
12-8
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
ESSI Block Diagram
12
When configured as an input, this signal can be used to receive frame sync signals from an
external source, or it can be used as a serial input flag. When SC1 is a serial input flag, it
controls status bit IF1 in the SSR.
When this signal is configured as a transmit data signal, it is always an output signal
regardless of the SCD1 bit value. As an output, it is fully synchronized with the other
ESSI transmit data signals (STD and SC0).
Note:
The source of the transmitter 0 drive-enable signal is the STD_OEN signal.
12.3.2.4
ESSI Serial Control Signal 2 (SC2)
This pin is used for frame sync I/O and can be configured as either an input or an output
pin. The direction of this signal is determined by the SCD2 bit of the SCR4 register. The
frame sync is used by the transmitter to synchronize the transfer of data in asynchronous
mode. It is used by both the receiver and transmitter in Synchronous mode. The frame
sync signal can be one bit, or one word in length. The start of the frame sync can occur one
bit before the transfer of data, or right at the start of the data transfer.
12.3.2.5 ESSI Transmit Data (STD)
The STD signal is used for transmitting data from the TXSR0 serial Transmit Shift
Register. STD is an output pin when data is being transmitted from the TXSR0 Shift
Register. The STD pin is tri-stated after transmitting the last data bit when another data
word does not follow immediately. If a data word follows immediately within a full clock
cycle, the STD pin is not tri-stated.
12.3.2.6 ESSI Receive Data (SRD)
The SRD signal receives serial data, then transfers the data to the ESSI Receive Data Shift
Register (RXSR).
12.4 ESSI Block Diagram
The following figure illustrates the block diagram of the ESSI. It consists of five control
registers to set-up the port, one status register, separate transmit and receive circuits with
FIFO registers, and separate serial clock and frame sync generation for the transmit and
receive sections.