JPH03230646A - Undershoot preventing circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] This invention relates to an undershoot avoidance circuit that avoids an undershoot that transiently drops beyond a low level when a switching waveform falls in a digital signal transmission circuit, and that does not become a load on a drive circuit. In order to avoid undershoot reliably and to reduce costs, one side of the resistor is connected to the positive power supply, the other side of the resistor R is connected to the anode side, and the cathode side is connected to the ground. It consists of a first diode and a second diode whose anode side is connected to a connection point between the first diode and a resistor, and whose cathode side is connected to a digital signal line.

[Industrial Field of Application] The present invention relates to an undershoot avoidance circuit that avoids an undershoot that transiently drops below a low level when a switching waveform falls in a digital signal transmission circuit.

[Prior Art] In a circuit that transmits a digital signal, undershoot occurs in the transmission waveform when the input impedance of the receiving circuit is high, and this undershoot causes malfunction in the receiving circuit.

Therefore, conventional methods to avoid undershoot include
(1) Matched termination and (2) installation of clamp diodes were implemented.

(1) Matched termination is implemented by attaching terminating resistors R1 and R2 to the digital signal line S before the receiver element M2 of the receiving circuit, and performing pull-up/pull-down to connect the transmission line. To match the clamp diode in (2), as shown in the circuit diagram in Figure 5, a diode D is connected between the digital signal line S at the front stage of the receiver element M2 and the ground. As shown in FIG. 6(a), the undershoot when the digital signal falls from the high level H to the low level L is cut off to a constant level.

[Problems to be Solved by the Invention] However, the conventional undershoot avoidance described above has the following drawbacks.

In the matched termination (1), the terminating resistor becomes a load on the driver circuit, so the driver element M1 has to have more driving ability than necessary, resulting in an increase in cost.

(2) When installing the clamp diode, see Figure 6 (b).
), since a forward voltage drop always occurs in the diode D, the undershoot cannot be made smaller than the forward voltage of the diode D, which poses a problem in terms of reliability. Furthermore, conventionally, silicon diodes or Schottky barrier diodes have been used, but the forward voltage of silicon diodes is 0.
．． Since the voltage is about 7V, undershoot occurs at -0.7V or more, and the forward voltage of the Schottky barrier diode becomes 0.
．． Since the voltage was about 3V, undershoot occurred at -0.3V or more. As described above, the Schottky barrier diode has a greater effect of avoiding undershoot than the silicon diode, but the Schottky barrier diode is expensive, so there is a problem of increased cost.

The present invention was created in view of these problems, and
It is an object of the present invention to provide an undershoot avoidance circuit that can reliably avoid undershoot without causing a load on a drive circuit, and can reduce costs.

[Means for Solving the Problem] The means in the present invention for achieving the above object are as follows:
As shown in the figure, the basic configuration of the present invention is a resistor R whose one end is connected to the positive power supply Vcc, and a first diode D1 whose anode side is connected to the other side of the resistor R and whose cathode side is connected to ground. and the first diode D on the anode side.
1 and a second diode D2 connected to the connection point of the resistor R and the cathode side connected to the digital signal line.

In the figure, Ml is a driver element of a drive circuit, and M2 is a receiver element of a receiving circuit.

[Function] The present invention eliminates the undershoot of the diode forward voltage in the diode clamp method. In the conventional circuit shown in FIG. 5, the diode does not turn on unless the potential of the digital signal line becomes equal to or less than the diode forward voltage VF, so an undershoot occurs by the amount of the forward voltage VF. Therefore,
If the diode is turned on when the potential of the digital signal line becomes Ov, the signal line will not fall below Ov, and no undershoot will occur.

That is, in the present invention, as shown in FIG. 1, when the output of the driver element M1 is at a high (H) level, the first diode DI is turned on from the power supply Vcc through the resistor R, and the second diode D2 is turned on in the opposite direction. It turns off because it becomes biased. At this time, the voltage at point X, which is the connection point between diodes DI and D2 and resistor R, becomes Qv+VF1 (forward voltage of diode D1). From this state, when the output of the driver element M1 changes to a low (L) level and the input of the receiver element M2 becomes less than Ov, the diode D2 is turned on. At this time, if the forward voltage VFI of the diode D1 and the forward voltage VF2 of the diode D2 are equal (VF1=VF2
), the input of receiver element M2 is Ov (Ov+VF
Clamped at IVF2=OV).

Undershoot is therefore avoided.

Then, after being clamped to Ov, the receiver element M2
The input returns to the L level voltage of the driver element M1, but since the L level voltage of the normal driver element M1 is higher than Ov, when the input of the receiver element M2 becomes equal to the output voltage of the driver element M1, the diode D2 is turned off.

Therefore, in the steady state at H and L levels, the undershoot avoidance circuit of the present invention
It does not become a burden.

In addition, in the case of VFI<VF2, O+ VFI-VF
2=-V u (V u is the difference between VFI and VF2), and undershoot occurs by -Vu. Also, VF
If I>VF2, Ov + VFI −VF2=
+Vu and is clamped before reaching Ov, which is preferable as a measure to avoid undershoot. However, if the L level of driver element M1 becomes < +Vu, diode D2 will be turned on even in a steady state, causing undershoot. The avoidance circuit becomes a load on the driver element M1. Therefore, in the undershoot avoidance circuit of the present invention, it is preferable to set VF1=VF2 as much as possible.

[Example] FIG. 2 is a configuration diagram of an embodiment of the present invention.

The undershoot avoidance circuit shown in FIG. 1 is the same as the circuit shown in FIG. 1 except that a capacitor C is connected thereto. As shown in Figure 2, the capacitor C is
It is connected between the point where the resistor R and the diodes DI and D2 are connected, and ground. This capacitor C functions to fix the potential at the X point to the forward voltage VFI of the diode D1 in response to high-speed switching of the driver element M1, thereby making it possible to more reliably avoid undershoot.

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

The embodiment shown in the figure is constructed by connecting four diodes to a bridge, similar to the rectifier circuit of a power supply.

As shown in the figure, the connection point al of the anode and cathode,
a2 is connected to the digital signal line S and the ground, respectively, the connection point b1 between the anodes is connected to a positive power supply Vcc through a resistor Ra, and the connection point b2 between cathodes is connected to a negative power supply 2 through a resistor Rb. It is connected to the. In this example,
When the output of driver element M1 is at H level, the bridge
Current flows through the diode and becomes a load on the driver element M1, but many bridge diodes with four diodes connected in advance are commercially available, which reduces component prices and simplifies manufacturing by eliminating the need to connect diodes. There is an advantage that it can be done.

[Effects of the Invention] As explained above, according to the undershoot avoidance circuit of the present invention, the digital signal is clamped at Ov by the resistor and two diodes, and undershoot is reliably avoided. Therefore, there is no need to increase the driving capacity more than necessary, and since undershoot can be avoided regardless of the forward voltage of the diode, inexpensive diodes can be used. Therefore, it is possible to contribute to improving reliability and reducing costs in a digital signal transmission circuit.

[Brief explanation of drawings]

Fig. 1 is a basic circuit diagram of the present invention, Fig. 2 is a circuit diagram of one embodiment of the present invention, Fig. 3 is a circuit diagram of another embodiment of the invention, and Fig. 4 is a circuit diagram of an example of matched termination. 5 is a circuit diagram of a conventional diode clamp, and FIG. 6 is an explanatory diagram of a conventional diode clamp. D, Di, D2--Diode, R, Ra, Rb, R1, R2--Resistor, Ml... Driver element, M2... Receiver element, S... Digital signal line. cc Basic circuit diagram of the present invention CC Circuit diagram of an embodiment of the present invention Fig. 2 1 Engineering/λ 1 \, λ

Claims (1)

[Claims] A resistor (R) whose one side is connected to a positive power supply (Vcc), and a first diode (D1) whose anode side is connected to the other side of the resistor (R) and whose cathode side is connected to ground. ), and the anode side is connected to the first diode (D1) and the resistor (R
) and connect the cathode side to the connection point of the digital signal line (
An undershoot avoidance circuit comprising: a second diode (D2) connected to S);