JPH049640Y2 - - Google Patents

Description

[Detailed explanation of the idea]

[Technical Field of the Invention] The present invention relates to a mode programming circuit for a microcomputer system that requires operational mode programming of a single-chip central processor unit (CPU), a multi-chip CPU, and the like. [Technical background of the invention and its problems] A conventional circuit of this type is shown in FIG. 4. In the figure, 1 is a microcomputer system (hereinafter abbreviated as microcomputer), 2 is a power supply circuit that supplies power to each part when the system power is turned on, and 3 is a reset input to the reset input RST to initial clear the microcomputer 1. Initial clear circuit that generates a signal, 4 is input IN
5 is an input switching circuit consisting of an analog switch and the like, and 6 is a mode setting switch. In the above, the system power is turned on and the power supply circuit 2
When power is supplied to each section, a reset signal is input from the initial clear circuit 3 to the microcomputer 1. Microcomputer 1 to which this reset signal is input
During that period, the port P is input and the operation mode is read. At this time, the input switching circuit 5 switches the input to a potential preset by the mode setting switch 6 and outputs it. As shown in the figure, when the switch 6 is off, _X0 is an H input, _X1 is undefined, and during the reset period, an H is output from X. The microcomputer 1 reads this from the port P, and after the reset period ends, operates according to the set operation mode. At the end of this reset period, since there is no longer any reset input to the input switching circuit 5, the input is set to _X1 shown in the figure, which is output from X and sent to port P of the microcomputer 1. In the conventional circuit described above, when an analog switch or the like is used as the input switching circuit 5, the noise margin, propagation delay time, feedthrough, etc. of the switching circuit section are
Since it is affected by crosstalk and other factors, circuit settings must take these into account. Also, this
When configured with FET discrete elements,
Space benefits will be very poor. This kind of thing also happens with ICs where the variety is limited. Furthermore,
In the figure, power is directly supplied from the power supply circuit 2 to the switching circuit 5, but when handling TTL levels (0V, 5V), a bias larger than this level is normally required to transmit this level. There were flaws. [Purpose of the invention] The present invention has been made in order to eliminate the drawbacks of the conventional ones mentioned above.In a microcomputer system in which the signal input port is also used as an input for mode setting, the signal input terminal,
a comparator means having a reference input terminal and an output terminal, the output terminal of which is connected to the signal input port; and at the time of mode setting, the voltage of the signal input terminal is set relative to the voltage of the reference input terminal according to the setting mode; setting to fix the output terminal of the comparator means at a predetermined level, input an input signal to the signal input terminal when the mode is not set, and output a signal corresponding to the input signal as it is to the output terminal of the comparator means. It is an object of the present invention to provide a mode programming circuit which does not have the above drawbacks by comprising a control means. [Embodiments of the invention] Examples of the invention will be described below based on the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 11 is a microcomputer, 12 is a power supply circuit, 13 is an initial clear circuit, and 14 is a comparator section. 1 to 4 respectively. 17 is an input control circuit, which outputs a signal at a preset level while the reset signal is input from the initial clear circuit 13, and outputs the signal input to the input IN when the reset signal is not input. Output. In the above configuration, when the system power is turned on and power is supplied to each part from the power supply circuit 12, a reset signal is inputted from the initial clear circuit 13 to the microcomputer 11. During the period during which this reset signal is being input, the microcomputer 11 uses the port P as an input to read the operating mode. At this time, a reset signal is also applied to the input control circuit 17 to shunt the input of the comparator section 14 to H or L. Then, the microcomputer 11 reads the output of the comparator section 14 at this time from the port P, and sets the operation mode. After the reset period in which the reset signal disappears, the microcomputer 11 operates according to the set operation mode. At this time, the input control circuit 17 transmits the signal from the input IN to the comparator section 14 as it is, and the comparator section 14 shapes the waveform and then sends the signal to the microcomputer 1.
input to port P of 1. FIG. 2 shows a specific example of the comparator section 14 and the input control circuit 17, and the operation of this circuit will be described below with reference to the time chart of FIG. 3 showing waveforms of each section. In FIG. 2, the current input control circuit 17
Assume that the switch SW inside is on the contact A side. In this state, the system power is turned on at time _t1 , and the voltage _Vcc supplied from the power supply circuit 12 to each part
rises as shown in FIG. 3a, and in response, the initial clear circuit 13 generates a reset signal as shown in FIG. 3b, which triggers the switch SW.
is input to the base of the transistor Q ₁ via the contact A of and the resistor R ₁ . Therefore, the transistor Q ₁ is turned on, and the signal input to the input IN (FIG. 3 c) is grounded through the capacitor C ₁ , the resistor R ₃ , and the transistor Q ₁ . The inverting input of the comparator 14a is set to approximately OV. Note that the comparator 14a
Since a constant level (R ₄ = R ₅ = V _cc /2) set by the voltage division by resistors R ₄ and R ₅ is applied to the non-inverting input of the comparator, the non-inverting input becomes larger and the comparator The output of 14a is fixed at H level. During the period without the above reset signal, the transistor
Since _Q1 is not turned on, the signal from the input IN is directly applied to the inverting input of the comparator 14a, and the waveform-shaped signal is output from the comparator 14a (Fig. 3d). Contrary to the above case, if L level input is required to port P during reset operation, the switch
Simply move the SW to the contact B side. When the switch SW is on the contact B side, the reset signal is sent to the inverter 17.
After being inverted by a, the signal is input to the base of the transistor _Q2 via the resistor _R2 , and the transistor _Q2 is turned on accordingly. Therefore, the inverting input of the comparator 14a becomes H level (5V),
This is larger than V _cc /2, so the output of comparator 14a is fixed at L level. In this case, the input to port P during the reset period is shown in Figure 3d.
as shown by the dotted line. By the way, the operation mode setting refers to setting the operation of the CPU of the microcomputer, as shown in the table below, for example.

[Effect of idea]

As mentioned above, in a microcomputer system in which the signal input port is also used as an input for mode setting, the operation mode setting is configured by a comparator and a control circuit that controls its input, so the signal input There is no switching circuit (FET, analog switch, etc.), which causes noise, propagation delay,
The effects of feed-through, crosstalk, etc. are eliminated, and the input/output is not affected by the control circuit even during normal operation. Basically, it can be configured with one resistor and one transistor, so it has good space advantages. There is no need to consider giving a special bias to the control circuit, and the probability of failure or malfunction is extremely reduced. Effects such as this can be obtained.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the principle of the circuit according to the present invention, Fig. 2 is a circuit diagram showing a part of the specific circuit in Fig. 1, and Fig. 3 shows the various parts in Figs. 1 and 2. A time chart showing waveforms is shown, and FIG. 4 is a block diagram showing an example of a conventional circuit. 11... Microcomputer, 13... Initial clear circuit, 14... Comparator section, 1
7...Input control circuit.

Claims (1)

[Claim for Utility Model Registration] A microcomputer system in which a signal input port is also used as an input when setting a mode, which has a signal input end, a reference input end, and an output end, and the output end is connected to the signal input port. a comparator means connected to a mode; and when setting a mode, the voltage at the signal input terminal is set with respect to the voltage at the reference input terminal according to the setting mode, and the output terminal of the comparator means is fixed at a predetermined level; A mode programming circuit comprising: control means for inputting an input signal to the signal input terminal and outputting a signal corresponding to the input signal as is to the output terminal of the comparator means when not set.