Automatic system for adjusting the output impedance of fast CMOS drivers

A CMOS integrated circuit comprising:

• a plurality of fast CMOS drivers including:
• a plurality of slaved drivers having an output impedance and connected to respective transmission lines and, through the respective transmission lines, to respective receivers, and
• a dedicated mismatch correction driver having an output impedance and connected to a reference transmission line equivalent to the transmission lines connected to said slaved drivers and terminated by a resistor having a predetermined resistance;
• a threshold comparator for output impedance adjustment of said dedicated mismatch correction driver;
• means for applying a first clock signal to an input of said mismatch correction driver for producing on said reference transmission line a periodic signal;
• means for applying the periodic signal produced on said reference transmission line to said comparator;
• means for applying a reference voltage to said comparator equal to half a supply voltage of output stages of said drivers;
• means for triggering said threshold comparator with a second clock signal having a frequency equal to said first clock signal but in quadrature therewith, whereby said threshold comparator compares said periodic signal produced on said reference transmission line with said reference voltage in correspondence with transitions of said second clock signal having said frequency equal to said first clock signal but in quadrature therewith so that each comparison is carried out at a center of a half-period at which a voltage at an input of said reference transmission line is at a maximum;
• a capacitor connected to an output of said threshold comparator and charged and discharged by a comparison result from said comparator;
• and
• respective buffer amplifiers connected to said output stages of said drivers and having inputs connected to a receiving voltage from said capacitor, one of said buffer amplifiers controlling the output impedance of said mismatch correction driver, others of said buffer amplifiers controlling output impedance of said slaved drivers.