WM8281ECSSLASHR datasheet(167/392 Pages) CIRRUS

167 / 392 page

WM8281

Rev 4.0

167

The Message Channel is allocated all of the Control space not used by the Framing Information or the

Guide Channel. The Message Channel enables SLIMbus devices to communicate with each other,

using a priority-based mechanism defined in the MIPI specification.

Messages may be broadcast to all devices on the bus, or can be addressed to specific devices using

their allocated Logical Address (LA) or Enumeration Address (EA). Note that, device-specific

messages are directed to a particular device (ie. Manager, Framer, Interface or Generic) within a

component on the bus.

DATA SPACE

The Data space can be organised into a maximum of 256 Data Channels. Each Channel, identified by

a unique Channel Number (CN), is a stream of one or more contiguous Slots, organised in a

consistent data structure that repeats at a fixed interval.

A Data Channel is defined by its Segment Length (SL) (number of contiguous Slots allocated),

Segment Interval (spacing between the first Slots of successive Segments), and Segment Offset (the

Slot Number of the first allocated Slot within the Superframe). The Segment Interval and Segment

Offset are collectively defined by a Segment Distribution (SD), by which the SLIMbus Manager may

configure (or re-configure) any Data Channel.

Each Segment may comprise TAG, AUX and DATA portions. Any of these portions may be 0-length;

the exact composition depends on the Transport Protocol (TP) for the associated Channel (see

below). The DATA portion must be wide enough to accommodate one full word of the Data Channel

contents (data words cannot be spread across multiple segments).

The Segment Interval for each Data Channel represents the minimum spacing between consecutive

data samples for that Channel. (Note - the minimum spacing applies if every allocated segment is

populated with new data; in many cases, additional bandwidth is allocated, as described below, and

not every allocated segment is used.)

The Segment Interval gives rise to Segment Windows for each Data Channel, aligned to the start of

every Superframe. The Segment Window boundaries define the times within which each new data

sample must be buffered, ready for transmission - adherence to these fixed boundaries allows Slot

allocations to be moved within a Segment Window, without altering the signal latency. The Segment

Interval may be either shorter or longer than the Frame length, but there is always an integer number

of Segment Windows per Superframe.

The Transport Protocol (TP) defines the flow control or handshaking method used by the Ports

associated with a Data Channel. The applicable flow control mode(s) depend on the relationship

between the audio sample rate (flow rate) and the SLIMbus CLK frequency. If the two rates are

synchronised and integer-related, then no flow control is needed; in other cases, the flow may be

regulated by

the use of a ‘Presence’ bit. The Presence bit can either be set by the source Device

(‘pushed’ protocol), or by the sink Device (‘pulled’ protocol).

The Data Channel structure is defined in terms of the Transport Protocol (TP), Segment Distribution

(SD), and the Segment Length (SL) parameters. Each of these is described above.

The Data Channel content definition includes a Presence Rate (PR) parameter (describing the

nominal sample rate for the audio channel) and a Frequency Locked (FL) bit (identifying whether the

data source is synchronised to the SLIMbus CLK). The Data Length (DL) parameter defines the size

of each data sample (number of Slots). The Auxiliary Bits Format (AF) and Data Type (DT)

parameters provide support for non-PCM encoded data channels; the Channel Link (CL) parameter is

an indicator that channel CN is related to the previous channel, CN-1.

For a given Root Frequency and Clock Gear, the Segment Length (SL) and Segment Distribution

(SD) parameters define the amount of SLIMbus bandwidth that is allocated to a given Data Channel.

The minimum bandwidth requirements of a Data Channel are represented by the Presence Rate (PR)

and Data Length (DL) parameters. The allocated SLIMbus bandwidth must be equal to or greater than

the bandwidth of the data to be transferred.

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