Interface circuits between powered down devices and a bus

An interface circuit for connection between a first, powered down circuit and an active bus, comprising:

• a first lead coupled to receive a first supply voltage applied to said powered down circuit;
• a first lead coupled to said first circuit;
• a second lead coupled to said bus;
• a pass device coupled between said first and second leads of said interface circuit and controlled by said first supply voltage;
• and
• a voltage limiting circuit coupled between said second lead of said interface circuit and a control terminal of said pass device.

A method for operating an interface circuit for connection between a first circuit designed for operation at a first supply voltage level and a lead operating at a second voltage level greater than said first supply voltage, comprising the steps of:

• coupling a first lead of said interface circuit to said first circuit;
• coupling a second lead of said interface circuit to said lead operating at said second voltage level;
• coupling a pass device between said first and second leads of said interface circuit;
• controlling said pass device by a first supply voltage applied to said first circuit;
• and
• limiting the voltage between said second lead of said interface circuit and a control terminal of said pass device.

An electronic system comprising:

• a first, powered down circuit;
• an active bus;
• and
• an interface circuit connected between said first circuit and said bus, said interface circuit comprising:
• a first lead coupled to receive a first supply voltage
• applied to said powered down circuit;
• a first lead coupled to said first circuit;
• a second lead coupled to said bus;
• a pass device coupled between said first and second leads of said interface circuit and controlled by said first supply voltage;
• and
• a voltage limiting circuit coupled between said second lead of said interface circuit and a control terminal of said pass device.

A method for operating an electronic system including a first circuit designed for operation at a first supply voltage level, a second circuit, and an interface circuit for connection between said first and second circuits operating at a second voltage level greater than said first supply voltage, comprising the steps of:

• coupling a first lead of said interface circuit to said first circuit;
• coupling a second lead of said interface circuit to said second circuit;
• coupling a pass device between said first and second leads of said interface circuit;
• controlling said pass device by a first supply voltage applied to said first circuit;
• and
• limiting the voltage between said second lead of said interface circuit and a control terminal of said pass device.