Motorola DSP56301 - Manual (Page 238)

Operating Modes
8 -2 DSP56301 User’s Manual
transmit and receive clock compatible with the Intel 8051 serial interface mode 0
synchronizes data. Asynchronous modes are compatible with most UART-type serial devices.
Standard RS-232 communication links are supported by these modes. Multidrop
Asynchronous mode is compatible with the MC68681 DUART, the M68HC11 SCI interface,
and the Intel 8051 serial interface.
8.1.1 Synchronous Mode
Synchronous mode (SCR[WD2–0]=000, Shift Register mode) handles serial-to-parallel and
parallel-to-serial conversions. In Synchronous mode, the clock is always common to the
transmit and receive shift registers. As a controller (synchronous master), the DSP puts out a
clock on the
SCLK pin. To select master mode, choose the internal transmit and receive clocks
(set TCM and RCM=0).
As a peripheral (synchronous slave), the DSP accepts an input clock from the SCLK pin. To
select the slave mode, choose the external transmit and receive clocks (TCM and RCM=1).
Since there is no frame signal, if a clock is missed because of noise or any other reason, the
receiver loses synchronization with the data without any error signal being generated. You
can detect an error of this type with an error detecting protocol or with external circuitry such
as a watchdog timer. The simplest way to recover synchronization is to reset the SCI.
8.1.2 Asynchronous Mode
Asynchronous data uses a data format with embedded word sync, which allows an
unsynchronized data clock to be synchronized with the word if the clock rate and number of
bits per word is known. Thus, the clock can be generated by the receiver rather than requiring
a separate clock signal. The transmitter and receiver both use an internal clock that is 16 times
the data rate to allow the SCI to synchronize the data. The data format requires that each data
byte have an additional start bit and stop bit. Also, two of the word formats have a parity bit.
The Multidrop mode used when SCIs are on a common bus has an additional data type bit.
The SCI can operate in full-duplex or half-duplex modes since the transmitter and receiver are
independent.
8.1.3 Multidrop Mode
Multidrop is a special case of asynchronous data transfer. The key difference is that a protocol
allows networking transmitters and receivers on a single data-transmission line.
Inter-processor messages in a multidrop network typically begin with a destination address.
All receivers check for an address match at the start of each message. Receivers with no
address match can ignore the remainder of the message and use a wakeup mode to enable the
receiver at the start of the next message. Receivers with an address match can receive the