JPH0535378A - Input and output circuit - Google Patents

Abstract

PURPOSE:To stabilize an operation just after an initialization by constituting a switching circuit of a resistor SR flip flop, and transistor which is turned on, and off by the output of the SR flip flop. CONSTITUTION:A microcomputer decides whether an input and output terminal 7 is used as an input terminal or as an output terminal by an initializing routine in an execution program, and when it is used as the input terminal, an input permitting signal E1 is defined as '1', and when it is used as the output terminal, an output permitting signal E0 is defined as '1'. Then, any processing is operated so that the reset of an SR flip flop 5 can be turned to '0', and an Nch transistor 2 can be turned off Then, when the input and output terminal 7 is used as the input terminal, a high impedance can be obtained, and when it is used as the output terminal, a level due to output data D1 can be outputted. And also, this input and output circuit 16 is equipped with input and output buffers 3 and 4, OR gate 6, and resistor 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input / output circuit, and more particularly to a microcomputer input / output circuit.

[0002]

2. Description of the Related Art In recent years, microcomputers have been used in a wide range of fields, and particularly in system control of consumer equipment, many control terminals are required.

Here, focusing on the terminals incorporated in the microcomputer, the number thereof is limited by the package and the like. From this, in order to effectively utilize the terminal, it is common to use both the input terminal and the output terminal. This input / output terminal is normally set to the input state, that is, the high impedance state by a hardware initialization process, and it is determined by the initial routine in the execution program whether this terminal is used as the input terminal or the output terminal. It

Therefore, there is no problem when this input / output terminal is fixedly used as an input terminal, but when it is used as an output terminal, the terminal is in a high impedance state until the above-mentioned initial routine is executed. , The state of the input buffer of the external input circuit connected to this terminal becomes undefined, and the operation immediately after the start of operation becomes unstable.

In order to avoid such a problem, conventionally, a circuit for pulling down to the ground potential of the device by a resistor having a relatively high resistance value outside the microcomputer, or
Alternatively, a circuit for pulling up to the power supply potential of the device is provided to perform pulldown or pullup processing temporarily or continuously.

A method for solving the above problem will be described with reference to FIG. Referring to FIG. 4, a conventional application system has an input / output circuit 16, an external input circuit 17, and a pull-down circuit 1.
It is composed of 8. The input / output circuit 16 includes the output buffers 3,
It is composed of an input buffer 4 and an input / output terminal 7.
The pull-down circuit 18 is composed of a resistor 13. The external input circuit 17 is composed of an input terminal 15 and an input buffer 14.

Next, the operation of this system will be described. By the system reset of the microcomputer,
Since the output enable signal E _O becomes inactive “0”,
The output buffer 3 is turned off and enters a high impedance state. However, the state of the input / output terminal 7 is pulled down to the ground potential by the resistor 13 outside the microcomputer. Here, the resistor 13 is set to have a higher resistance than the output impedance of the output buffer 3 when the output buffer 3 is active. Therefore, when the output permission signal E _O becomes active "1" and a high level is output, a ratio circuit is formed, but the interface with the external input circuit 17 can be logically and normally performed.

[0008]

However, in the conventional microcomputer, it is necessary to externally attach a pull-down resistor or a pull-up resistor for stabilizing the device state after reset. In that case, the cost of the resistor and the connection are required. The man-hour problem occurs. Moreover, in the case of a small device, it is difficult to secure a space for installing the resistor.

[0009]

The input / output of the present invention is an input / output circuit of a microcomputer having an input / output combined terminal capable of programmable switching between input and output of data. An input circuit for inputting, an output circuit for outputting data from the input / output combined terminal, and a series connection between the input / output combined terminal and one of a high-level power supply terminal and a low-level power supply terminal of the microcomputer. A resistor circuit and a switching circuit, wherein the switching circuit is brought into a conducting state by the initialization means of the microcomputer, and one of the input control signal of the input circuit and the output control signal of the output circuit is used as a trigger. It is characterized by transitioning to the cutoff state.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an optimum embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIG. 2 is a detailed circuit diagram in which the circuit diagram of FIG. 1 is made into a more specific circuit. Referring to FIGS. 1 and 2, the input / output circuit 16 includes an output buffer 3, an input buffer 4, an input / output terminal 7, an OR gate 6, an SR flip-flop 5, a resistor 1 and an Nch transistor 2.

The output buffer 3 includes an inverter 30 and an NA.
It is composed of an ND 31, a NOR 32, a Pch transistor 33 and an Nch transistor 34.

The input buffer 4 is composed of inverters 40 and 41, a Pch transistor 42 and Nch transistors 43 and 44.

The SR flip-flop 5 is an inverter 5
It is composed of 0 and 51 and NANDs 52 and 53.

Next, the operation of this embodiment will be described. The system reset of the microcomputer, the output enable signal E _O is to become inactive "0", the output buffer 3, the Pch transistor 33 and Nch transistor 34 are both turned off.

Further, the SR flip-flop 5 is reset by the system, and the input enable signal E _I and the output enable signal E I are set.
_Since both _O are inactive "0", the NAND 53 is "1" in response to the high level of the reset signal R.
Becomes Therefore, the Nch transistor 2 is turned on, and the input / output terminal 7 is pulled down to the ground potential via the resistor 1.

Next, the microcomputer determines whether to use this input / output terminal 7 as an input terminal or an output terminal according to an initial routine in the execution program. The permission signal E _{I is set} to “1”, and when used as an output terminal, the output permission signal E _{O is set} to “1”. Then, the SR flip-flop 5 is reset to "0" by performing any one of the processes, so that the Nch transistor 2 is turned off, and the input / output terminal 7 becomes high impedance when used as an input terminal and becomes an output terminal. When used, the level according to the output data D _O is output.

Next, a second embodiment of the present invention will be described with reference to FIG. Referring to FIG. 3, in the second embodiment, the terminal state after reset is set to the Pch transistor 2
It differs from the first embodiment in that it is pulled up to the power supply potential at 0.

Since the SR flip-flop 5 is inactive "0" for both the input enable signal E _I and the output enable signal E _O due to the system reset, the NAND 52 becomes "0" in response to the high level of the reset signal R. .. Therefore, the Pch transistor 20 is turned on, and the input / output terminal 7 is pulled up to the power supply potential via the resistor 1.

[0019]

As described above, according to the present invention,
Since it is not necessary to externally attach a pull-down resistor or a pull-up resistor for fixing the state after reset, the cost of the resistor can be reduced. Moreover, in the case of a small-sized device, it is not necessary to secure a space for installing a resistor, so that the space can be effectively used, and the cost of the entire device can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a detailed circuit diagram of FIG.

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

FIG. 4 is a block diagram of a conventional input / output circuit.

[Explanation of symbols]

 1, 13, resistors 2, 34, 43, 44 Nch transistor 3 output buffer 4, 14 input buffer 5 SR flip-flop 6 OR gate 7 input / output terminal 15 input terminal 16 input / output circuit 17 external input circuit 18 pull-down circuit 20, 33, 42 Pch transistor 30, 40, 41, 50, 51 Inverter 31, 52, 53 NAND 32 NOR

Claims (1)

Claim: What is claimed is: 1. An input / output circuit of a microcomputer comprising an input / output combined terminal capable of programmable switching between data input and output, and an input circuit for inputting data from the input / output combined terminal. An output circuit for outputting data from the input / output combined terminal, and a resistance circuit and a switching circuit connected in series between the input / output combined terminal and one of the high-order power supply terminal and the low-order power supply terminal of the microcomputer. The switching circuit is rendered conductive by the initialization means of the microcomputer, and transitions to a cutoff state by using one of the input control signal of the input circuit and the output control signal of the output circuit as a trigger. An input / output circuit characterized by the above.