JP2008203988A - Security protection function-equipped microcomputer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a security protection function-equipped microcomputer wherein protection to security management information to an incorporated rewritable nonvolatile memory and the other resources is enhanced. <P>SOLUTION: This security protection function-equipped microcomputer 1 has: a PROM 11 that is the only-once rewritable nonvolatile having an area written with a security flag; and an access control part 12 controlling access from a CPU 13 to the various resources incorporated by the security protection function-equipped microcomputer 1 according to setting of the security flag written in the PROM 11. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a microcomputer with a security protection function.

  The ROM installed in the microcomputer is a rewritable non-volatile memory such as a flash memory, the program to be executed by the microcomputer is written in the rewritable non-volatile memory, and the program is rewritten as necessary to develop the program. There is an easy microcomputer.

  In such a microcomputer, rewriting of the non-volatile memory is permitted at the time of program development, but reading, rewriting and erasing of data written to the non-volatile memory are prohibited after the development of the program is completed in order to protect the program. It is necessary to manage security for security.

  For this reason, conventionally, the flash memory is provided with a programming area and a programming management area for writing a write flag, a read flag, and an erase flag, which are management information for the programming area. In such a case, there has been proposed a microcomputer in which the CPU refers to the management information in the programming management area to determine whether or not the programming area can be programmed (see, for example, Patent Document 1).

  However, the above-described method has a problem that the management information itself for the area to be programmed is written in a rewritable flash memory, and thus the security for the management information is not sufficiently protected.

Further, since the object of security protection is limited to the area where the flash memory is programmed, there is a problem that the security of other resources such as a RAM built in the microcomputer cannot be protected.
JP 2000-181898 A (page 3-4, FIG. 1)

  SUMMARY OF THE INVENTION An object of the present invention is to provide a microcomputer with a security protection function in which protection against security management information for a built-in rewritable nonvolatile memory and other resources is enhanced.

  According to one aspect of the present invention, a non-volatile memory that can be written only once to write a security flag for a plurality of built-in resources, and access to the resource is controlled according to the setting of the security flag written to the non-volatile memory. There is provided a microcomputer with a security protection function characterized by having an access control means.

  According to the present invention, since security management information is written in a non-volatile memory that can be written only once, the security management information can be prevented from being rewritten, and the security protection function of the microcomputer can be enhanced.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a block diagram showing an example of the configuration of a microcomputer with a security protection function according to an embodiment of the present invention.

  The microcomputer 1 with a security protection function of this embodiment includes a PROM 11 that is a non-volatile memory that can be rewritten only once and has a security flag write area, and a microcomputer with a security protection function according to the setting of the security flag written in the PROM 11. And an access control unit 12 that controls access from the CPU 13 to various resources built in the computer 1.

  Here, a flash memory 14, a RAM 15, and an I / O area 16 are shown as examples of resources whose access is controlled by the access control unit 12. Among these, a program executed by the microcomputer 1 with a security protection function is written in the flash memory 14 which is a rewritable nonvolatile memory.

  Note that the resources subject to access control are not limited to these, and other resources can also be subject to access control.

  The access control unit 12 controls transmission of a read / write signal issued from the CPU 13 to each resource, and suppresses transmission of the read / write signal to a resource for which access is prohibited. Data input / output to / from each resource is performed via a data bus line.

  The PROM 11 to which the security flag is written has a plurality of bits, and the bit to which the security flag has been written indicates prohibition of access to a resource assigned in advance to the bit. If the security flag is not written, access to the resource assigned to that bit is permitted.

  The security flag is set by sending security data from the external device 100 to the CPU 13 and controlling the writing of the PROM 11 bit by bit in accordance with the designation of the security data.

  Since the PROM 11 can be written only once and cannot be rewritten, once the security flag is set, the security flag cannot be canceled.

  FIG. 2 is a diagram illustrating an example of the configuration of the security flag. Here, it is assumed that the security flag writing area of the PROM 11 is 8 bits, and each bit indicates prohibition of access as shown below.

  The security flag of bit 0 specifies prohibition of rewriting of the flash memory 14. For example, if a security flag is written to bit0 after a program is written to the flash memory 14, the flash memory 14 is thereafter in a rewrite-inhibited state, and the program written to the flash memory 14 is protected. .

  The security flags of bit 1 to bit 4 indicate prohibition of access to resources other than the flash memory 14.

  Bit 1 indicates prohibition of access to the RAM 15, and bit 2 indicates prohibition of access to the I / O area.

  Bit 3 indicates prohibition of data transfer from the RAM 15 to the I / O area 16, and bit 4 indicates prohibition of data transfer from the I / O area 16 to the RAM 15. When prohibiting data transfer, the access control unit 12 prohibits read access of the data transfer source and prohibits write access of the data transfer destination.

  In this way, by designating prohibition of access to resources other than the flash memory 14, in addition to protecting the program, it is possible to prevent the data stored in the RAM 15 and the I / O area 16 from leaking outside. it can.

  The security flags of bit 5 and bit 6 are set in order to protect the PROM 11 in which the security flag is written.

  Bit 5 prohibits reading of the security flag written in the PROM 11. This makes it impossible to read how the security flag is set.

  Bit 6 prohibits additional writing of a security flag to the PROM 11. If only a part of the PROM 11 is written, the remaining bits can be written after that. However, writing to the remaining bits can be prohibited by setting this bit 6 security flag. Thereby, the state of the security flag once set can be held.

  Bit 7 indicates that debugging is prohibited. Normally, when performing on-chip debugging or the like, an external debugging tool is connected to check the internal operation of the microcomputer. By setting this bit7 security flag, the microcomputer 1 with security protection function can be externally debugged. Deny connection to the tool. This prevents the internal operation state of the microcomputer 1 with a security protection function from being viewed through the external debug tool.

  The area for writing the security flag is not limited to 8 bits, and can be increased or decreased to the required number of bits. Further, the security function assigned to each bit is not limited to the above example, and can be arbitrarily set by the user.

  FIG. 3 is a flowchart showing a procedure for setting the security flag shown in FIG.

  When security data is transmitted from the external device 100 to start writing the security flag (step S01), the CPU 13 checks whether the bit 6 security flag of the PROM 11 has been set (step S02).

  If the security flag for bit 6 has already been set (Y), additional writing of the security flag to the PROM 11 is prohibited, so that a new security flag cannot be set, and the security flag setting process is terminated as it is. To do.

  If the bit 6 security flag is not set (N), thereafter, the CPU 13 checks, for each bit, whether there is a security flag setting request in the transmitted security data, and if there is a setting request. For example, the bit is written and a security flag is set (steps S03 to S16).

  When all the bits have been processed, the CPU 13 returns the data set in the PROM 11 to the external device 100 (step S17), and ends the security flag setting process.

  FIG. 4 is a flowchart showing an example of an access control processing procedure performed by the access control unit 14. Here, an example of processing for a rewrite request to the flash memory 14 is shown.

  When there is a rewrite request from the CPU 13 to the flash memory 14 (step S21), the access control unit 14 reads the security flag of bit0 of the PROM 11 (step S22), and whether or not the security flag of bit0 is set to prohibit rewriting. Is confirmed (step S23).

  If the bit0 security flag is set to prohibit rewriting (Y), the access control unit 14 prohibits access to the flash memory 14 (step S24), and if the bit0 security flag is not set to prohibit rewriting (step S24). N) The access control unit 14 permits access to the flash memory 14 (step S24), and ends the access control process.

  According to the present embodiment, since the security flag is written in the non-volatile memory (PROM) that can be written only once, the security flag once written can be prevented from being rewritten, and the security protection function of the microcomputer is enhanced. can do.

  In addition to the protection of programs executed by the microcomputer, the PROM to which the security flag is written has a plurality of bits, and a security function for resources incorporated in the microcomputer is assigned to each bit. Various kinds of stored data can be protected, and the security protection function of the microcomputer can be further improved.

  In addition, since the user can arbitrarily set the security function assigned to each bit of the PROM, the security level can be set according to the use of the microcomputer.

The block diagram which shows the example of a structure of the microcomputer with a security protection function based on the Example of this invention. The figure which shows the example of a structure of a security flag. The flowchart which shows the example of the setting procedure of a security flag. The flowchart which shows the example of the process sequence of an access control part.

Explanation of symbols

1 Microcomputer with security protection function 11 PROM
12 Access control unit 13 CPU
14 Flash memory 15 RAM
16 I / O area

Claims (5)

Non-volatile memory that can be written only once to write security flags for multiple built-in resources;
An access control means for controlling access to the resource according to the setting of the security flag written in the nonvolatile memory. The nonvolatile memory has a plurality of bits to be written for each bit, and the bit to which the security flag is written indicates prohibition of access to a predesignated resource. The microcomputer with a security protection function according to claim 1. 3. The microcomputer with security protection function according to claim 2, wherein the security flag includes a flag indicating prohibition of writing to the nonvolatile memory. 3. The microcomputer with security protection function according to claim 2, wherein the security flag includes a flag indicating prohibition of reading from the nonvolatile memory. 2. The microcomputer with security function according to claim 1, wherein the access control means inhibits access to the resource by inhibiting transmission of a read / write signal issued from a CPU.

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