Audio playback

AN2394

4/13

2

Audio playback

The audio playback requires two dedicated timers:

●

Timer used as system timer, SysTimer: generates an interrupt at a programmable rate

(fixed by the .WAV file sample rate value).

●

Timer used in PWM mode. This timer should be able to produce high frequency PWM

signal.

The .WAV file data can be stored in a compatible SPI-based Flash (requiring an SPI

interface), in internal Flash memory (if the chosen STR7/SRT9 MCU has sufficient capacity)

or on any other external memory type supported by the microcontroller .

The audio playback flow is presented in the following figure:

Figure 1.

Audio playback flow

To playback the audio, the microcontroller reads each 8-bit sample and then outputs it to a

digital-to-analog converter (DAC) at the desired sample rate. The analog wave produced by

the DAC is amplified and fed to a speaker to produce the sound.

As the STR7/STR9 MCU doesn't have an on-chip DAC, an alternative method is used to

implement 1 channel DAC. Such a method is the use of the built-in Pulse Width Modulation

(PWM) module to generate a signal whose pulse width is proportional to the amplitude of

the sample data. The PWM output signal is then integrated by a low-pass filter to remove

high frequency components, leaving only the low-frequency content. The output of the low-

pass filter provides a reasonable reproduction of the original analog signal.

2.1

STR7/STR9 PWM description

This section introduces the basic concepts of the PWM mode of the STR7/STR9 TIMER

(TIM).

Pulse Width Modulation (PWM) consists of a signal with a fixed period whose duty cycle is

variable. The duty cycle represent the ratio of ON period to the period of the signal.

For the TIM, the PWM period and duty cycle are determined by the value of the OCAR and

OCBR registers.

The PWM output signal period is fixed by the OCBR which determines the maximum value

that the counter can count before starting a new period:

STR7/STR9

PWM

Low-Pass

filter

Audio

Amplifier

Speaker

External Memory,

example: SPI Flash

.WAV

.WAV

Internal

Flash

OR