JPH0795020A - Pull-up circuit - Google Patents

Abstract

PURPOSE:To obtain the pull-up circuit in which a voltage level does not become high when a control signal becomes an 'L' level, while improving a rise waveform of the control signal. CONSTITUTION:In this pull-up circuit in which a pull-up resistance is connected between a control signal line 2 and a power source VDD, the pull-up resistance is constituted of a series circuit of two resistances R1, R2, and an emitter and a collector of a first switching transistor Q1 are connected in parallel to both ends of one resistance R1. By turning ON and OFF this transistor Q1, a pull-up resistance value can be switched. Between the control signal line 2 and a reference potential point, two resistances R3, R4 for detecting a voltage of the control signal line 2 are also connected in series. By turning ON a second switching transistor Q2 at when a detected voltage of its connecting point rises to a prescribed voltage, a first switching transistor Q1 is turned ON, and the value of the pull-up resistance is set small. In such a way, a rise waveform can be made steep.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an integrated circuit (hereinafter referred to as I
The control signal line for controlling C) has a logic level "
The present invention relates to a pull-up circuit for the purpose of supplying H "(hereinafter," H "level).

[0002]

2. Description of the Related Art In an IC, a reference voltage called a threshold level is determined, and a voltage higher than that is required.
If it is applied to the input terminal, it is determined to be "H" level, and if it is less than that, it is determined to be "L" level.

In practice, it is unstable to operate at a voltage just above or below the threshold level, and even a little noise causes the IC to malfunction.

Therefore, conventionally, the control signal voltage VOH when the signal at the input terminal of the controlled IC is in the "H" level is
In order to raise the voltage to near the power supply voltage (for example, 5 V), the control signal line is connected to the power supply line via a resistor.

FIG. 4 shows a conventional pull-up circuit. In FIG. 4, the control signal output from the control IC 1 is supplied to the input terminal of the controlled IC 3 through the control signal line 2. Between the control signal line 2 and the power supply VDD,
A pull-up resistor R10 is connected.

However, in the above method, the power supply VDD
When a current is supplied from the control IC 1 to the control signal line 2 through the pull-up resistor R10 and a signal of "H" level is output from the control IC 1, the control signal waveform rises with a constant time constant. Therefore, when a large number of controlled ICs are connected to the control signal line 2, the capacitance component increases and the waveform becomes blunt, and the rising characteristic deteriorates. As a result, there is a malfunction in that the control signal is not accurately transmitted to the controlled IC 3 or the control signal cannot be received.

Therefore, in order to improve the rising characteristics of the signal waveform, the time constant may be reduced by reducing the constant of the pull-up resistor. This time, however, when the control signal is set to the "L" level. The control signal voltage V0L to the controlled IC 3 becomes large, and the "L" level becomes a level considerably higher than the ground level. As a result, the controlled IC will malfunction.

[0008]

As described above, in the conventional pull-up circuit, the rising waveform of the control signal is blunted. To improve this, it is necessary to reduce the value of the pull-up resistor. Because of the control signal
There is a problem that the voltage level becomes high when the L "level is reached.

Therefore, the present invention has been made in view of such problems.
The control signal is improved while improving the rising waveform of the control signal.
It is an object of the present invention to provide a pull-up circuit that does not increase the voltage level when it is set to the L ″ level.

[0010]

According to a first aspect of the present invention, there is provided a pull-up resistor including a plurality of resistors provided between a power supply and a control signal line, and the pull-up resistor. Switch means provided to be connected to switch the pull-up resistance value by supplying a switching voltage, and a voltage provided between the previous control signal line and the reference potential point to detect the voltage of the control signal line. It is provided with a detecting means and means for creating a switching voltage when the voltage detected by the voltage detecting means exceeds a predetermined voltage and supplying the switching voltage to the switch means.

A pull-up circuit according to a second aspect of the present invention is the pull-up circuit according to the first aspect, wherein the voltage detecting means is connected in series between the control signal line and a reference potential point. And a capacitor is connected in parallel to both ends of the first resistor.

[0012]

According to the invention described in claim 1, when the voltage of the control signal line reaches a predetermined voltage when the control signal rises and falls, the pull-up resistor connected to the control signal line is It is possible to switch the value. When the control signal rises to the "H" level, the switch means is turned on to switch to a small pull-up resistance value, the rising characteristic of the control signal is improved to be steep, and the control signal line is set to the "L" level. At this time, when the voltage value reaches a predetermined value, the switch means is turned off and the pull-up resistance value is set to a large value, and the control signal voltage can be decreased.

According to the second aspect of the present invention, by connecting a capacitor to both ends of the first detection resistor connected to the control signal line, the rise and fall of the detection voltage can be accelerated, resulting in a switching voltage. Is rapidly created, the resistance value can be switched quickly by the switching means, and noise on the control signal line can be removed.
Therefore, the rising and falling characteristics of the control signal voltage supplied to the controlled IC can be made steeper.

[0014]

EXAMPLES Examples will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a pull-up circuit according to an embodiment of the present invention.

In FIG. 1, the control signal output from the control IC 1 passes through the control signal line 2 and the controlled IC 3
, 3n are supplied to the respective input terminals. Power supply VDD
And a control signal line 2 between the pull-up resistor R
1 and R2 are connected in series, and resistors R3 and R4 for detecting the voltage of the control signal line 2 are connected in series between the control signal line 2 and the reference potential point. Resistance R
At both ends of 1, the emitter of PNP transistor Q1
The collector paths are connected in parallel, a resistor R5 is connected between the emitter and base of Q1, and the base of Q1 is resistor R6.
And the collector-emitter path of the NPN transistor Q2 are connected in series to the reference potential point. Furthermore, the resistance R
A small-capacitance capacitor C is connected to both ends of 3 to speed up the rise and fall of the detection voltage to speed up the switching of the transistor Q2 and
It is possible to remove noise mixed in the control signal line 2.

In the above structure, the voltage of the control signal line 2 is divided by the resistors R3 and R4, and this is applied to the base of the transistor Q2. By appropriately selecting the resistance values of the resistors R3 and R4, the voltage value at which the control signal line 2 turns on the transistor Q2 can be arbitrarily selected.

This voltage value is "L" on the control signal line 2.
The noise is mixed in the control signal line 2 in the level state, and is determined. That is, if this voltage value is set too low, the noise mixed in the control signal line 2 will cause the transistor Q2 to turn on accidentally,
Conversely, if it is set too high, it takes time for the signal to reach the voltage value, and the effect of improving the rising characteristics of the signal waveform cannot be obtained.

By turning on the transistor Q2,
The size of the pull-up resistor formed by the resistors R1 and R2 is switched. That is, when the transistor Q2 is turned on, a bias supply circuit is formed by the resistors R5 and R6, and both ends of the resistor R5, that is, the transistor Q1.
A bias voltage required to turn on Q1 is applied between the emitter and the base of the transistor, and the transistor Q1 is turned on to short-circuit both ends of the resistor R1. This allows
When the control signal line 2 rises to "H" level,
When the switching transistors Q2 and Q1 are turned on, the pull-up resistance value is switched to a small value, and the rising characteristic of the control signal voltage is improved.

On the other hand, when the control signal line 2 is set to "L" level, when the voltage value reaches a predetermined value, the switching transistor Q2 is turned off and the switching transistor Q1 is turned off to set a large pull-up resistance value. As a result, the control signal voltage can be kept small.

At the rising edge of the control signal, the timing at which the pull-up resistance value switches, that is, the base voltage at which the transistor Q2 turns on, must not be set too high as described above. This is because the threshold level for detecting the "H" level among the controlled ICs 3a to 3n when the "H" level is output to the control signal line 2 is lower than the threshold level of the controlled IC. Should also be set small. For example, if the controlled ICs 3a, ..., 3n are connected to the control signal line 2 and the lowest value VIHmin of the voltage VIH for detecting the "H" level among these devices is 2.8V, The voltage Von at which the transistor Q2 turns on should also be set to a value lower than 2.8V. If the base voltage when the transistor Q2 is turned on is VB, the resistance value may be selected so as to satisfy the following equation.

VIHmin> Von VB = Von.R4 / (R3 + R4) R2 << R3 FIG. 2 shows a control signal waveform at the time of rising in the embodiment of FIG. When the control IC 1 outputs, for example, a "H" level signal, it rises with a constant time constant until the transistor Q2 turns on as shown by the solid line in FIG. 2, and the voltage of the control signal line 2 turns on Q2. When the voltage reaches Von, Q2 turns on and Q1 turns on, the pull-up resistance becomes a small resistance value of R2, and the control signal voltage rises rapidly. At this time, the control signal voltage is controlled IC
The voltage rises rapidly beyond the "H" level detection voltage VIH to near the power supply voltage VDD.

Therefore, when the control signal output from the control IC 1 is at the "H" level, the rising edge of the control signal voltage supplied to the controlled ICs 3a, ..., 3n becomes steeper than the conventional waveform and is controlled. Since the voltage exceeds the "H" level detection voltage VIH of the IC and rises to near the power supply voltage VDD, the controlled IC operates without malfunction.

The control signal from the control IC 1 is "L".
When it becomes the level, the base voltage of the transistor Q2 also drops rapidly, Q2 turns off and Q1 also turns off, the pull-up resistance becomes R1 + R2 and becomes a large resistance value, and the control signal voltage to the controlled IC becomes high. Surely descends to near the ground level.

Therefore, when the control signal output from the control IC 1 becomes "L" level, the controlled ICs 3a, ...,
Since the control signal voltage supplied to 3n falls close to the ground level, the controlled IC operates without malfunction.

Incidentally, when there is no capacitor C at both ends of the detection resistor R3 in FIG. 1, there is a rise similar to the case where the capacitor C is present when it rises to "H" level as shown by the alternate long and short dash line in FIG. However, the control signal voltage when the rise is a little slower than when the capacitor C is present and when it falls to the "L" level becomes a level slightly higher than the ground level. As a result, the capacitor C
It can be said that when there is no signal, the rising and falling waveforms of the signal deteriorate as compared with the case where the capacitor C exists. The waveform of the dotted line in FIG. 2 shows the waveform of the control signal voltage of the conventional example (FIG. 4).

FIG. 3 is a circuit diagram showing another embodiment of the present invention. In the embodiment of FIG. 1, power supply VDD and control signal line 2
While the pull-up resistors R1 and R2 are connected in series with the pull-up resistors R1a and R2a between the power supply VDD and the control signal line 2 in this embodiment shown in FIG.
Are connected in parallel. Other configurations are the same as those in FIG. Also in this case, when the transistor Q2 is turned on and the transistor Q1 is turned on in the process that the control signal from the control IC1 rises to the "H" level, the pull-up resistor becomes a resistance value in which R1a and R2a are connected in parallel, and has a small resistance value. The rising waveform of the signal voltage supplied to the controlled IC can be made steep. on the other hand,
When the control signal falls to "L" level, the transistor Q2 turns off and the transistor Q1 turns off, and the pull-up resistor has a high resistance value only for R2a, and the "L" level supplied to the controlled IC is You can prevent it from rising above the ground level.

[0027]

As described above, according to the present invention, the control signal is "L" while improving the rising waveform of the control signal.
It is possible to prevent the voltage level from becoming high when the level is set. In particular, in a television receiving system or the like in which a large number of controlled devices are connected to the control signal line and noise from a noise source such as a deflection circuit is easily mixed, the rise of the control signal while reducing the noise of the control signal line , It is possible to improve the fall characteristic.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a pull-up circuit according to an embodiment of the present invention.

FIG. 2 is a control signal waveform diagram illustrating the operation of FIG.

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

FIG. 4 is a circuit diagram showing a conventional pull-up circuit.

[Explanation of symbols]

 1 ... Control IC 2 ... Control signal lines 3a to 3n ... Controlled IC Q1 ... Switch transistor (switch means) Q2 ... Switch transistor R1, R2 ... Pull-up resistors R3, R4 ... Voltage detection resistance (voltage detection means) Bias resistance of R5, R6 ... Q1

Claims (2)

[Claims] 1. A pull-up resistor composed of a plurality of resistors provided between a power supply and a control signal line, and a pull-up resistance value provided by connecting to the pull-up resistor and supplying a switching voltage. Switch means for switching, voltage detection means for detecting the voltage of the control signal line provided between the previous control signal line and the reference potential point, and the voltage detected by this voltage detection means is predetermined. A pull-up circuit comprising: means for generating a switching voltage when the voltage exceeds the voltage and supplying the switching voltage to the switching means. 2. The voltage detecting means comprises first and second resistors connected in series between the control signal line and a reference potential point, and a capacitor is connected in parallel across both ends of the first resistor. The pull-up circuit according to claim 1, which is characterized in that.