ASAHI KASEI

[AK5370]

MS0027-E-00

8

2000/05

n Overview of AK5370

The AK5370 is the advanced A/D converter, which converts analog signal of condenser microphone into USB audio class formats.

This

device is suitable for USB microphone.

The AK5370 includes the following blocks:

1.

AD converter

2.

Pre-Amplifier with variable gain/attenuation

3.

Serial Interface Engine

4.

Control block of Audio Device Class

Support Mute, Gain/Attenuation, Sampling Frequency Control, and AGC (only decode the request)

5.

USB transceiver

AK4170 compliant

6.

FIFO memory for bus synchronization

Memory Size (16bit wide x 100 depth)

7.

PLL

Synchronize Audio Stream data with USB bus I/F

The AK5370 supports 5 sampling frequencies (8kHz, 11.025kHz, 22.05kHz, 44.1kHz, 48kHz) mute control, volume control of Feature

Unit in Audio Device Class.

In addition to this, as the AK5370 decodes the AGC request, and puts “H” or “L” level to the external pin,

AGC pin, it is easy to implement AGC with external circuit.

In order to support these frequencies, the AK5370 incorporates PLL.

In addition to this, as the AK5370 consumes low power, it is

possible to operate as bus-powered device.

n Class & Descriptor Overview

USB peripheral usually belongs to a group, “Class”.

The AK5370 belongs to Audio Device Class.

The Class consists of four

Descriptors, Device Descriptor, Configuration Descriptor, Interface Descriptor, and Endpoint Descriptor.

The AK5370 has Audio

Specific Interface Descriptor and Endpoint Descriptor.

(1) Device Descriptor

Device Descriptor describes the product information; product name, manufacturer, etc.

In addition to this, Device Descriptor has

Endpoint0 fifo-buffer size.

At enumeration phase, host reads 8 bytes from the top of Device Descriptor at first.

The host knows by

reading bMaxPacketSize0 in the Device Descriptor, and uses this value for the following transactions, and does not send the bigger data

at once.

(2) Configuration Descriptor

Configuration Descriptor informs the host of the number of Interface, Bus-powered/Self-Powered device, etc.

This descriptor is the same as Standard Interface Descriptor.

(3) Interface Descriptor

There are two Interfaces in Audio Class; Audio Control (AC) Interface, and Audio Streaming (AS) Interface.

Each Audio Interface includes Standard Interface Descriptor and Class-Specific Interface.

Standard AC Interface Descriptor describes interface’s class.

Class-Specific AC Interface Descriptor is a concatenation of all the descriptors that are used to fully describe the audio function.

As the AK5370 has three logical audio functions; Input Terminal, Output Terminal, and Feature Unit (mute, volume, AGC), each

terminal and unit has their own Descriptor.

Not only each descriptor has their own information, but also has connection information between the blocks.

For example, bSourceID field in Output Terminal Descriptor is 0x03, which is ID of Feature Unit.

This means that Feature Unit is

connected to Output Terminal.

Standard AS Interface has the interface class name (AUDIO) and sub-class name (AUDIO_STREAMING), the related Endpoint number,

etc.

Class-Specific AS Interface Descriptor mainly shows Audio Data Format information that follows as Descriptor.

There is a note for Alternate Setting:

Alternate setting 0 is a zero-bandwidth setting, used to relinquish the claimed bandwidth on the bus when the microphone is not in use. It

is the default setting after power-up. The zero bandwidth is implemented by specifying that this alternate setting of the interface has no

endpoints associated with it (bNumEndpoints=0).

(4) Endpoint Descriptor