JPH03257557A - Microcomputer - Google Patents

Abstract

PURPOSE:To protect the secrecy of a program by attaining a constitution where a coincidence detecting circuit outputs a grant signal when the coincidence is secured between the ROM codes and the external input codes by an amount equal to the number of bytes set previously. CONSTITUTION:A coincidence detecting circuit 6 compares the codes equivalent to three bytes and outputs an overflow signal only when the coincidence is secure among all codes. Then the circuit 6 is set in a test mode when the logic level of a test terminal 3 is set high. When the terminal 3 is set at a high level, the addresses of a ROM 1 are successively inputted to the circuit 6 at and after an address O. At the same time, the codes equal to the ROM codes equivalent to three bytes are successively inputted via an external input port 5 synchronously with input of the ROM codes. Thus the overflow signal given from the circuit 6 is inputted to a control circuit 2, and the ROM codes are outputted to an external output port 4 at and after the 4th code in a ROM code output state.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a microcomputer with a built-in read-only memory.

[Conventional technology]

FIG. 2 shows a block diagram of a conventional microcomputer. In the figure, (1) is read-only memory (
(2) is a control circuit that controls the output of the FIOM code, (8) is a test terminal for externally switching to the test mode, and (4) is an external output port.

Next, the operation will be explained. When the logic level for entering the test mode is input to the test terminal (8), the control circuit (2) outputs the contents of the ROM (1) in order from address O to the external output port (4) as an E'lOM code. Ru.

[Problem to be solved by the invention]

Conventional microcomputers are configured as described above, so it is easy to read ROM by changing the logic level of the test terminal.
There was a problem that since the code could be read out, confidential information could be revealed.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a microcomputer that can easily hide the ROM code from those who do not know the ROM code.

[Means to solve the problem]

The microcomputer used in this (2) Q Ming is 40
When reading the M code, the ROM code cannot be read unless a code that matches the ROM code by a predetermined number of bytes is input from the outside in synchronization with the ROM code.

[Effect]

The match detection circuit according to the present invention outputs a permission signal when the ROM code and external input code match by a preset number of bytes, allowing the ROM code to be read.

〔Example〕

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

In FIG. 1, items (1) to 4) are the same as those shown in the conventional example of FIG. 2, so their explanation will be omitted.

(6) is an external input port, and (6) is a coincidence detection circuit that compares the codes input from the ROM (1) and the external hole capo (15!) and detects a coincidence.

Here, the match detection circuit (6) compares 3 bytes worth of codes and outputs an overfollow signal only when all codes match, and the logic level of the test terminal (8) is set to H.
Enter test mode when it becomes ``high''.

Next, the operation will be explained.

Put the microcomputer into operation state and connect the test terminal (8
) is set to Eight level, the data is sequentially input to the coincidence detection circuit (6) starting from address 0 of the ROM (1). At this time, in synchronization with the input of this ROM code, the same code as the 3-byte ROM code is input to the external input port (5).
When input from the 4th byte, the overfollow signal from the coincidence detection circuit (6) is input to the control circuit (2), becomes the ROM code output state, and is output from the 4th byte of the ROM code to the external output port (4).

〔Effect of the invention〕

As described above, according to the present invention, the test mode prevents a person who does not know the correct ROM code from reading out the ROM code, thereby protecting the confidentiality of the program.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a microcomputer according to an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional microcomputer. (1) is the ROM, (2) is the control circuit, (8) is the test terminal, (4) is the external output port, +5) is the external input port, and (6) is the match detection circuit. The same symbols indicate the same or equivalent parts.

Claims (1)

[Claims] A match detection circuit that compares the code output from the read-only memory and the code input from the outside in synchronization with the code by a predetermined number of bytes or more, and a match signal from this circuit that allows the code to be read-only to the outside. A microcomputer equipped with a control circuit that controls whether or not to output the code output from memory.