12-10
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Functional Description
12
12.5 Functional Description
The ESSI has two basic operating modes. Table 12-5 lists these operating modes and
some of the typical applications in which they can be used. These distinctions result in the
basic operating modes allowing the ESSI to communicate with a wide variety of devices.
These modes can be programmed by several bits in the ESSI control registers. For
The ESSI 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.
The frame sync occurs at a periodic interval. The length of the frame is determined by the
DC and WL field bits in either the SRXCR or STXCR register, depending on whether data
is being transmitted or received. The number of words transferred per frame depends on
the mode of the ESSI.
12.5.1 Normal Mode
Normal mode is the simplest mode of the ESSI. It is used to transfer one word per frame.
A frame sync occurs at the beginning of each frame. The length of the frame is determined
by the following factors:
The period of the serial bit clock (PSR, PM bits for internal clock or the frequency
of the external clock on the SCK or SC0 pin)
The number of bits per sample (WL) bits
The number of time slots per frame (DC) bits
Table 12-5.
ESSI Operating Modes
TX, RX Sections1
1.
In synchronous mode, the transmitter and receiver use a common clock and frame sychronization signal. In
the Asynchronous mode the transmitter and receiver operate independently, on their own clocks and frame
syncs.
Serial Clock
Mode2
2.
In the Normal mode, the ESSI only transmits during the first time-slot of each I/O frame. In the Network mode,
any number from 2 to 32 data words of I/O per frame can be used. The Network mode is typically used in star
or ring time division multiplex networks with other processors or codecs, allowing interface to TDM networks
without additional logic.
Typical Application
Asynchronous
Continuous
Normal
Multiple Asynchronous Codecs
Asynchronous
Continuous
Network
TDM Codec or DSP Networks
Synchronous
Continuous
Normal
Multiple Synchronous Codecs
Synchronous
Continuous
Network
TDM Codec or DSP Network