Motorola DSP56000 Read/Write Control Signals, Program Memory Select PS, Data Memory Select DS

PORT A INTERFACE

be subdivided into three additional groups: read/write control (RD and WR), address space selection (including program memory select (PS), data memory select (DS), and X/ Y select) and bus access control.

The read/write controls are self-descriptive. They can be used as decoded read and write controls, or, the write signal can be used as the read/write control and the read signal can be used as an output enable (or data enable) control for the memory. Decoding in this fashion simplifies the connection to high-speed random-access memories (RAMs). The address space selection signals can be considered as additional address signals, which extend the addressable memory from 64K words to 192K words

Note: Depending on system design, unused inputs should have pullup resistors for two

reasons: 1) floating inputs draw excessive power, and 2) a floating input can cause erro- neous operation. For example, during RESET, all signals are three-stated. Output pins PS and DS may require pullup resistors because, without them, the signals may become ac- tive and may cause two or more memory chips to try to simultaneously drive the external data bus, which can damage the memory chips. A pullup resistor in the 50K-ohm range should be sufficient.

8.2.1 Read/Write Control Signals

The following paragraphs describe the Port A read/write control signals. These pins are three-stated during reset and may require pullup resistors to prevent erroneous operation of a memory device or other external components.

8.2.1.1Program Memory Select (PS)

This three-state output is asserted only when external program memory is referenced.

8.2.1.2Data Memory Select (DS)

This three-state output is asserted only when external data memory is referenced.

8.2.1.3X/Y Select (X/Y)

This three-state output selects which external data memory space (X or Y) is referenced

by DS.

8.2.2 Port A Address and Data Bus Signals

The following paragraphs describe the Port A address and data bus signals. These pins are three-stated during reset and may require pullup resistors to prevent erroneous operation.

MOTOROLA PORT A 8 - 5