Motorola DSP56309 User Manual page 205

Enhanced Synchronous Serial Interface (ESSI)
Operating Modes
7.5.4 Operating Modes: Normal, Network, and On-Demand
The ESSI has three basic operating modes and several data/operation formats. These
modes can be programmed using the ESSI control registers. The data/operation formats
available to the ESSI are selected by setting or clearing control bits in the CRA and CRB.
These control bits are WL[2:1], MOD, SYN, FSL[1:0], FSR, FSP, CKP, and SHFD.
7.5.4.1 Normal/Network/On-Demand Mode Selection
To select normal mode (or network mode), clear (or set) the MOD bit in the CRB. In
normal mode, the ESSI sends or receives one data word per frame (per enabled receiver
or transmitter). In network mode, two to thirty-two time slots per frame can be selected.
During each frame, zero to thirty-two data words can be received or transmitted (from
each enabled receiver or transmitter). In either case, the transfers are periodic.
Normal mode is typically used to transfer data to or from a single device. Network mode
is typically used in TDM networks of codecs or DSPs with multiple words per frame.
Network mode has a sub-mode called on-demand mode. Setting the MOD bit in the CRB
for network mode, and setting the frame rate divider to 0 (DC = $00000) selects the
on-demand mode. This sub-mode does not generate a periodic frame sync. A frame sync
pulse is generated only when data is available to transmit. The frame sync signal
indicates the first time slot in the frame. The on-demand mode requires that the transmit
frame sync be internal (output) and the receive frame sync be external (input). For
simplex operation, synchronous mode could be used; however, for full-duplex
operation, asynchronous mode must be used. Data transmission that is data driven is
enabled by writing data into each TX. Although the ESSI is double-buffered, only one
word can be written to each TX, even if the transmit shift register is empty. The receive
and transmit interrupts function normally, using TDE and RDF; however, transmit
underruns are impossible for Ôon- demandÕ transmission and are disabled. This mode is
useful for interfacing to codecs requiring a continuous clock.
7.5.4.2 Synchronous/Asynchronous Operating Modes
The transmit and receive sections of the ESSI interface can be synchronous or
asynchronous. The transmitter and receiver use common clock and synchronization
signals in synchronous mode; they use separate clock and sync signals in asynchronous
mode. The SYN bit in CRB selects synchronous or asynchronous operation. When the
SYN bit is cleared, the ESSI TX and RX clocks and frame sync sources are independent. If
the SYN bit is set, the ESSI TX and RX clocks and frame sync are driven by the same
source (either external or internal). Since the ESSI is designed to operate either
synchronously or asynchronously, separate receive and transmit interrupts are
provided.
7-40 DSP56309UM/D MOTOROLA