12-18
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Functional Description
12
An interrupt can occur after the reception of each data word or the programmer can poll
the RDR flag. The ESSI program response can be one of the following:
Read SRX and use the data
Read SRX and ignore the data
Do nothing—the receiver overrun exception occurs at the end of the current time
slot
12.5.3 Synchronous/Asynchronous Operating Modes
The transmit and receive sections of the ESSI may be synchronous or asynchronous.
During asynchronous operation the transmitter and receiver have their own separate clock
and sync signals. When operating in the Synchronous mode the transmitter and receiver
use common clock and synchronization signals, specified by the transmitter configuration.
The SYN bit in SCR2 selects synchronous or asynchronous operation.
Since the ESSI is designed to operate either synchronously or asynchronously, separate
receive and transmit interrupts are provided. During the synchronous operation, the
receiver and transmitter operate in lock step with each other. Overhead may be reduced by
Table 12-9.
Note
Source Signal
Destination
Signal
Description
6
——
Example of a five time slot frame, receiving data from time slots 0 and 2.
The receive hardware will obtain data on the SRD pin every bit time.
The software must determine which data belongs to each time slot and
discard the unwanted time slot data.
7SC0
—
Receive clock timing from which it is derived.
8SC1
—
Example with bit length frame sync and standard timing (RFSI=0,
RFSL=1, and REFS=0). Frame timing begins with the rising edge of
SC1.
9SRD
RXSR
Register
Data on the SRD pin is sampled on the falling edge of SC0 and shifted
into the RXSR register.
10
RXSR
Register
SRX
Register
At the word clock, the data in the RXSR register is transferred to the
SRX register.
11
RDR Status Flag
and Receive
Interrupt
—
This flag is set for each word clock (time slot) indicating data is available
to be processed. The software must keep track of the time slots as they
occur so it knows which data to keep.
If the receive interrupts are enabled (RIE=1) an interrupt will be
generated when this status flag is set. The software reads the SRX
register to clear the interrupt.1
1.
Section 12.12 provides a complete description of the interrupt process.