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Enhanced Synchronous Serial Interface (ESSI)
7.4.2 Synchronous/Asynchronous Operating Modes
The transmit and receive sections of the ESSI interface are 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 CRB[SYN] bit 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
operates either synchronously or asynchronously, separate receive and transmit interrupts are
provided.
Transmitter 1 and transmitter 2 operate only in Synchronous mode. Data clock and frame sync
signals are generated internally by the DSP or obtained from external sources. If clocks are
internally generated, the ESSI clock generator derives bit clock and frame sync signals from the
DSP internal system clock. The ESSI clock generator consists of a selectable fixed prescaler with
a programmable prescaler for bit rate clock generation and a programmable frame-rate divider
with a word-length divider for frame-rate sync-signal generation.
7.4.3 Frame Sync Selection
The transmitter and receiver can operate independently. The transmitter can have either a bit-long
or word-long frame-sync signal format, and the receiver can have the same or another format.
The selection is made by programming the CRB FSL[1–0], FSR, and FSP bits.
7.4.4 Frame Sync Signal Format
CRB[FSL1] controls the frame sync signal format.
If CRB[FSL1] is cleared, the receive frame sync is asserted during the entire data transfer
period. This frame sync length is compatible with Freescale codecs, serial peripherals that
conform to the Freescale SPI, serial A/D and D/A converters, shift registers, and
telecommunication pulse code modulation serial I/O.
If CRB[FSL1] is set, the receive frame sync pulses active for one bit clock immediately
before the data transfer period. This frame sync length is compatible with Intel and
National Semiconductor Corporation components, codecs, and telecommunication pulse
code modulation serial I/O.
7.4.5 Frame Sync Length for Multiple Devices
Mixing frame sync lengths is useful in configuring systems in which data is received from one
type of device (for example, codec) and transmitted to a different type of device. CRB[FSL0]
controls whether RX and TX have the same frame sync length.
If CRB[FSL0] is cleared, both RX and TX have the same frame sync length.
7-10
DSP56311 User's Manual, Rev. 2
Freescale Semiconductor
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