Interface circuit for single-chip microprocessor

Claim 1

An interface circuit for a microprocessor having internal ROM memory with a first program therein, control input means for selecting operation in first and second modes, said microprocessor in the first mode operating in accordance with the first program in internal memory, said microprocessor in the second mode operating in accordance with a second program in external memory, an address-data port which in the first mode is operable as a data input or output port and in the second mode is operable in a multiplexing mode alternately to output address signals and to input or output program instructions or data from external memory and memory mapped devices, the interface circuit comprising, comprising: (a) an interface input and/or output port for functioning as the input and/or output port of the microprocessor; (b) switch means connected between the address-data port of the microprocessor and the interface input and/or output port so as to control conduction or shutting-off of passage of signals between the address-data port of the microprocessor and the interface input and/or output port; (c) connector means for connecting the address-data port of the microprocessor to external memory means, for connecting said control input means of the microprocessor to external circuitry to operate the microprocessor in the second mode, for connecting external circuitry which operates the switch means to shut-off passage of signals through the switch means, and for connecting an external I/O unit between the address-data port and the interface input and/or output port; and (d) biasing means for operating the switch means to render the switch means conductive in the absence of the connection of the external switch means operating circuitry by the connector means.