Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
2000-06-28
2002-03-05
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S087000, C323S284000
Reexamination Certificate
active
06353522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a switching state detecting device for a switch and to an electronic apparatus, and particularly relates to a switching state detecting device for a switch capable of detecting the switching state of a switch with lower electric power consumption and high precision, and to an electronic apparatus using this device.
2. Background Art
With common electronic apparatuses, various operations are performed by switches, but with electronic apparatuses which require low electrical power consumption, ways of keeping electric power consumed by detecting circuits to a minimum have been devised, in that the switching states of the switches are not continuously detected, but intermittently detected.
Description will be made regarding a configuration for detecting the switching state of switches used for such electronic apparatuses, with reference to FIG.
17
. As shown in the Figure, one end of the switch SW regarding which the switching state is to be detected is grounded at a reference level Vdd at the high-potential side, and the other end is connected to a detecting circuit
900
. Now, the detecting circuit
900
is made up of an n-channel field-effect transistor
910
and a latch circuit
930
, with the drain of the transistor
910
being connected to the other end of the switch SW, and the source thereof connected to the negative side power source voltage Vss. Also, sampling pulses SP are supplied to the gate of the transistor
910
.
The latch circuit
930
latches the voltage level of the signal line A connected to the other end of the switch SW with the trailing edge of the sampling pulse SP, and outputs a signal Out indicating the switching state of the switch SW.
With such a detecting circuit
900
, the transistor
910
is on only during the period wherein the sampling pulse SP is at the “H” level, and the signal line A is pulled down to the power source voltage Vss by the on resistance thereof. According, the voltage level of the signal line A maintains the power source voltage Vss in the event that the switch SW is open during the period wherein the sampling pulse SP is at the “H” level, but conversely makes transition to the ground level in the event that the switch SW is closed.
Accordingly, a signal Out according to the switching state of the switch SW can be output by the latch circuit
930
latching the voltage level of the signal line A with the leading edge of the sampling pulse SP. Then, processing corresponding to the instruction of the switch is executed by a later circuit (omitted in the drawings) based on this signal out.
According to such a detecting circuit
900
, no electrical current is constantly flowing between the drain/source of the transistor
910
, so the electricity consumed in the detecting circuit
900
can be kept low.
Now, depending on the electronic apparatus applied, the power source voltage Vss may not be constant but vary with a certain width. For example, in the case of an electronic apparatus comprising an electricity generating mechanism and a battery mechanism, wherein the electricity generated by the electricity generating mechanism is stored in the battery mechanism, and the electricity stored in the battery mechanism is used as the power source, fluctuation of the power source voltage Vss due to the battery state is presupposed.
Now, general transistors have a nature wherein the lower the voltage between the source/drain is, the greater the on resistance thereof is, i.e., in straightforward terms, the resistance value properties as to voltage are non-linear. On the other hand, pulling down the signal line A with high resistance tends to make the voltage level thereof unstable. Accordingly, a transistor
910
of a type with a small on resistance must be used in order to stabilize the voltage level of the signal line A, whether the voltage between source/drain is low, or whether the difference between the power source voltage Vss and ground level is small.
However, with a configuration wherein the signal line A is pulled down with a transistor having small on resistance, the electric power consumption of the detecting circuit
900
has to increase, so not only does this go against the original object of lowering electric power consumption, but further creates a problem in that the range of power source voltage capable of detecting the switching state of the switch is restricted due to transistor properties.
This problem is marked in cases wherein the sampling rate is increased, in order to detect the switching state of the switch SW with high precision. The reason is as follows. That is, the line for the signal line A has parasitic capacity owing to the transistor
910
, pads for extending lines for mounting, wiring, etc. Now, in the event that a charge is stored in this parasitic capacity due to some reason in the state of the switch SW being open, when the sampling pulse SP is at the “H” level the level of the signal line A changes over time according to a time-constant owing to the parasitic capacity and pulldown resistance. Accordingly, the signal line A is not determined at the “L” level state wherein the switch SW is open, until after pulling down for a certain amount of time. Thus, in order to raise the sampling rate, the pulse width of the sampling pulse SP must be secured to a certain level in order to take sufficient time for the level of the signal line A to be determined. This means nothing else than extending the amount of on time at the transistor
910
.
The present invention has been made in light of the above problems, and it is an object thereof to provide a switching state detecting device for a switch capable of realizing both widening the power source voltage range capable of detecting the switching state of the switch, and improving detection precision of the switching state of the switch, and an electronic apparatus using this device.
DISCLOSURE OF INVENTION
A first form of the present invention comprises: a switch connected at one end to a ground line or a power source; a resistive element connected between the other end of the switch and a power source or a ground line; and a control unit for controlling the value of the resistive element based on a power source voltage which is the difference between the voltage level of the power source and the ground level of the ground line; wherein signals equivalent to the switching state of the switch corresponding to the voltage level at the other end of the switch are output.
Further, the first form of the present invention comprises a judging unit for judging, or determination unit that determines the voltage level at the other end of the switch and outputting signals equivalent to the switching state of the switch.
Further, the determination unit of the first form of the present invention performs determination of the voltage level at predetermined intervals.
Also, the control unit of the first form of the present invention controls the resistance value of the resistive element such that the resistance value of the resistive element does not exceed the predetermined upper limit resistance value.
Also, the control unit of the first form of the present invention controls the resistance value of the resistive element such that the resistance value of the resistive element is within the resistive element value range stipulated by the predetermined upper limit resistance value and lower limit resistance value.
Also, the resistive element of the first form of the present invention is a variable resistive element that changes resistance value based on the power source voltage; and wherein the control unit controls the resistance value of the resistive element to a virtual resistance value when an absolute voltage level of the power source is higher than a predetermined reference voltage; and wherein the control unit controls the resistance value of the resistive element to be smaller than the virtual resistance value when the absolute voltage level of the power source is lower than the predetermined re
Huynh Kim
Seiko Epson Corporation
Watson Mark P.
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