Jameco Electronics 2000, 3000 manual Slave Applications, 3.Slave Port Status Register SPSR adr =

Bits 1,0—This 2-bit field sets the priority of the slave port interrupt. The interrupt is disabled by (0,0).

Table 13-3describes the slave port status register. The status register has 6 bits that are set if the particular register is full. That means that the register has been written by the processor that can write to it but it has not been read by the processor that can read it. The bits for SPD0R are used to control the slave interrupt and the handshaking lines as shown in Figure 13-3.

Table 13-3. Slave Port Status Register (SPSR) (adr = 0x023)

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
	1—set by	1—set by	1—set by		1—set by	1—set by	1—set by
1—set by				1—set by			slave
	master	master	master		slave write	slave write
master				slave write			write to
	write to	write to	write to		to SPD2R.	to SPD1R.
write to				to SPD0R.			SPD0R.
	SPD2R.	SPD1R.	SPD0R.		Cleared	Cleared
SPD0R.				Cleared by			Cleared
	Cleared	Cleared	Cleared		when	when
Cleared by				master			when
	when slave	when slave	when slave		master	master
slave write	reads	reads	reads	write to	reads	reads	master
to SPSR.	register.	register.	register.	SPSR.	register.	register.	reads
							register.

13.3 Applications and Communications Protocols for Slaves

The communications protocol used with the slave port depends on the application. A slave processor may be used for various reasons. Some possible applications are listed below.

Keep in mind that the Rabbit can also be operated as a slave processor via a serial port and some of the protocols will work well via a serial communications connection. If a serial connection is used, the protocol becomes more complicated if errors in transmission need to be taken into account. If the physical link can be controlled so that transmission errors do not occur, a realistic possibility if the interconnection environment is controlled, the serial protocol is simpler and faster than if error correction needs to be taken into account.

13.3.1 Slave Applications

•Motion Controller—Many types of motion control require fast action, may be com- pute-intensive or both. Traditional servo system solutions may be overly expensive or not work very well because of system nonlinearities. The basic communications model for a motion controller is for the master to send short messages—positioning com- mands—to the slave. The slave acknowledges execution of the commands and reports exception conditions.

•Communications Protocol Processor—Communications protocols may be very com- plex, may require fast responses, or may be compute-intensive.

•Graphics Controller—The Rabbit can be used to perform operations such as drawing geometric figures and generating characters.

•Digital Signal Processing—Although the Rabbit is not a speciality digital signal pro- cessor, it has enough compute speed to handle some types of jobs that might otherwise

206 Rabbit 3000 Microprocessor