Static information storage and retrieval – Read/write circuit – Data refresh
Reexamination Certificate
2000-12-29
2002-08-20
Nguyen, Viet Q. (Department: 2818)
Static information storage and retrieval
Read/write circuit
Data refresh
C365S185250, C365S185240
Reexamination Certificate
active
06438056
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a device for refreshing the memory content of at least one memory cell of a read-only memory. The present invention also relates to a digital controller for controlling and/or regulating certain functions in a motor vehicle and having a microcomputer for processing a control program stored at least partially in a read-only memory.
BACKGROUND OF THE INVENTION
A digital controller is used in a motor vehicle to control and/or regulate certain functions in the motor vehicle. In particular, controllers are used to control/regulate the internal combustion engine, the transmission, the brakes, the heating, and the air conditioning.
The controller includes a microcomputer having a microprocessor, a program memory, a variable memory and input and output circuits. The microprocessor is the central control unit and processing unit, i.e., a central processing unit (CPU). The program memory contains a sequence of commands to be processed by the microprocessor, that is, the control program. Generally, the program memory is a read-only memory because the control program must be preserved even after a failure of the operating voltage. The variables generated during the processing of the control program and needed for running the control program are stored in the variable memory. The variable memory is usually composed of a read-write memory with random access (random access memory, RAM). Communication with peripheral devices such as a data display unit, a keyboard, a mass memory, etc. can take place over the input and output circuits. For example, three bus systems are provided for communication between the microprocessor with the program memory, the data memory and the input and output circuits. The microprocessor indicates a desired memory address over an address bus. Over a control bus, the microprocessor determines whether reading or writing is to be performed. The data exchange takes place over a data bus.
Read-only memories are also denoted as ROMs. In addition to being provided as the program memory, read-only memories are also used for other functions in a microprocessor. In the normal case, read-only memories can only be read, so they are suitable for storing tables and programs. The content of the read-only memories is preserved for a finite period of time when the operating voltage is turned off. This period of time is in the range of approximately several years.
An EEPROM (electronically erasable ROM) is a special type of ROM. This component is a memory that can be programmed and erased electrically. The stored information is preserved even after the power supply is turned off.
Immediately after programming the memory cells of a read-only memory, the charge status is at a programming charge. Because of parasitic effects in the memory cell, the charge of the memory cells drops continuously starting from the programming charge during the finite period of time until it is ultimately below a reading charge. The reading charge is the charge of a memory cell necessary for the memory sense amplifier to be able to detect the correct status (programmed/erased). A drop in the charge status of memory cells below the reading charge level results in information stored in the memory cells being read incorrectly. Then, a program code cannot be executed correctly, resulting in a malfunction or even total failure of the microcomputer and thus, of the controller.
SUMMARY OF THE INVENTION
The present invention provides a method and device to aid in preventing a malfunction or failure of a microcomputer accessing a read-only memory due to data loss in a read-only memory.
The present invention provides a method that may include the following steps:
determining the instantaneous charge status of the memory cell;
comparing the instantaneous charge status of the memory cell with a charge threshold which is above a reading charge necessary for correct detection of the content of the memory cell; and
increasing the charge status of the memory cell if the instantaneous charge status of the memory cell is below the charge threshold.
The charge status of the memory cell of the read-only memory is charged up to a programming charge (or an erase charge) as part of the programming or reprogramming of the memory cell. Because of parasitic effects, for example, traps in the oxides, in the memory cell, the charge status of the memory cell drops slowly over a finite period of several years starting from the programing charge/erase charge. The variation in the discharge over time may depend greatly on ambient parameters, for example, the ambient temperature, and therefore can be predicted only inadequately. Therefore, according to the present invention, the charge status of the memory cell during this period of time is compared with a charge threshold which is above the reading charge, in order to correctly detect the programming/erase state of a memory cell. When the charge status of the memory cell drops below the reading charge, data loss can occur in the memory cell. Therefore, the charge status of the memory cell is increased as soon as the instantaneous charge status of the memory cell drops below the charge threshold, that is, before data stored in the memory cell can be lost.
Through the method according to the present invention, the lifetime of the data in a read-only memory can be significantly prolonged. This is of interest in particular for those read-only memories which are part of a microcomputer of a controller for a motor vehicle, because the information stored in the read-only memory is correctly preserved even when there is a loss of charge of the memory cells due to the refresh.
The charge threshold is set so far above the reading charge that the information stored in the memory cell can still be read reliably at the charge threshold even in the worst case, e.g., at especially high temperatures.
For each memory cell, for each block having a plurality of memory cell or for each read-only memory having a plurality of blocks, the method according to the present invention determines the optimum time for raising the charge status of the memory cell. If the charge status of a memory cell has dropped below the charge threshold, the charge status of the memory cells of the block in which this memory cell can be arranged or the charge status of all the memory cells of the read-only memory can be raised. The method according to the present invention also makes it possible to take into account tolerances in manufacturing the memory cells of a read-only memory or component scattering with regard to the finite period of time for which the maintenance of charge of the memory cell is guaranteed.
The method according to the present invention can be used universally for various read-only memories. Except for a refresh circuit, no additional hardware is required in the read-only memory or in the microcomputer in order to implement this method. The refresh circuit can be integrated into the read-only memory.
According to an embodiment of the present invention, the charge status of the memory cell can be raised to the programming charge of a freshly programmed memory cell. According to this embodiment, the charge status of a memory cell whose charge status has been raised can again be above the reading charge for a finite period of time. Thus, the charge of the memory cell can be maintained for additional years.
According to another embodiment of the present invention, to increase the charge status of the memory cell, the content of the memory cell is stored temporarily in another memory cell, the memory cell is erased and the content of the additional memory cell is programmed again into the memory cell. To a certain extent, in increasing the charge status of a memory cell, the memory cell is in a way reprogrammed with its own memory content. The programming operation takes place in a conventional manner according to the related art by applying a programming voltage to the programming voltage pin of the read-only memory.
To determine the instantane
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