# Patent

## Title
Automatic test entry termination in a memory device

## Abstract

A memory device has a control register comprising a test mode disable bit. The test mode is initially enabled. If the device does not receive an appropriate key or command as the next command received, the test mode is disabled. If the appropriate key is received, the test mode is continued to be enabled until it is expressly disabled by the user.

## Background
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to memory devices and in particular the present invention relates to test modes in memory devices.
BACKGROUND OF THE INVENTION
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices. Most electronic devices are designed with a single flash memory device.
NAND flash memory devices are becoming popular due to the high memory densities possible at a relatively low cost. The NAND architecture connects 8, 16 or 32 memory cells in series on a single bit line.
NAND flash memory devices can typically be sold to an end user, sold to a memory card manufacturer for combination with a memory controller on memory cards such as MMC cards and SD cards, or combined with a controller by the memory manufacturer to produce a memory card. In any case, the memory device must be tested before the memory device can be sold.
The manufacturing and testing of NAND memory devices is a complicated process. Many tests and functions are needed to make the part reliable. To accomplish this, the memory manufacturers have provided test modes to perform various test functions on the memory device. Some test functions could stress the part beyond its datasheet limits in order to determine if anything will break. These test modes should not be available to the end user of the memory device or the end user of the memory card.
To keep the end user from accessing the test modes, and possibly destroying the part, the manufacturers typically make it difficult to enter these modes. This can be accomplished by using complicated command sequences combined with high voltages on specific pins. This makes it very difficult for end users to enter the test modes since these voltages are not available on the memory cards or end systems. However, this might also make it difficult for some end users to test the parts if they have to generate the high voltages.
If the memory device is sold to an end user, it is desirable to disable any low voltage test mode. If the memory devices are put on cards at the memory manufacturer, it is desirable to have a low voltage test mode available since high voltages are not normally available on the memory card. Also, the testing of the functionality of a NAND flash memory device might be done on a memory card to make the cost more favorable.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved memory device test entry termination.
SUMMARY
The above-mentioned problems with memory testing and other problems are addressed by the present invention and will be understood by reading and studying the following specification.
The present invention encompasses a flash memory device comprising a memory array. The memory array has a plurality of non-volatile memory cells. A control register in the device has a test mode disable bit that controls disablement of a test mode such as a low voltage test mode. A controller circuit is coupled to the control register. The controller circuit sets the disable bit in response to receiving a command that is not a predetermined key.
Further embodiments of the invention include methods and apparatus of varying scope.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of one embodiment of a memory system of the present invention.
FIG. 2 shows a flowchart of one embodiment of an automatic test entry termination method of the present invention.
FIG. 3 shows a flowchart of another embodiment of an automatic test entry termination method of the present invention.



DETAILED DESCRIPTION
In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.
FIG. 1 illustrates a functional block diagram of a memory device 100 of one embodiment of the present invention that is coupled to a processor 110. The processor 10 may be a microprocessor, a processor, or some other type of controlling circuitry. The memory device 100 and the controller 110 form part of an electronic system 120. The memory device 100 has been simplified to focus on features of the memory that are helpful in understanding the present invention.
The memory device includes an array of memory cells 130. The memory cells are non-volatile floating-gate memory cells and the memory array 130 is arranged in banks of rows and columns. In one embodiment, the memory array is a NAND-type architecture. In another embodiment, the memory array is a NOR-type architecture. The present invention is not limited to any one type of memory array architecture.
An address buffer circuit 140 is provided to latch address signals provided on address input connections A0–Ax 142. Address signals are received and decoded by a row decoder 144 and a column decoder 146 to access the memory array 130. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array 130. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.
The memory device 100 reads data in the memory array 130 by sensing voltage or current changes in the memory array columns using sense/latch circuitry 150. The sense/latch circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array 130. Data input and output buffer circuitry 160 is included for bi-directional data communication over a plurality of data connections 162 with the controller 110). Write circuitry 155 is provided to write data to the memory array.
Control circuitry 170 decodes signals provided on control connections 172 from the processor 110. These signals are used to control the operations on the memory array 130, including data read, data write, and erase operations. In one embodiment, the control circuitry 170 is a microcontroller that executes the embodiments of the automatic test entry termination methods of the present invention.
Chip select generation circuitry 125 generates the chip select signals for the memory device 100. This circuitry 125 uses the address connections 142 from the processor 110 to generate the appropriate chip select signal depending on the address present on the address connections 142.
The flash memory device illustrated in FIG. 1 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.
FIG. 2 illustrates a flowchart of one embodiment of the automatic test entry termination method of the present invention. In this embodiment, the high voltage testing is performed prior to the memory device being mounted in memory card or other product. For example, the high voltage testing may be done in a test fixture that can provide the required voltages to stress the part.
After the initial testing is complete 201, the low voltage test mode is enabled 203. This may be accomplished by setting a test mode enable bit in the control register 180 illustrated in FIG. 1. If the memory device does not receive a key 205 as its next command, the low voltage test mode is disabled 207. This can be accomplished by setting a test mode disable bit in the control register 180 of FIG. 1, by resetting the enable bit to another state, or some other way of flagging a disabled test mode.
In the embodiment of FIG. 2, the key received by the memory device is a unique program command. For example, if a normal program command is a 40H, a command of 49H may be used to not only perform a program command but also to keep the low voltage test mode from being disabled. If the normal program command of 40H was received as the next command after the test mode enable bit was set, the device would permanently disable the low voltage test mode such that it is no longer available to an end user.
If the special program command was received, the program operation is initiated 209. The low voltage test mode can still be initiated at any time after that 211. In this embodiment, if the low voltage test mode is enabled by receipt of the key, the test mode will not be disabled until a command is received to expressly disable the mode 213. In other words, the device may go through multiple read and program operations prior to the test mode finally being initiated and then disabled 213.
FIG. 3 illustrates a flowchart of an alternate embodiment of the automatic test entry termination method of the present invention. This embodiment enables the low voltage test mode 303 after the initial test operations are complete 301.
The key in this embodiment is a sequence that does not affect the device's operation but only informs the part that the low voltage test mode should not be disabled. For example, the key may be the special program command 49H with data of FFFF. The microcontroller of the present invention does not recognize this command and data to initiate a program operation but instead causes the microcontroller to not disable the test mode. In an alternate embodiment, the key is a sequence of multiple commands.
If the next command received after the low voltage test mode is enabled is not the required key 305, the test mode is disabled 307 such that it is no longer available to an end user. If the key is received 305, the low voltage test mode is initiated 309. The test mode can then be disabled 313 at some subsequent time as discussed previously. This can be accomplished by setting a test mode disable bit in the control register 180 of FIG. 1, by resetting the enable bit to another state, or some other way of flagging a disabled test mode.
CONCLUSION
The embodiments of the present invention enable a memory device to have a test mode that is available to anyone having a key. If the key is the next command received by the memory device after enablement of the test mode, the test mode remains available. If any command besides the key is received, the test mode is disabled. The present invention prevents an unauthorized end user from having access to the test modes of the part, thereby potentially damaging the part.
The embodiments of the present invention are not limited to any one type of memory technology. For example, the scratch control memory array may be implemented in a NOR-type flash memory device, a NAND-type flash memory device, or any other type memory device that can be constructed with such a memory array.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.

## Claims

1. A flash memory device comprising:
a memory array comprising a plurality of non-volatile memory cells;
a control register comprising a test mode disable bit; and
a controller circuit, coupled to the control register, for setting the test mode disable bit in response to receiving a command, other than an intervening predetermined key, after a low voltage test mode has been enabled such that the command is received immediately after enabling the low voltage test mode without the intervening predetermined key.

2. The device of claim 1 wherein the flash memory device is a NAND-type device.

3. The device of claim 1 wherein the flash memory device is a NOR-type device.

4. The device of claim 1 wherein the test mode disable bit comprises a non-volatile memory cell.

5. The device of claim 1 wherein the predetermined key is a program command.

6. The device of claim 1 wherein the predetermined key is a sequence of commands.

7. The device of claim 1 wherein the controller circuit enables execution of the test mode in response to receiving the predetermined key.

8. A flash memory device comprising:
a memory array comprising a plurality of non-volatile memory cells;
a control register comprising a test mode disable bit; and
a controller circuit, coupled to the control register, for setting the test mode disable bit in response to receiving a command, other than an intervening predetermined command sequence, after a low voltage test mode has been enabled such that the command is received immediately after enabling the low voltage test mode without the intervening predetermined command sequence.

9. The device of claim 8 wherein the predetermined command sequence is a command and a data word.

10. The device of claim 8 wherein the predetermined command sequence is a plurality of commands.

11. The device of claim 8 wherein the controller circuit enables execution of the test mode in response to receiving the predetermined command sequence.

12. An electronic system comprising:
a processor that controls operation of the electronic system; and
a flash memory device, coupled to the processor, comprising:
a memory array comprising a plurality of non-volatile memory cells;
a control register comprising a test mode disable bit; and
a controller circuit, coupled to the control register, for setting the test mode disable bit in response to receiving a command, other than an intervening predetermined key, after a low voltage test mode has been enabled such that the command is received immediately after enabling the low voltage test mode without the intervening predetermined key.