Patent ID: 8427198

Claim:
A method of calibrating an impedance for a data transfer circuit using a calibration circuit having a plurality of main transistors respectively switched between a high impedance state and a low impedance state by a respective bit value signal of an N-bit calibration value, said plurality of main transistors being arranged in parallel with each other and a rounding transistor to provide a combined impedance, said method comprising the steps of: (a) setting a default value for an impedance of said rounding transistor and a current N-bit calibration value to a first value in a sequence of N-bit calibration values corresponding to a monotonically changing combined impedance; (b) detecting if said combined impedance has crossed a threshold value and one of: (i) if said combined impedance has not crossed said threshold impedance, then setting said current N-bit calibration value to a next value in said sequence and repeating step (b); and (ii) if said combined impedance has crossed said threshold value, then: (1) selecting a value for said N-bit calibration value and changing said impedance of said rounding transistor from said default value to a changed value to provide a further combined impedance with a magnitude between said combined impedance that crossed said threshold value and a preceding impedance value that did not cross said threshold value; (2) detecting if said further combined impedance value remains as crossed said threshold value and one of: (A) if said further combined impedance remains as crossed said threshold value, then setting as a determined N-bit calibration value that value of said calibration N-bit value corresponding to said preceding combined impedance that did not cross said threshold value; and (B) if said further combined impedance does not remains as crossed said threshold value, then setting as a determined N-bit calibration value that value of said calibration N-bit value corresponding to said combined impedance that did cross said threshold value.