Mode selection circuit for low-cost integrated circuits such as microcontrollers

A circuit, comprising:

• a below-ground detector circuit having an input coupled to a first signal terminal, and an output for providing an output voltage in a first logic state when a voltage on said first signal terminal is less than a voltage on said ground voltage terminal by at least a predetermined amount, wherein said below-ground detector circuit comprises:
• a transistor having a first current electrode, a control electrode, and a second current electrode coupled to said first signal terminal;
• a bias circuit for biasing said first current electrode and said control electrode of said transistor such that said transistor remains non-conductive during power supply transients, and becomes conductive after said power supply transients when a voltage on said first signal terminal is less than a voltage on said ground voltage terminal by at least said predetermined amount;
• an output capacitor having a first terminal coupled to said first current electrode of said transistor, and a second terminal coupled to said ground voltage terminal;
• and
• an inverting buffer having an input coupled to said first current electrode of said transistor, and an output terminal for providing said output voltage;
• a first output circuit for providing a first logic signal in an active state of referenced to a first voltage domain in response to said output voltage being in said first logic state;
• and
• an enable circuit coupled to a second signal terminal for disabling said first output circuit when said second signal terminal is in a second logic state.

A method of operating an integrated circuit, comprising:

• detecting whether a voltage on a first input/output port terminal of the integrated circuit is less than a voltage on a ground voltage terminal by at least a predetermined amount, wherein said detecting comprises:
• biasing a first current electrode and a control electrode of a transistor such that said transistor remains non-conductive during power supply transients, and becomes conductive after said power supply transients when a voltage on said first input/output port terminal is less than a voltage on said ground voltage terminal by at least said predetermined amount;
• filtering a voltage at said first current electrode of said transistor using an output capacitor;
• buffering said voltage at said first current electrode of said transistor;
• and
• providing a first logic signal in response to said buffering, and
• enabling said detecting in response to a logic state of a second input/output port terminal;
• activating said first logic signal referenced to a first voltage domain;
• and
• changing an operation of a first circuit of the integrated circuit in response to an activation said first logic signal.

An integrated circuit comprising a below-ground detector circuit, said below-ground detector circuit comprising:

• a transistor having a first current electrode, a control electrode, and a second current electrode coupled to a first signal terminal of the integrated circuit;
• a bias circuit for biasing said first current electrode and said control electrode of said transistor such that said transistor remains non-conductive during power supply transients, and becomes conductive after said power supply transients when a voltage on said first signal terminal is less than a voltage on said ground voltage terminal by at least a predetermined amount;
• an output capacitor having a first terminal coupled to said first current electrode of said transistor, and a second terminal coupled to a ground voltage terminal;
• and
• an inverting buffer having an input coupled to said first current electrode of said transistor, and an output terminal for providing an output voltage.