CM6308 C-Media Electronics, Inc., CM6308 Datasheet - Page 22

• Manufacturer Part Number: CM6308
• Description: Cm6308 Usb Audio Chip Specification
• Manufacturer: C-Media Electronics, Inc.

Date: 09/04/2007
The SPI interface is used to transfer control data between the CM6308 and external codecs. It
is not a standard interface. Every vendor has its own slightly different implementation, but
generally speaking, all of them comprise four signals, spi_cen, spi_clock, spi_data_o,
spi_data_i. Their meanings are as follows.
Our goal is to design a robust SPI interface that can be suitable for all existing codecs. After
analyzing the SPI interfaces of several codecs, we have written down the following differences
among them.
For difference 1 listed above, we have designed a 4-wire SPI interface, which is able to
accommodate the 3-wire SPI interfaces as well. For difference 2 and 3, control bits in the SPI
interface of the CM6308 are used to be initiated a 2-byte or 3-byte data transfer, and maintain
SPI clock high or low at codec latching data. All of these can be observed in Figs. A ~ E.
1). An SPI interface that can read data from and write data to a codec has 4 wires, but some
2). An SPI transaction length is 2 or 3 bytes depending on the codec.
3). Some codecs latch control data on the SPI clock’s high state, but others latch control data
4). The highest SPI clock frequencies are different for many codecs.
7.2 Serial Peripheral Interface
codecs only support input data. In other words, the data in the codec registers can not be
retrieved by audio processor. This kind of codec only requires 3 wires.
on the SPI clock’s low state
spi_cen: the SPI chip enable signal that is used to inform a codec when it should latch
onto the data.
spi_clock: the SPI clock signal.
spi_data_o: the SPI data output to codec.
spi_data_i: the SPI data input from codec.
7.2.1
The SPI Design Goal and SPI Transactions
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Audio and Voice Applications
Single-Chip USB Solution for
CM6308
Version: