must contain audio data as well as some basic file management structure to allow the ST7 to pick the correct audio during playback.

The serial Flash is used to store the audio files in binary format. For this purpose, it is necessary to have the audio files in .wav format which allows them to be converted to a .bin file to be programmed into the serial Flash.

For conversion to binary, the .wav files should have an 8 kHz sampling frequency and 8-bit quantization format. These files are input into the provided WAV Converter utility to generate a single .bin file.

Figure 3. Sound recording flow

Original Sound Sampling Quantifying

Recording process: Can be done on a standard PC, in a recording studio, or just some existing audio samples

Standard 8kHz/8-bit

WAV

.WAV files

WAV

WAV File Storage Conversion to Binary File Burn binary file to M25P

PC Processing: Convert the .WAV files into a propriety binary file used to reprogram the onboard serial Flash

011011011 001010101

BIN

Serial Flash Memory

Once this process is completed, the board is ready to play customized sounds provided by the user.

A sound is selected using the Index push-button, and then heard by pressing the Play button.

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Audio reproduction flow AN2400

3 Audio reproduction flow

Figure 4. Audio reproduction flow

MCU reads data from Serial Flash through SPI

Micro Data Processing: Complete source code free available from ST

Train of Pulses PWM timer feature Filter

6th Order RC

Analog Filtering

Low-Pass

Filter quality/cost tradeoff can be achieved by populating or not the filter capacitors

Reconstructed signal

to Speaker

Reproduction of “phone

quality” audio

Data, which is stored in the serial Flash, is read by the microcontroller through the SPI interface. This data is then fed through the timer registers to generate a PWM output whose duty cycle varies according to the data value.

The sequential path of the data flow from the serial Flash to the speaker is shown in

Figure 5.

Figure 5. Flow of data from external serial flash to external speaker

Internal

SPI

CLOCK

PWM Timer Feature

LITE TIMER

w/ WATCHDOG

PORT A

12-BIT AUTO-

RELOAD TIMER

PA7:0

(8 bits)

External

Filter/Speaker

FLASH

MEMORY

ADDRESS AND DATA BUS

(1 or 1.5K Bytes)

RAM

(128 Bytes)

DATA EEPROM

(128 Bytes)

External Serial Flash

SPI Data Tr an s fe r

RESET

PB4:0

(5 bits)

1 MHz. RC OSC

PLL x 4 or x 8

LVD/AVD

POWER SUPPLY

CONTROL

8-BIT CORE

ALU

PORT B

8-BIT ADC

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AN2400 Audio reproduction flow

The microcontroller initiates the communication when it selects the Flash and starts reading the data on a full duplex, synchronous basis. Once the microcontroller reads the data, it generates a train of pulses using the PWM feature of the 12-bit auto reload timer. This is explained with the help of an example:

Audio samples of 8 kHz, 8-bit format are to be reproduced. This data is coded in an 8bit format (with values from 0 to 255).

To reproduce the audio with sampling rate of 8 kHz, the microcontroller outputs each value in every 1/8000 sec (every 125 µs). This means that the period of the PWM is set at 125µs.

If the coded value of the audio signal is “1”, the microcontroller needs to generate the PWM signal with a HIGH output for 1 count and LOW for the rest of the 255 counts. This is shown in the figure below:

Figure 6. Waveform depicting the microcontroller output

BIN

Flash

256 256 256 256

255

(256 - x)

Data value of 1 is read from Flash and ART generates a PWM pulse with a duty cycle high of length 1 with 255 low.

Data value of x is read from Flash and ART generates a PWM pulse with a duty cycle high of length x and (256-x) low.

Varying duty cycles

Figure 7 below shows how the duty cycle register generates this output.

This process uses a duty cycle register, a 12-bit auto reload register and an up-counter.

When an up-counter overflow occurs (OVF event), the ATR value is loaded into the upcounter, the preloaded duty cycle is transferred to the duty cycle register. The PWM0 signal is then set to a high level. Finally, when the upcounter matches the DCRx value, the PWM0 signal is set to a low level.

To use this sequence for audio generation, the following steps are taken:

1. When the up-counter reaches 0xFFF, an overflow event occurs. At this point, the PWM output becomes high.

2. The ATR register value is loaded into the up-counter which in this example is 0xF00. 0xF00 is selected to maintain a range of 0-FF i.e. 0xF00-0xFFF for an 8-bit resolution .wav file.

3. The DCR lower 8-bit register (DCR0L) is loaded with a value that the microcontroller has read from the serial Flash. The DCR0H register value remains 0x0F. In this

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