JPH02265314A - Input circuit in semiconductor integrated circuit - Google Patents

Abstract

PURPOSE:To easily change a threshold level of an inverter by providing an inverter receiving an input signal from an external terminal whose threshold level is varied while the structure of the inverter is programmed in terms of software, and a changeover circuit to vary the structure of the inverter. CONSTITUTION:Assuming that an input signal 7 has a waveform as shown in figure (a), then a threshold level of an inverter 2 is halved with respect to a power voltage VDD when transistors(TRs) A3, B4 of a changeover circuit 5 are respectively turned off by using selection signals 9, 10. When the input signal 7 is larger than the threshold level of the inverter 2, the output 8 of the inverter goes to a high level as shown in figure (b), and when the input signal 7 is smaller than the threshold level conversely, the output 8 of the inverter goes to a low level. Assuming that the threshold level of the inverter 2 as 0.4VDD when the TR A3 of the changeover circuit 5 is turned off and the TR B4 is turned off with the selection signals 9, 10, then the output 8 of the inverter is a waveform as shown in figure (c).

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to an input circuit in a semiconductor integrated circuit.

[Conventional technology]

FIG. 3 is a circuit diagram showing an example of the configuration of an input circuit of a conventional semiconductor integrated circuit, in which (1) is an external terminal, (6J is an internal circuit,
(7) is the input signal, (8) is the output of the inverter, and α is the inverter.

Figure 4 shows the input signal (7) that enters the external terminal (1) in Figure 3.
) and an inverter output (8). The operation will be explained below with reference to FIGS. 3 and 4.

, the input signal f'r (7) from the external terminal (1) is
High or Low depending on the threshold value of inverter O
The output of the inverter (8) is converted and the internal circuit (6
) is output to. Figure 4 shows this situation. If the threshold of the inverter (b) is o, 5vno, the input signal (7) is the threshold of the inverter (b).
, 5VDD, the inverter output (8) is High, and when the input signal (7) is smaller than the threshold of the inverter (B), the inverter output (8) is Low.
It becomes w.

[Problem to be solved by the invention]

As an example, assume that in a CR reset circuit using a capacitor C and a resistor R, the electric charge charged in this capacitor cannot be completely discharged when the battery is replaced due to a bypass capacitor or the like. At that time, the terminal voltage of the capacitor of the CR reset circuit may be higher than the threshold voltage of the reset circuit, and the reset does not occur even though the battery has been replaced.

Conversely, if the threshold constant voltage of the reset circuit is high, in applications such as battery backup, a reset will occur when switching from a low voltage battery to a commercial power source (generally higher than the voltage of the backup battery). Sometimes.

If it is desired to change the threshold value of the inverter in response to such an inconvenience, the threshold value of the inverter cannot be changed because it is fixed.

This invention was made to solve the above-mentioned problems, and an object of the invention is to make it possible to change the threshold value of an inverter by changing the structure of the inverter.

[Means to solve the problem]

The input circuit in the semiconductor circuit according to the present invention is provided with a switching circuit for recombining transistors constituting an inverter.

[Effect]

In the input circuit of the semiconductor circuit according to the present invention, the switching circuit for changing the transistors constituting the inverter changes the transistors using a selection signal, thereby making it possible to change the threshold value of the inverter.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. 1st
The figure is a circuit diagram showing the configuration of an input circuit of a semiconductor integrated circuit. In the figure (1), (63, (7), (8
) is the same as that shown in the conventional example of FIG. 3, and therefore its explanation will be omitted. (2) is an inverter, (3) is transistor A, (4) is transistor B, (5) is a switching circuit, (9) and σO are selection signals. Figure 2 (a) to (d)
) is a waveform diagram showing the relationship between the input signal (7) entering the external terminal (1) in FIG. 1 and the output (8) of the inverter. The operation will be explained below with reference to FIGS. 1 and 2.

Assume that the input signal (7) has a waveform as shown in FIG. 2(a). Transistor A (3) and transistor B (
Inverter (2) when 4) are respectively OFF L/
The threshold value of is set to 1/2 of the power supply voltage VDD. When the input signal (7) is larger than the threshold value of the inverter (2), the output (8) of the inverter becomes High as shown in Figure 2 (b), and conversely, when the input signal (7) is larger than the threshold value, the inverter output (8) becomes High. If it is small, the inverter output (8) will be LOW.

Selection signal <97, (switching circuit (5
) transistor A (3) is turned off, transistor B (
4) is ONL, and the threshold value of the inverter (2) is 0.4VDD, the output of the inverter (8) in response to the input signal (7) is shown in Fig. 2(c). It becomes a waveform.

The selection signal (ll), (10 causes the switching circuit (
5) transistor A (3) is turned on, transistor B (
4) is OFF L, and the threshold value of inverter (2) is 0.3VDD, then input signal (7)
On the other hand, the output (8) of the inverter has a waveform shown in FIG. 2(d).

The output (3) of the inverter in the above three cases is transmitted to the internal circuit (61).

〔Effect of the invention〕

As described above, since the structure of the inverter can be changed, the threshold value of the inverter can be changed.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the input circuit configuration of a semiconductor integrated circuit according to an embodiment of the present invention, and FIGS.
A waveform diagram showing the relationship between the input signal and the output of the inverter in the circuit shown in the figure, Figure 3 is a circuit diagram showing the configuration of the input circuit of a conventional semiconductor integrated circuit, and Figure 4 is the input signal and output of the inverter shown in Figure 3. FIG. In the figure, (1) is the external terminal, (2) is the inverter, (
3) is transistor A, (4) is transistor B1 (5
) is the switching circuit, (67 is the internal circuit, (7 is the input signal, (August is the output of the inverter, (9), (IG is the selection signal. In the figure, the same symbols are the same or equivalent. Show parts.

Claims (1)

[Claims] In the input circuit of a semiconductor integrated circuit, the threshold value of the inverter that receives an input signal from an external terminal can be changed by programming the structure of the inverter using software, and the structure of the inverter can be changed. 1. An input circuit in a semiconductor integrated circuit, comprising: a switching circuit.