DSP56303 datasheet(142/292 Pages) FREESCALE

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DSP56303 User’s Manual, Rev. 2

7-10

Freescale Semiconductor

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.

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