Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2000-01-03
2001-12-25
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S082000, C326S080000, C326S112000
Reexamination Certificate
active
06333643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hotplug tolerant I/O circuit suitable for a device receiving a signal with a voltage higher than its supply voltage, and more particularly to the hotplug tolerant I/O circuit enabling the device to be hotplugged to a cable transmitting such a signal. Here, the term “hotplug” refers to externally connecting to the device a data signal having a voltage higher than the supply voltage of the device under the condition in which the supply voltage is not supplied to the device.
2. Description of Related Art
Conventionally, such devices that are supplied with a data signal of a voltage higher than their power supply voltage VDD have been widely used. Even when the data signal with a voltage higher than the power supply voltage VDD is supplied from the outside to the devices under the condition in which the power supply voltage VDD is being supplied to the devices, these devices carry out in their I/O circuit such control that protects transistors constituting the I/O circuit from being supplied with an excessive voltage, and prevents current on a data signal cable from flowing into the power supply (VDD and GND) of the I/O circuit.
FIG. 6
is a circuit diagram showing a conventional I/O circuit in a device. In
FIG. 6
, the reference numeral
60
designates the I/O circuit;
61
designates an I/O transistor circuit in which three transistors MP
1
, MN
1
and MN
2
are connected in series;
62
designates a floating gate signal generator;
63
designates a floating well signal generator; and Gl-G
3
designate a gate circuit.
Next, the operation of the conventional I/O circuit will be described.
The conventional I/O circuit as shown in
FIG. 6
is incorporated in a device, and functions as an I/O interface. The I/O circuit is connected to a signal cable of an external device via I/O pins under the condition in which the power supply voltage VDD is being supplied.
In this case, because the power supply voltage VDD is being applied, the I/O transistor circuit
61
is not supplied with an excessive voltage. Thus, the transistors in the I/O transistor circuit
61
are protected from damage, and no current will flow from the data signal cable to the power supply (VDD and GND) of the I/O circuit.
The conventional I/O circuit with the foregoing configuration has the following problems. First, when the signal with the voltage higher than the power supply voltage VDD is supplied from the external device to the device via the signal cable and I/O pins under the condition in which the power supply voltage VDD is not applied to the device, the transistors MP
1
, MN
1
and MN
2
constituting the I/O transistor circuit
61
are supplied with an excessive voltage, and are destroyed. This will disable the I/O circuit, and have an adverse effect on the data on the signal cable because of a current flowing from the signal cable to the power supply (VDD and GND) of the I/O circuit.
SUMMARY OF THE INVENTION
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a hotplug tolerant I/O circuit capable of implementing such control that can protect the transistors constituting the I/O circuit from being supplied with an excessive voltage, and prevent a current from flowing from the signal cable to the power supply (VDD and GND) of the I/O circuit, even when a signal of a voltage higher than the power supply voltage VDD is supplied from the external device via the signal cable and I/O pins, under the condition in which the power supply voltage VDD is not supplied to the device.
According to an aspect of the present invention, there is provided a hotplug tolerant I/O circuit incorporated in a first device, the hotplug tolerant I/O circuit comprising: a first voltage generator for generating a first control voltage from an input signal supplied from a second device, when the first device is connected to the second device under a condition in which a power supply voltage is not supplied to the first device; and an I/O circuit transistor section to which the first control voltage generated by the first voltage generator is supplied instead of the power supply voltage.
Here, the first voltage generator may comprise a first transistor connected to an I/O pin receiving the input signal; a diode connected to the first transistor; and a second transistor connected between the diode and the power supply voltage.
The hotplug tolerant I/O circuit may further comprise a second voltage generator for generating a second control voltage from the input signal supplied from the second device, when the first device is to connected to the second device under the condition in which the power supply voltage is not supplied to the first device, wherein the first control voltage may be supplied to one of a plurality of transistors constituting the I/O circuit transistor section, and the second control voltage may be supplied to another transistor constituting the I/O circuit transistor section.
The second voltage generator may comprise a first transistor connected to the I/O pin receiving the input signal; a diode connected to the first transistor; and a second transistor connected between the diode and the power supply voltage.
The first transistor and the second transistor may be a PMOS transistor each.
The first transistor may be an NMOS transistor and the second transistor may be a PMOS transistor.
REFERENCES:
patent: 4800303 (1989-01-01), Graham et al.
patent: 5266849 (1993-11-01), Kitahara et al.
patent: 5410267 (1995-04-01), Haycock et al.
patent: 5450025 (1995-09-01), Shay
patent: 5528447 (1996-06-01), McManus et al.
patent: 5546019 (1996-08-01), Liao
patent: 5990705 (1999-11-01), Lim
patent: 6043681 (2000-03-01), Lim
Kurooka Kazuaki
Moriguchi Yasuo
Burns Doane , Swecker, Mathis LLP
Mitsubishi Denki & Kabushiki Kaisha
Paik Steven S.
Tokar Michael
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