Cirrus Logic CobraNet Audio Routing Primer - Manual (Page 7)

Appendix B – SSI Interface Theory
There are typically four Synchronous Serial Interfaces in a CobraNet TM interface. This number can differ
depending on product and/or configuration. SSI interfaces are configured at the factory to carry two to eight
audio channels each, in each direction. The number of audio channels multiplexed on an SSI will be reflected
in the rate of the audio bit clock (SCLK). The word clock (FS1) will run at the configured sample rate of 48-
kHz or 96-kHz. Each doubling of audio channels on the SSI will result in a doubling of the SCLK rate. For 2
channels per SSI, the SSI interface can be connected directly to D to A and A to D converters in most cases.
For higher channel counts the SSI data and/or SLCK must be multiplexed or de-multiplexed in order to
interface to the converters. The SSI interface always assumes use of a 32 bit sample size; in actual use the
relevant data within each sample will be 16, 20 or 24 bits.
The SSI carries multiplexed audio data in one of three modes. The mode is fixed in firmware at the factory
and is described in detail in the Hardware Reference Manual or Data Sheet applicable to the particular
CobraNet
TM interface in use.
Example:
A CM-2 module (based on the CS18101) chip is typically configured to handle 8 audio input channels and 8
audio output channels. There are four SSI interfaces, each of which is capable of carrying 2 audio channels in
each direction.
Therefore, the interface’s word clock (FS1 signal) will be running at the configured sample rate of 48-kHz or
96-kHz and the bit clock (SCLK) will be running at 64 times FS1 (or at FS64). This SCLK rate allows
multiplexing of two audio channels of up to 32 bits each on each SSI channel in each direction; or one
channel per half cycle of FS1. SSI channels 1, 2, 3 and 4 will each contain audio channels 1 and 2 for each
direction. i.e. 4 SSI channels x 2 audio channels per SSI x 2 directions = 8 channels in and 8 channels out, or
8 x 8. Refer to the Hardware Reference Manual or Data Sheet applicable to the particular CobraNet
TM
interface in use for more detail.
CobraNet TM Audio Routing Primer Rev. 1.2 ©Copyright 2004 Cirrus Logic, Inc
Page 7 of 9