General description

AN2682 - Application note

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Figure 2.

I2S protocol signal description and configuration

1.2

SPI protocol

The SPI is chosen to implement this solution because its protocol is the most similar to the

I2S protocol.

It uses four pins:

●

Two pins (MOSI and MISO) to transmit and receive serial data

●

One pin (SCLK) for data clocking (either rising or falling edge modes are available)

●

One pin (nSS) to choose between master and slave modes.

When nSS is tied to Gnd, the SPI peripheral functions in Slave mode: it receives the data

clock (SCLK) from the master which controls how and when data are transmitted or

received.

When nSS is tied to Vcc, the SPI peripheral functions in master mode and SCLK is then an

output.

MISO is the Master Input in master mode and the Slave Output in slave mode.

MOSI is the Master Output in master mode and the Slave Input in slave mode.

There are two main differences between the I2S and SPI protocols:

●

The I2S clock is continuous while the SPI clock is generated only during word

transmission and is stopped afterwards.

●

The data format in SPI protocol is fixed to 16 or 8 bits only, while the data in I2S can

have any length.

Due to these two differences, the SPI peripheral cannot be configured in master mode in

this case. This is due to the fact that in SPI master mode, data are transmitted with a delay

between the words. Besides, many difficulties would arise when other tasks are performed

by the microcontroller. So the solution uses the SPI in slave mode.

Transmitter = Master

Receiver = Master

Controller = Master

SCK

WS

SD

SCK

WS

SD

SCK

WS

SD

Transmitter

Receiver

Transmitter

Controller

Transmitter

Receiver

Receiver