Automatic cutoff frequency adjusting circuit and portable digital assistant

A portable digital assistant comprising an automatic cutoff frequency adjusting circuit,

• wherein the automatic cutoff frequency adjusting circuit comprising:
• a voltage/current converter circuit;
• a charge circuit;
• a discharge circuit;
• a digital capacitance having a plurality of electrostatic capacitances series-couples to corresponding switches;
• a comparator for comparing a voltage inputted to the digital capacitance with a reference voltage;
• and
• a capacitance control circuit for controlling the digital capacitance,
• wherein the voltage/current converter circuit converts the reference voltage into a DC current,
• wherein the DC current is inputted to the charge circuit,
• wherein the digital capacitance is coupled to the charge circuit and the discharge circuit through a switch,
• wherein a reset signal is inputted to the switch and the capacitance control circuit,
• wherein all or some of the electrostatic capacitances of the digital capacitance are coupled in parallel by switching the corresponding switchers,
• wherein the digital capacitance is coupled to the discharge circuit when the reset signal is at a first level,
• wherein the digital capacitance is coupled to the charge circuit when the reset signal is at a second level, and
• wherein the capacitance control circuit measures a time until the comparator detects that a voltage between the terminals of the digital capacitance is higher than the reference voltage after the reset signal has become the second level, and controls the digital capacitance by repeating, under a predetermined condition, processing for obtaining a next setting value of the digital capacitance, based on a measurement result, a target value of the digital capacitance, and a current value of the digital capacitance.