Method and circuit for checking integrated circuit chips

In an integrated circuit chip the method of inputting data from on-chip logic and off-chip logic to an on-chip checking circuit and selectively outputting said data from said chip, comprising the steps of:

• passing on-chip logic output data directly to an input of said checking circuit;
• and
• passing on-chip logic output data through a multistate means capable of being in a high-impedance state or a low-impedance state, selectively, to another input of said checking circuit and to an off-chip output;
• whereby when said multistate means is in a high impedance state, data from said logic is prevented from reaching said off-chip output thereby permitting off-chip data to be placed at said off-chip output and checked with respect to said on-chip logic output at said checking circuit.

An integrated circuit chip comprising:

• a logic circuit having at least one output;
• an error check logic having first and second inputs for checking signals on said inputs;
• first means for connecting the output of said logic circuit to the first input of said error check logic;
• gating means capable of being in a first state and a second state for inhibiting, when in said first state and not inhibiting, when in said second state, signals from passing therethrough from an input to an output thereof;
• second means for connecting the output of said logic circuit to the input of said gating means;
• an off-chip data output;
• and
• third means for connecting the output of said gating means to said second input of said error check logic and to said off chip data output.