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SSI Operating Modes

8.4 SSI Operating Modes

The SSI has three basic operating modes, with the option of asynchronous or synchronous protocol, as follows:

Normal mode

Asynchronous protocol

Synchronous protocol

Network mode

Asynchronous protocol

Synchronous protocol

Gated clock mode

Synchronous protocol only

These modes can be programmed by several bits in the SSI control registers. Table 8-6 lists these operating modes and some of the typical applications in which they can be used:

Table 8-6. SSI Operating Modes

TX, RX Sections

Serial Clock

Mode

Typical Applications

 

 

 

 

 

 

 

 

Asynchronous

Continuous

Normal

Multiple synchronous codecs

 

 

 

 

Asynchronous

Continuous

Network

TDM codec or DSP networks

 

 

 

 

Synchronous

Continuous

Normal

Multiple synchronous codecs

 

 

 

 

Synchronous

Continuous

Network

TDM codec or DSP networks

 

 

 

 

Synchronous

Gated

Normal

SPI-type devices; DSP to MCU

 

 

 

 

The transmit and receive sections of the SSI can be synchronous or asynchronous. In synchronous mode, the transmitter and the receiver use a common clock and frame synchronization signal. In asynchronous mode, the transmitter and receiver each has its own clock and frame synchronization signals. Continuous or gated clock mode can be selected. In continuous mode, the clock runs continuously. In gated clock mode, the clock is only functioning during transmission.

Normal or network mode can also be selected. In normal mode, the SSI functions with one data word of I/O per frame. In network mode, any number from 2 to 32 data words of I/O per frame can be used. Network mode is typically used in star or ring time division multiplex networks with other processors or codecs, allowing an interface to time division multiplexed networks without additional logic. Use of the gated clock is not allowed in network mode. These distinctions result in the basic operating modes that allow the SSI to communicate with a wide variety of devices.

The SSI supports both normal and network modes, and these can be selected independently of whether the transmitter and receiver are synchronous or asynchronous. Typically these protocols are used in a periodic manner, where data is transferred at regular intervals, such as at the sampling rate of an external codec.

Both modes use the concept of a frame. The beginning of the frame is marked with a frame sync when programmed with a continuous clock. The frame sync occurs at a periodic interval. The length of the frame is determined by the DC[4:0] bits in either the SCRRX or SCRTX register, depending on whether data is being transferred or received. The number of words transferred per frame depends on the mode of the SSI.

In normal mode, one data word is transferred per frame. In network mode, the frame is divided into anywhere between 2 and 32 time slots, where in each time slot 1 data word can optionally be transferred.

Synchronous Serial Interface

8-23

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