Flip-flop design and technique for scan chain diagnosis

A circuit comprising:

• a plurality of sequential circuits connected in a scan chain, a scan output terminal of one of said sequential circuits being electrically coupled to a scan input terminal of a next sequential circuit, at least one of said sequential circuits comprising:
• a first circuit connected to receive a scan enable signal for causing a scan output signal of at least one of said sequential circuits to be forced to a predetermined logic state by transitioning said scan enable signal between logic states, comprising a NOR gate having a first input terminal connected to said scan enable signal and a second input terminal connected to a delayed scan enable signal.

A circuit comprising:

• a plurality of sequential circuits connected in a scan chain, a scan output terminal of one of said sequential circuits being electrically coupled to a scan input terminal of a next sequential circuit, at least one of said sequential circuits comprising:
• a plurality of circuits connected to receive a scan enable signal for causing the scan output signal of at least one of said sequential circuits to be forced to a predetermined logic state by transitioning said scan enable signal between logic states, wherein alternative ones of said sequential circuits have scan output signals which can be reset upon transitioning said select enable signal between logic states, and alternate ones of said sequential circuits have scan output signals which can be set upon transitioning said select enable signal between logic states.

A method for diagnosing a scan chain, said scan chain comprising a series of sequential circuits, each of said sequential circuits having a scan enable signal applied to it to select a scan chain mode of operation, each of said sequential circuits having a scan output terminal and a scan input terminal, said method comprising the steps of:

• transitioning said scan enable signal between logic states to force an output signal at a scan output terminal of a plurality of said sequential circuits to be at a predetermined logic state, without set or reset controls.

A method for diagnosing a scan chain, said scan chain comprising a series of sequential circuits, each of said sequential circuits having a scan enable signal applied to it to select a scan chain mode of operation, each of said sequential circuits having a scan output terminal and a scan input terminal, said method comprising the steps of:

• transitioning said scan enable signal between logic states to force a scan output terminal of a plurality of said sequential circuits to be at a predetermined logic state, wherein alternate ones of said sequential circuits in said scan chain have scan output signals being at a logical 1 as the predetermined state and alternative ones of said sequential circuits in said scan chain have scan output signals at a logical 0 as the predetermined state.