Method and apparatus for enabling and disabling a test mode of operation of an electronic memory device without additional interconnects or commands

Apparatus for providing a test mode of operation and functional mode of operation of an electronic device wherein the device includes a number of inputs and outputs, test circuitry for carrying out the test mode of operation of the device and functional circuitry for carrying out the functional mode of operation of the device, the apparatus comprising:

• (a) output selection and driver circuitry for selectively applying the outputs of the test circuitry to the output terminals of the device, when in test mode of operation, or the outputs of the functional circuitry to the output terminals of the device, when in functional mode of operation,
• (b) control circuitry to cause the output selection and driver circuitry to switch between the test circuitry when in test mode of operation and the functional circuitry when in functional mode of operation,
• (c) power sensing circuitry operative directly after power up of the device, for determining when power to the device becomes stable,
• (d) the power sensing circuitry including circuits for activating the control circuitry for the test mode of operation, during which the device outputs are controlled by the test circuitry,
• (e) delay circuitry being activated by the power sensing circuitry for providing a delay function during which the test mode of operation cannot be deactivated and
• (f) monitoring circuitry, the inputs of which are connected to predetermined ones of the inputs for monitoring the inputs of the device until one of a number specified conditions is detected and
• (g) the monitoring circuitry including an output connected to the control circuitry to cause the test mode of operation to be deactivated and the functional mode of operation to be activated, after which the device outputs are controlled by the functional circuitry and wherein at least one of the number of specified conditions is defined by a combination of input signals which cause the state of at least one part of the functional circuitry of the device to become a known state and wherein at least one of the number of specified conditions corresponds to a command which causes data to be written into the functional circuitry of the electronic device.

A method of allowing a test mode of operation and subsequent functional mode of operation of an electronic device wherein the device includes a number of inputs and outputs, test circuitry for carrying out the test mode of operation and functional circuitry for carrying out the functional mode of operation, the method comprising the following steps:

• (a) in response to powering up of the device and after power to the device becomes stable, activating the test mode of operation of the device, during which the outputs of the device are controlled by test circuitry, but during which the functional circuitry operates normally in all respects to input stimulus, except that it does not control the outputs of the device;
• (b) preventing the test mode of the test circuitry from being deactivated for a predetermined period of time;
• and,
• (c) after the predetermined period of time has elapsed, deactivating the test mode of operation of the device upon occurrence of one of a number of specified conditions and activating the functional mode of operation of the device wherein the outputs of the device are controlled by the functional circuitry and wherein at least one of the number of specified conditions is defined by a combination of input signals which cause the state of at least one part of the functional circuitry of the device to become a known state and wherein at least one of the number of specified conditions corresponds to a command which causes data to be written into the functional circuitry of the electronic device.