JPH053428A - Digital output circuit - Google Patents

Abstract

PURPOSE:To apply an output signal of positive logic directly to a light emitting diode of a photocoupler and to simplify the circuit configuration when the output signal of a CPU is outputted to an external device via the photocoupler. CONSTITUTION:A D flip-flop 10 comprising a CMOS element in which an absolute of a high level output current is larger than an operating current of photocouplers 1, 2 is provided as a latch circuit. An output signal from the CPU is inputted to the D flip-flop 10 as positive logic. Then an output signal from the D flip-flop is inputted to an anode terminal of a light emitting diode 13 of the photocoupler via a current limit resistor 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital output circuit for outputting an output signal of a CPU in a microcomputer to an external device via a photo coupler.

[0002]

2. Description of the Related Art Normally, the output of a logic circuit such as a microcomputer is divided into a negative logic output in which a low (L) level indicates an active state and a positive logic output in which a high (H) level indicates an active state. is there. FIG. 2A is a drive circuit diagram for driving the load 2a with the negative logic output circuit 1a having a negative logic output. In this drive circuit, a + 5V power source is connected to one end of the load 2a. When the output signal of the negative logic output circuit 1a becomes low (L) level, the output current (load current) I _OL flows from the + 5V power source in the arrow direction.

On the other hand, FIG. 2B is a drive circuit diagram for driving the load 2b by a positive logic output circuit 1b having a positive logic output. In this drive circuit, one end of the load 2b is grounded. When the output signal of the negative logic output circuit 1a becomes high (L) level, the output current (load current) I _OH flows in the direction of the arrow.

However, in an IC (integrated circuit) generally composed of a TTL bipolar logic circuit, the current value of the high-level output current I _OH is remarkably higher than that of the low-level output current I _OL. It is a small value. ｜ I _OH ｜ << I _OL

On the other hand, in the CPU (central processing unit) incorporated in the microcomputer, the control signals such as the write signal WR, the read signal RD and the reset signal RST are generally negative logic outputs, but the data signal D, The address signal A is generally a positive logic output.

Further, when controlling the operation of various devices by the output signal of the microcomputer, it is necessary to electrically disconnect the external device from the microcomputer by, for example, a difference in ground potential or noise countermeasures. There may be. As described above, a photocoupler is used as the simplest method of electrically blocking and transmitting only a signal.

However, in the photocoupler, since it is necessary to energize the light emitting diode by the input signal,
The current value of the input signal must be larger than the operating current value of the light emitting diode. This operating current value is significantly larger than the current value of the positive logic high level output current I _OH described above. Therefore, it is impossible to directly drive the photocoupler with the positive logic output signal output from the data terminal of the CPU via, for example, the latch circuit. Therefore, the digital output circuit shown in FIG. 3 is used to drive the photocoupler with the output signal of the CPU (actual fairness 2-3706).
No. 7).

That is, the 8-bit positive logic data signals D0 to D7 output from the respective data terminals of the CPU (not shown) are applied to the respective data terminals of the 8-bit D-type flip-flop 3. Then, the negative logic write signal WR applied from the CPU to the clock terminal CI.
Changes to L level, it takes in each data signal D0 to D7 applied to each data terminal and outputs it from each output terminal Q. When the negative logic reset signal RST applied to the reset terminal R changes to L level, the D-type flip-flop 3 is reset.

Each positive logic output signal output from each output terminal Q of the D-type flip-flop 3 is input to the next inverter circuit 4. The inverter circuit 4 inverts the signal level of the input signal. Therefore, each output signal of the D flip-flop 3 is converted by the inverter circuit 4 from a positive logic signal to a negative logic signal.

Each negative logic output signal output from the inverter circuit 4 is applied to the ground terminal of each light emitting diode 7 of each photocoupler 6 via the current limiting resistor 5. The anode terminal of the light emitting diode 7 is connected to the + 5V power source. Since the low level output current I _OL in the output signal of negative logic is larger than the drive current of the light emitting diode 7, the light emitting diode 7 operates sufficiently. When the light emitting diode 7 emits light, the phototransistor 8 becomes conductive and the FE
T9 is on / off controlled according to the signal. Therefore, an external device (not shown) connected to the output terminal of the FET 9 operates.

[0011]

However, in the digital output circuit configured as shown in FIG. 3, in addition to the usual bipolar D-type flip-flop 3 as a latch circuit, the purpose is to invert the output. The inverter circuit 4 used only was necessary. Therefore, not only the circuit structure becomes complicated, but also the manufacturing cost increases.

The present invention has been made in view of such circumstances, and an output signal of the positive theory can be directly applied to a light emitting diode of a photocoupler, an inverter circuit in a conventional circuit can be removed, and a circuit configuration can be small and simple. It is an object of the present invention to provide a digital output circuit that can be made compact and can reduce the manufacturing cost.

[0013]

In order to solve the above problems, the present invention provides a digital output circuit for outputting an output signal of a CPU to an external device through a photo coupler,

A D-type flip-flop formed by a CMOS device in which the absolute value of the high-level output current is larger than the operating current value of the photocoupler is provided as a latch circuit, and the output signal from the CPU is positively logically supplied to the D-type flip-flop. Then, the output signal of the D-type flip-flop is input to the anode terminal of the light emitting diode of the photocoupler via the current limiting resistor.

[0015]

[Function] Recently, high-speed CMOS (complementary metal-oxid)
e-semiconductor; complementary MOS) -IC was developed,
The high-speed CMOS-IC is gradually replacing the logic circuit formed of the conventional bipolar IC. This high-speed CMOS-IC has low power consumption, which is a characteristic of the prior art, and other characteristics that conventional bipolar ICs do not have. That is, in the case of the conventional TTL bipolar IC, as described above, the current value of the high level output current I _OH is much smaller than the current value of the low level output current I _OL . However, in this high-speed CMOS-IC, the current value of the high level output current I _{OH has} a value substantially equal to the current value of the low level output current I _OL .

The value of the positive logic high level output current I _OH is larger than the operating current value of the photocoupler. Therefore, the CPU
Even if the output signal of the positive logic, which is output from the device and is latched by the D-type flip-flop formed by the CMOS element, is directly applied to the light emitting diode of the photocoupler without level conversion, this photocoupler has the positive logic. It operates normally with the output signal of. As a result, it is not necessary to use an inverter circuit for converting a positive logic output signal into a negative logic output signal as in the conventional circuit.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a digital output circuit of the embodiment. For example, 8-bit positive logic data signals D0 to D output from respective data terminals of the CPU (not shown)
7 is applied to each data terminal of the D-type flip-flop 10 having an 8-bit structure. Then, C is applied to the clock terminal CI.
When the negative logic write signal WR applied from PU is changed to L level, each data signal D0 to D7 applied to each data terminal is taken and output from each output terminal Q. When the negative logic reset signal RST applied to the reset terminal R changes to L level, the D-type flip-flop 10 is reset.

The D-type flip-flop 10 is composed of the above-mentioned high speed CMOS-IC element. Therefore, each high level output current I in each positive logic output signal output from each output terminal Q of the D-type flip-flop 10
The value of _OH is larger than the operating current value of a normal photocoupler.

Each output terminal Q of the D-type flip-flop 10
Each output signal of the positive logic output from each is applied to the anode terminal of each light emitting diode 13 of each photocoupler 12 via each current limiting resistor 11. Light emitting diode 1
The cathode terminal of 3 is grounded. As described above, the high level output current I in the positive logic output signal of the D-type flip-flop 10 composed of the high speed CMOS-IC element
_{Since OH} is larger than the drive current of the light emitting diode 13, the light emitting diode 13 operates sufficiently. Light emitting diode 13
When is emitted, the phototransistor 14 becomes conductive.

The emitter signal of the phototransistor 14 is applied to the gate terminal of the FET 16 via the resistor 15. A resistor 17 is inserted between the gate and source of the FET 16. A collector voltage is applied to the collector of the phototransistor 14 from an external power supply 18 via a connection terminal 19. In addition, the positive terminal of the external power source 18 and F
An external load 21 as an external device is connected via a connection terminal 20 between the drain terminals of the ET 16. The source of the FET 16 is connected to the negative side terminal of the external power supply 18 via the connection terminal 22.

Therefore, the resistors 15 and 17 and the FET 16 connected to the photocoupler 12 are connected to the photocoupler 1.
At 2, a buffer circuit for an output insulated from an output signal from the CPU is configured.

In the digital output circuit having such a configuration, when an arbitrary output signal of the CPU changes to the high (H) level, the corresponding data terminal of the D-type flip-flop 10 becomes the high (H) level and the write signal. When the WR changes to the low (L) level, the D-type flip-flop 10
The output signal of the corresponding output terminal Q changes to a high (H) level. Then, the high level output current I _OH in the positive logic output signal flows through the light emitting diode 13 of the corresponding photocoupler 12.

As a result, the light emitting diode 13 operates,
The phototransistor 14 becomes conductive. Therefore, the emitter current flows and the FET 16 becomes conductive. And the external power supply 1
A current is supplied from 8 to an external load 21 as an external device, and the external load 21 operates.

As described above, since it is possible to directly drive the photocoupler 12 with the positive logic output signal of the D-type flip-flop 10 composed of the high-speed CMOS-IC device, the conventional digital circuit shown in FIG. The inverter circuit 4 used in the output circuit can be removed. As a result, the circuit configuration can be simplified. When the circuit configuration is simplified, the entire circuit can be downsized and the reliability of the circuit operation can be improved. Further, the manufacturing cost can be significantly reduced.

[0026]

As described above, in the digital output circuit of the present invention, the high speed CMOS device is used for the D-type flip-flop which latches the output signal of the CPU.
Therefore, the positive theoretical output signal can be directly applied to the light emitting diode of the photocoupler. As a result, the inverter circuit in the conventional circuit can be removed, the circuit configuration can be made small and simple, and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a digital output circuit according to an embodiment of the present invention,

FIG. 2 is a schematic diagram for explaining a difference between a general negative logic output signal and a positive logic output signal,

FIG. 3 is a circuit diagram showing a conventional digital output circuit.

[Explanation of symbols]

10 ... D-type flip-flop, 11 ... Current limiting resistor, 1
2 ... Photo coupler, 13 ... Light emitting diode, 14 ... Photo transistor, 16 ... FET, 18 ... External power supply, 21
… External load.

Claims (1)

Claim: What is claimed is: 1. A digital output circuit for outputting an output signal of a CPU to an external device via a photo coupler, wherein a CMOS element in which an absolute value of a high level output current is larger than an operating current value of the photo coupler. The D-type flip-flop formed by the above is provided as a latch circuit, the output signal from the CPU is input to this D-type flip-flop in positive logic, and the output signal of the D-type flip-flop is passed through the current limiting resistor to the photo sensor. A digital output circuit for inputting to an anode terminal of a light emitting diode of a coupler.