JPH0124661Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an input/output port circuit for a central processing unit (CPU), and in particular, to an input/output port (I/O port) circuit.
The pull-up resistor in the input mode of the port) is the digital output of the numerical output in the output mode.
This invention relates to a circuit improved so that it can be used as a weighting resistor for analog conversion.

For data input to an I/O port group in input mode, analog output corresponding to the numerical data may be required in output mode. For example, in a control circuit system of a telephone, a dial tone corresponding to each dial value is generated in response to a dial value input when the circuit is connected. On the other hand, in recent years, it has become not uncommon for ordinary so-called push-button telephones to use microcomputers to perform various necessary functions. This tendency is particularly strong in cordless telephones and the like. In such telephones, the above-mentioned function is such that when the central processing unit is input with dial numerical values operated by the user, the central processing unit adjusts the corresponding frequency and intensity according to each numerical value. This is accomplished by outputting a dial tone signal.
As is well known, this dial tone signal is composed of a combination of multiple frequencies as an international standard, and this method is also known as Dual Tone Multi-Frequency.
It is called a method. However, since the dial tone information output from the central processing unit according to such a predetermined method is still a digital value at the output port of the central processing unit, it is necessary to convert it into an analog value. Naturally, this requires a digital-to-analog (D/A) converter.

However, conventionally, even in such cases,
The data input system and the dial tone information output system are considered completely separate and distinct, and generally the input and output ports have different terminal arrangements, and each system has its own auxiliary circuit. It ended up requiring a lot of effort.

In such an input/output circuit system, the present invention
It was created with the aim of providing a circuit that can perform data input and analog conversion output with a rational configuration around the input/output ports without requiring a digital-to-analog converter via a separate dedicated port group. .

The accompanying drawings illustrate an embodiment of the invention.
In this case, the input data is assumed to be 8 bits, so the input/output port I/O is also 8 bits. Of course, in general, it can be considered as n bits.

This input/output port I/O reads data or outputs data according to each input/output mode signal. In this embodiment, the above-mentioned dial data is considered, and therefore, the information to be read is the ON state of each switch SW1 to SW8 corresponding to the dial, and the numerical data to be output is the dial data. This is tone information.

The operation of this circuit will be explained in detail below. When reading the state of the switch, a specific output port of the central processing unit 1, for example, output port 01, is set to a low level "L". Then, the transistor 2 inserted as a kind of analog switch turns on, equivalently closing the output line to the subsequent operational amplifier 3, which will be described later.

Therefore, the other end of each resistor R1 to R8, whose one end is connected to each of input/output ports #0 to #7, is connected to the power supply V _CC
Therefore, in this input mode, each of the resistors R1 to R8 functions as a normal pull-up resistor for the input/output port I/O. Therefore, the state of each switch can be input to the central processing unit 1 via the input/output port I/O in the corresponding input mode.

On the other hand, in the numerical data output mode, the previously described output port 01, which is provided separately, transitions from a low level "L" to a high level "H", so that the transistor 2 is turned off. Therefore, the other end of each resistor, whose one end is connected to each of the input/output ports #0 to #7, is now isolated from the power supply and is connected in a kind of wired-OR configuration for calculation. This is equivalent to being connected to the signal input of amplifier 3.

In such a state, each of the resistors R1 to R8 connected to each input/output port I/O can function as a weighting resistor in digital-to-analog conversion by appropriately determining their relative values. can. In other words, if the least significant bit is port #7, then the value of the resistor R7 connected to port #6 of the next bit is doubled compared to the resistance value connected to this port, and then the value of the resistor R7 connected to port #6 of the next bit is If the value of resistor R5 connected to #5 is set to four times, etc., output port group #7~
The numerical output appearing at #0 is input to the operational amplifier 3 as a corresponding converted analog value, and a dial tone signal as predetermined analog information that has been appropriately amplified is obtained at the output of the operational amplifier.

Note that the digital-to-analog conversion circuit itself using a plurality of weighted resistor groups each having a different resistance value is well known, so conversely, if there is an electronic switch like transistor 2 in the above embodiment, this electronic When the switch is in input mode, the power is
In the output mode, the resistance groups R1 to R8 connected to V _CC are disconnected from the power supply, and instead, these resistance groups become the weighting resistance group in the digital-to-analog conversion circuit at connection nodes (in the illustrated embodiment). In this case, if the connection is switched to the positive-phase input of the operational amplifier 3, then the present invention is satisfied. Of course, these resistance groups R1 to R8
It is clear that when the resistors function as pull-up resistors in the input mode as described above, there is no problem in circuit operation even if they are weighted.

In the end, according to the present invention, in order to combine the pull-up resistor necessary for data input and the weighting resistor necessary for analog value conversion of the output digital value, a group of resistors for both of these functions is used as in the conventional method. There is no need to provide a separate circuit, and the circuit can be greatly simplified and the cost can be greatly reduced.

[Brief explanation of drawings]

The drawing is a schematic diagram of an embodiment of the present invention. In the figure, 1 is the central processing unit, 2 is the analog
Switch or transistor, 3 is an operational amplifier,
I/O is an input/output port, and R1 to R8 are resistors.

Claims (1)

[Claims for Utility Model Registration] A group of resistors each having one end connected to each of a plurality of input/output ports of the central processing unit and individually weighted in a predetermined manner; opposed to the one end of the resistor group; The other end is connected to a power source in input mode, while disconnected from the power source in output mode, and the connection is switched to a connection node where the resistance group becomes a weighted resistance group in the digital-to-analog conversion circuit. An input/output port circuit for a central processing unit comprising an electronic switch;