Gate Circuit for Bus Signal Lines

1. Field of the Invention

The present invention relates to a gate circuit for bus signal lines used upon a LSI (Large Scale Integration) device. More particularly, the present invention relates to a gate circuit having a failure detecting circuit for making the choice of bus signal lines when a processing unit controls the gates of the gate elements.

2. Description of the Prior Art

FIG. 5 shows a gate circuit for bus signal lines with a tri-status bus circuit which has been broadly used upon a LSI device of a conventional microcontroller.

In the drawing, a gate circuit for bus signal lines includes the tri-status gates as gate elements 1-3, bus signal lines 1a-3a, gate signal lines 1b-3b, functional block of a microcontroller F0-F5 showing Micro Read Only Memory F0, Data Path F1, Interrupt Controller F2, Timer F3, Random Access Memory F4, and Universal Asynchronous Receiver Transmitter F5 respectively, inner bus IBUS which mutually connects with F0-F5, and control circuits G1-G3.

Referring to FIG. 5, the gate circuit for bus signal lines includes three tri-status gates.

These tri-status bus circuits supply the control signals C1-C3 to the gates of the said tri-status gates through the gate signal lines 1b-3b for making the choice of input signals provided through bus signal lines 1a-3a and control each of the states of the tri-status bus gates 1-3.

Then, for example, in the case that the control signals, C1-C3, are switched to the "1" state, the tri-status gates 1-3 turn to the "ON" state control micro code signal MIC, output from micro ROM F0, is designed not to provide two control signals at the same time and also not to turn a plurality of the tri-status gates 1-3 to the "ON" state. FIG. 6 shows an example of a PLA circuit consisting of control circuits G1-G3 as shown in FIG. 5. These circuits decode micro code signal MIC from micro ROM F0 and thereby output control signals C1-C3.

The above mentioned tri-status bus circuit does not include any detecting circuit for each of the tri-status gates 1-3. Any failure occurring in control circuits G1-G3 etc. provide error control signals C1-C3 to gate signal lines 1b-3c. Even when a plurality of tri-status gates 1-3 have turned to the "ON" state simultaneously, any failure that has not been detected, causes the problem that the reliability of data to be output to inner bus IBUS is decreased.

The primary object of the present invention is to provide a failure detecting gate circuit wherein the elements of each gate, for example, a tri-status gate, are under the control of the control signals.

Another object of the present invention is to provide a failure detecting circuit composed of a simple circuit though the number of gate signal lines which control the gate of the gate elements may be increased.

For the accomplishment of the above mentioned objects of the present invention, there is included a failure detecting circuit providing an output signal which indicates a failure when a plurality of the tri-status gates turn into "ON" state at the same time.

The above and other objects, features, and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.

FIG. 1 shows an embodiment of a microcontroller LSI using the gate circuit of the bus signal lines of the present invention;

FIG. 2 shows a relationship between the memorized contents of the ROM and the address when a failure detecting circuit is composed of ROM;

FIG. 3 shows another example of construction of a failure detecting circuit;

FIG. 4 illustrates an example of multiplexing signals indicating failure conditions with normal signals;

FIG. 5 shows an example of microcontroller LSI using a conventional gate circuit; and

FIG. 6 is a block diagram showing an example of construction of a control circuit as shown in FIG. 5.

FIG. 1 shows an embodiment of a microcontroller LSI using the bus signal lines of the present invention.

In the drawing, the same reference numerals designate like parts. The reference numeral 4 is a memory composed of ROM as a failure detecting circuit, and the ROM is supplied control signals for the address signals C1-C3 to the input terminals A1-A3 for the tri-status gates. The ROM 4 as illustrated in FIG. 2, reads out its memorized contents at the address 0-7 in response to the address signals supplied to the input terminals A1-A3. When a plurality of the control signal C1-C3 are in the state "1", a plurality of the tri-status gates 1-3 go to the "ON" state, and output signal CHK indicating failure state "ON" is available to read out. For instance, at the time the control signal C1 and C2 on "1" state are respectively supplied to the input terminals A1 and A2. Control signal C3 are in the "0" state, the is supplied to the input terminal A3. The memorized content "1" at the sixth address is read out, so any failure in the processing unit is detected. Referring to the tri-status bus circuit illustrated in FIG. 1, when any failure has occurred on the control signal C1-C3 and a plurality of the tri-status gates 1-3 go to the "ON " state, output signal CHK "1" appears and a failure on control signals C1-C3 is detected.

According to the above description of the failure detecting circuit, the ROM is used in the detecting circuit. Even when the number of gate signal lines which gate the gate elements of tri-status gates has been increased, only the increase of the capacity of the ROM should be required and a simple failure detecting circuit may be obtained.

While there has been described what is, at present, considered to be a preferred embodiment of the present invention, it will be understood that the number of the tri-status gates may be available more than three.

Although in the above mentioned description a ROM is used in a failure detecting circuit, it will also be understood that various modifications of the circuit may be available without using a ROM. FIG. 3 illustrates an example of the embodiment of logic element as a failure detecting circuit. In this circuit, when an input signal to either any two of input terminals A1-A3 goes to the "ON" state, output signal CHK is output. In short, a detecting function will be required to detect the "ON" state of a plurality of the tri-status gates caused by control signals C1-C3.

It is to be understood that in the above mentioned embodiment of the present invention, tri-status gates are used as the gate elements. The above embodiment is applicable to the other circuit elements, for instance, transfer gates, controlling the state of gate elements by supplying a control signal to the logic element. And as to an output signal, CHK, indicating a failure condition as shown in FIG. 4, both an output signal CHK and a normal signal of a microcontroller LSI are designed to be input to a multiplex circuit 5. The output of multiplex circuit 5 is the CHK signal which issued as a test mode signal, TM, in case of H-level signal. In case of L-level signal a normal signal is output. Consequently a failure in a LSI device of bus signal lines is to be detected without large additional increases in hardware.