USB7056 datasheet(30/55 Pages) MICROCHIP

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USB7056

DS00002675F-page 30

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7.3.1

MODES OF OPERATION

The USB audio class operates in three ways: Asynchronous, Synchronous and Adaptive. There are also multiple oper-

ating modes, such as hi-res, streaming, etc.. Typically for USB devices, inputs such as microphones are Asynchronous,

and output devices such as speakers are Adaptive. The hardware is set up to handle all three modes of operation. It is

recommended that the following configuration be used: Asynchronous IN; Adaptive OUT; 48Khz streaming mode; Two

channels: 16 bits per channel.

7.3.1.1

Asynchronous IN 48KHz Streaming

In this mode, the codec sampling clock is set to 48Khz based on the local oscillator. This clock is never changed. The

data from the codec is fed into the input FIFO. Since the sampling clock is asynchronous to the host clock, the amount

of data captured in every USB frame will vary. This issue is left for the host to handle. The input FIFO has two markers,

a low water mark (THRESHOLD_LOW_VAL), and a high water mark (THRESHOLD_HIGH_VAL). There are three reg-

isters to determine how much data to send back in each frame. If the amount of data in the FIFO exceeds the high water

mark, then HI_PKT_SIZE worth of data is sent. If the data is between the high and low water mark, the normal MID_P-

KT_SIZE amount of data is sent. If the data is below the low water mark, LO_PKT_SIZE worth of data is sent.

7.3.1.2

Adaptive OUT 48KHz Streaming

In this mode, the codec sampling clock is initially set to 48Khz based on the local oscillator. The host data is fed into the

OUT FIFO. The host will send the same amount of data on every frame, i.e. 48KHz of data based on the host clock. The

codec sampling clock is asynchronous to the host clock. This will cause the amount of data in the OUT FIFO to vary. If

the amount of data in the FIFO exceeds the high water mark, then the sampling clock is increased. If the data is between

the high and low water mark, the sampling clock does not change. If the data is below the low water mark, the sampling

clock is decreased.

7.3.1.3

Synchronous Operation

For synchronous operation, the internal clock must be synchronized with the host SOF. The Frame SOF is nominally

1mS. Since there is significant jitter in the SOFs, there is circuitry provided to measure the SOFs over a long period of

time to get a more accurate reading. The calculated host frequency is used to calculate the codec sampling clock.

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