12-14
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Functional Description
12
12.5.2 Network Mode
Network mode is used for creating a Time Division Multiplexed (TDM) network, such as
a TDM codec network or a network of DSPs. This mode only operates with continuous
clock mode. A frame sync occurs at the beginning of each frame. In this mode, the frame
is divided into more than one time slot. During each time slot, one data word can be
transferred. Each time slot is then assigned to an appropriate codec or DSP on the network.
The DSP can be a master device controlling its own private network, or a slave device
connected to an existing TDM network, occupying a few time slots.
The frame sync signal indicates the beginning of a new data frame. Each data frame is
divided into time slots and transmission and/or reception of one data word can occur in
each time slot (rather than in just the frame sync time slot as in the Normal mode). The
frame rate dividers, controlled by the DC bit field, select two to thirty-two time slots per
frame. The length of the frame is determined by these factors:
The period of the serial bit clock (PSR, PM[7:0] bits for internal clock, or the
frequency of the external clock on the SCK and/or SC0 pins)
The number of bits per sample (WL) bits
The number of time slots per frame (DC) bit fields
Data can be transmitted in any time slot while in the Network mode. The distinction of the
Network mode is, each time slot is identified with respect to the frame sync (data word
time). This time slot identification allows the option of transmitting data during the time
slot by writing to the STX register or ignoring the time slot by writing to STSR. The
receiver is treated in the same manner, except data is always being shifted into the RXSR
and transferred to the SRX register. The core reads the SRX register, either using the data
or discarding it.
figures illustrate Receive and Transmit frames of five time-slots for each. The numbered
circles and arrows in the figure identify discussion notes contained in Table 12-8 and
12.5.2.1 Network Mode Transmit
The transmit portion of the ESSI is enabled when the ESSIEN and the TE bits in the SCR2
are both set. However, when the TE bit is set, the transmitter is enabled only after
detection of a new frame boundary. Software has to find the start of the next frame by
checking the TFS bit of the SSR. A normal start-up sequence for transmission is to do the
following: