Patent ID: 7030641

Claim:
A programmable fuse state determination system, comprising: a programmable fuse; a current source arranged to provide a fuse current through said fuse which produces a voltage at a first node that varies with the resistance of said fuse; a threshold detection circuit which produces an output having a first state when said first node voltage is greater than a threshold voltage and having a second state when said first node voltage is less than said threshold voltage, one of said states indicating that said fuse is blown and the other of said states indicating that said fuse is intact; and a test circuit which has ‘normal’ and ‘test’ modes, said test circuit arranged to establish a first relationship between the resistance of said fuse and said threshold voltage when in said ‘normal’ mode and to establish a second, different relationship between the resistance of said fuse and said threshold voltage when in said ‘test’ mode, such that a higher fuse resistance is required for said threshold detection circuit output to indicate that said fuse is blown in said ‘test’ mode than in said ‘normal’ mode; wherein said current source comprises a first transistor (Q 1 ) having its current circuit connected between said first node and a circuit common point and which conducts said fuse current in response to a bias voltage applied to Q 1 's control input, said test circuit comprising: a ‘fuse test’ input for receiving a ‘fuse test’ signal which has a ‘normal’ state corresponding to said ‘normal’ mode and a ‘test’ state corresponding to said ‘test’ mode; and an output which provides said bias voltage to Q 1 , said test circuit arranged to provide a first bias voltage when said fuse test signal is in said normal state such the fuse current conducted by Q 1 has a first value I fuse,1 , and to provide a second bias voltage when said fuse test signal is in said test state such that the fuse current conducted by Q 1 has a second value I fuse,2 , said test circuit arranged such that I fuse,1 >I fuse,2.