Active pullup circuitry for open-drain signals

A method for biasing an open-drain signal line, the method comprising:

• providing a pullup current to the signal line;
• monitoring a slew rate of a signal on the signal line;
• and
• providing additional pullup current to the signal line responsive to the monitored slew rate exceeding a slew rate threshold.

A method of reducing signal rise time on an open-drain signal line, the method comprising:

• providing pullup current to the signal line;
• monitoring a voltage on the signal line;
• determining if a slew rate of the voltage on the signal line exceeds a predetermined slew rate threshold value;
• and
• providing additional pullup current to the signal line if the slew rate is determined to exceed the predetermined slew rate threshold value.

Circuitry for pulling up an open-drain signal line, the circuitry comprising:

• level monitoring circuitry for providing a first signal indicative of a voltage on the open-drain signal line;
• slew rate monitoring circuitry for providing a second signal indicative of a slew rate of the voltage on the signal line;
• a first source of current for providing a pullup current to the signal line;
• and
• control circuitry for controlling the pullup current responsive to the first and second signals.