Method and circuitry for enabling and permanently disabling test mode

Abstract

A method of enabling access to a test mode of a semiconductor memory in response to user commands. The method enables test mode access only when a number of 'keys' are presented in the proper sequence via the memory device pins. During the first phase of the unlocking process, an array controller determines whether the correct confirmation codes were input via the address and data pins. If they were, the array controller proceeds to the second phase of the unlocking process. During the second phase voltage levels on selected control pins are checked for a transition to a first voltage level. If the control pins transition as required, the array controller proceeds to the third phase. During the third phase, the array controller waits a limited time for receipt of a second test mode enable command. The second test mode enable command must be followed by correct confirmation codes. If the third phase is successfully completed, the array controller writes to a test mode enable access register. As a result, an enable test mode signal becomes active, which allows the user interlace to respond to subsequently issued test mode commands. Also described is a method of eliminating access to the test mode of the semiconductor memory device, which includes a nonvolatile instruction memory.