UM0290 datasheet(64/76 Pages) STMICROELECTRONICS

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USB voice demos

UM0290

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7.4.1

General characteristics

●

USB characteristics:

–

Endpoint 0: used to enumerate the device and to respond to the class specific

requests. The maximum packet size of this endpoint is 64 bytes.

–

Endpoint 1 (IN): This endpoint is used to transfer the audio stream data converted

from the STR7/9 to the PC host with a maximum packet size up to 22 bytes.

●

Audio characteristics:

–

Audio data format: Type I / PCM8 format / Mono.

–

Audio data resolution: 8 bits.

–

Sample frequency: 22 kHz.

●

Hardware requirements:

To record the audio streams, the analog signal issued from the microphone is amplified

using a differential preamplifier. The resulting signal is converted by the built-in ADC

(channel 12). Figure 20 shows the hardware components used in the microphone

demo.

Figure 20.

Microphone hardware blocks

7.4.2

Implementation

Hardware implementation

The microphone hardware is already implemented in both the STR75x and the STR91x

evaluation board. There is no need to add any external hardware to run the demo in this

board.

Firmware implementation

The aim of the microphone demo is to convert the signal issued from the microphone and

send it via USB to the PAC host.

The audio sampling frequency declared in the

USB Micro Audio Type I Format Interface

Descriptor is 22 kHz (see USB microphone descriptors on page 70). This means that the

host asks the device for 22 audio streams (bytes) each of 1ms using endpoint 1. These

audio streams represent the digital value of the audio signal picked up by the microphone

every 45.45 µs.

To manage this functionality, the built-in ADC (channel) is configured in continuous

conversion mode and the Timer 1 is configured to generate an interrupt every 45.45 µs.

STR7/9

Differential Mi-

crophone Pre-

amplifier

Microphone

USB

ADC

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