Semiconductor memory with sequential clocked access codes for test mode entry
First Claim
1. An integrated circuit having a normal operating mode, and having a special operating mode enabled by an enable signal, comprising:
- a first terminal for receiving a mode initiate signal indicating entry into a special operating mode;
a second terminal for receiving a mode select code;
an evaluation circuit, having inputs coupled to said first and second terminals, and having an output, for presenting a match signal responsive to a mode select code, received at said second terminal in conjunction with said mode initiate signal, matching a preselected value; and
an enable circuit, having an input coupled to the output of said evaluation circuit, and having an output for presenting said enable signal responsive to receipt of a plurality of said match signals.
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Accused Products
Abstract
An integrated circuit having a normal operating mode and a special operating mode, such as a special test mode, is disclosed. The special test mode is enabled by a series of signals, such as overvoltage excursions at a terminal, rather than by a single such excursion, so that it is less likely that the special test mode is entered inadvertently, such as due to noise or power-down and power-up of the device. The circuit for enabling the test mode includes a series of D-type flip-flops, each of which are clocked upon detection of the overvoltage condition together with a particular logic level applied at another terminal; multiple series of flip-flops may be provided for multiple special test modes. In addition, sequential codes may be used for further security. Logic for evaluating both a sequence of codes received in parallel from a number of address terminals, and also a sequence of serial codes received at single address terminal, are disclosed. Additional features include the provision of a power-on reset circuit which locks out the entry into the test mode during power-up of the device. Acknowledgment of the entry into test mode is provided by the presentation of a low impedance at output terminals while the device is not enabled; chip enable of the device causes the device to exit the test mode. Once in test mode, the output enable terminal of the device can provide a chip enable function.
65 Citations
31 Claims
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1. An integrated circuit having a normal operating mode, and having a special operating mode enabled by an enable signal, comprising:
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a first terminal for receiving a mode initiate signal indicating entry into a special operating mode; a second terminal for receiving a mode select code; an evaluation circuit, having inputs coupled to said first and second terminals, and having an output, for presenting a match signal responsive to a mode select code, received at said second terminal in conjunction with said mode initiate signal, matching a preselected value; and an enable circuit, having an input coupled to the output of said evaluation circuit, and having an output for presenting said enable signal responsive to receipt of a plurality of said match signals. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A method for enabling a special operating mode of a circuit, comprising:
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receiving a plurality of mode initiate pulses at a first terminal of said circuit; receiving a mode select code at a second terminal in conjunction with receipt of each of said mode initiate pulses, said mode select codes indicative of a special operating mode to be enabled; and generating a special mode enable signal responsive to said plurality of received mode select codes matching preselected values; wherein said special mode enable signal is not generated responsive to receipt of the first of said plurality of mode initiate pulses. - View Dependent Claims (15, 16, 17, 18, 19, 20)
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21. A circuit for enabling a special operating mode, comprising:
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a first terminal for receiving a mode initiate signal; a second terminal for receiving mode select codes; a plurality of latches connected in series with one another, a first one of said plurality of latches having a known logic state connected to its data input, and a last one of said plurality of latches generating an enable output at its output; an evaluation circuit for presenting a clock signal to a first of said plurality of latches responsive to a first mode select code, received in conjunction with a mode initiate signal, matching a first predetermined value, and wherein said evaluation circuit presents a clock signal to a second of said plurality of latches responsive to a second mode select code, received in conjunction with another mode initiate signal, matching a second predetermined value. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29, 30, 31)
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Specification