Abstract:
Different reference voltages are employed in different logical electrical circuits. In the conventional arts, a reference voltage circuit is only employed in a specific logical electrical circuit or a reference voltage circuit with fuse can changes a reference voltage by fusing the fuse. Nevertheless, the reference voltage circuit with fuse is still only employed in a specific system of logical circuit regardless of the fuse is fused or unfused. A select reference voltage circuit for a logical electrical system of the present invention can solve the problem. Therefore, the select reference voltage method for a logical electrical system can be employed in different systems having respective different system voltages.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a circuit of selecting reference voltage, and more particularly to employ a compare circuit to decide a state of an input signal, and a reference voltage select circuit selects a suitable voltage according to the state of the input signal. 
     2. Description of the Prior Art 
     A dynamic random access memory (DRAM) decides upon a logical signal of an input signal according to a voltage of the input signal. When a system outputs a logical signal “1”, a voltage of the logical signal “1” is an input high voltage V IH ; when the system outputs a logical signal “0”, a voltage of the logical signal “0” is an input low voltage V IL . 
     In some applications, for example, a low power synchronous dynamic random access memory (LPSD), a signal voltage is an input high voltage or an input low voltage that is determined by a system voltage. If a ratio of the signal voltage and the system voltage is larger than 0.8, the signal voltage is determined as the input high voltage V IH , and if the ratio of the signal voltage and the system voltage is smaller than 0.2, the signal voltage is determined as the input low voltage V IL . In the DRAM, a logical signal is the input high voltage V IH  or the input low voltage V IL , that is determined according to a reference voltage V REF , it is determined voltage that. If the input signal is higher than the reference voltage V REF , it is determined as “1”; and if the input signal is lower than the reference voltage V REF , it is determined as “0”. In general, the reference voltage V REF  is an average of the input high voltage V IH  and the input low voltage V IL  for determining the logical signal under an optimum state due to a toleration for a variation resulting from a noise being equal between the input high voltage V IH  and the input low voltage V IL . 
     Different systems have respective different system voltages and so the input high voltage V IH , the input low voltage V IL  and the reference voltage V REF  are changed according to the system voltage. For example, a system voltage of first system is 2.5V, an input high voltage V IH  thereof is 2.0V, an input low voltage V IL  thereof is 0.5V and a reference voltage V REF  is 1.25V; a system voltage of second system is 1.8V, an input high voltage V IH  thereof is 1.44V, an input low voltage V IL  thereof is 0.36V and a reference voltage V REF  is 0.9V. Therefore, a design of a different reference voltage is needed for suiting to a different system. In the conventional arts, a reference voltage circuit with a fuse can change a reference voltage of the circuit. For example, if the fuse is unfused, the reference voltage provided by the circuit is suitable for the first system voltage 2.5V, and if the fuse is fused, the reference voltage provided by the circuit is suitable for the second system voltage 1.8V. But the circuit needs an extra process of fusing the fuse to change the reference voltage thereof and further the circuit only suits with the system voltage 1.8V when the fuse is fused. 
     Hence, aimed at the aforementioned problem of the conventional arts, the present invention employs a design of a select reference voltage circuit to select different reference voltages for suiting with different systems having different system voltages. 
     SUMMARY OF THE INVENTION 
     In the conventional arts, a reference voltage circuit is only applied in a system having a special system voltage, or the reference voltage circuit with a fuse needs the extra process of fusing the fuse and the circuit only suits with a system having a special system voltage regardless of the fuse is fused or unfused. It is an objective for present invention to provide a select reference voltage circuit for a logical electrical system to select a suitable reference voltage according to a system voltage. 
     It is another objective for present invention to employ a select reference voltage circuit for a logical electrical system to determine a state of a system voltage, and a reference voltage select circuit can select a suitable reference voltage according to the state. 
     It is a still another objective for present invention to provide a select reference voltage circuit for a logical electrical system to determine a suitable reference voltage for a logical system only during a short time after the logical system turns on. Hence, the determined reference voltage is not changed under an unstable system voltage. 
     It is another objective for present invention to provide a select reference voltage circuit for a logical electrical system that turns off after a suitable reference voltage is determined for economizing on consumption power. 
     As aforementioned, the present invention provides a select reference voltage circuit for a logical electrical system comprising a compare circuit and a reference voltage select circuit. The compare circuit receives both a system voltage and a base voltage of a logical electrical system and compares the system voltage with the base voltage to output a compare signal. The reference voltage select circuit receives the compare signal and selects a reference voltage coupling with the compare signal to output to the logical electrical system. Therefore, the device can select a suitable reference voltage for a different logical electrical system to output. A square wave generate circuit is further employed to receive a turn-on signal from the logical electrical system and generate a pulse signal. The compare circuit and the reference select circuit turn off when receive a turn-off signal of the pulse signal dropping down for avoiding changing the determined reference voltage due to a unstable system voltage. Moreover, a consumption power of the circuit can be efficiently economized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A to FIG. 1D are diagrams of a circuit apparatus of one preferred embodiment in the present invention; 
     FIG. 2A to FIG. 2B are diagrams of operating example of one preferred embodiment in the present invention; 
     FIG. 3A to FIG. 3C are diagrams of timing relation of a square wave generate circuit, a system turn-on signal and a system turn-on ready signal; and 
     FIG. 4A to FIG. 4B are diagrams of a circuit apparatus of another preferred embodiment in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Some sample embodiments of the invention will now be described in greater detail. Nevertheless, it should be recognized that present invention can be practiced in a wide range of other embodiments besides those explicitly described, and the scope of the present invention is expressly not limited expect as specified in the accompanying claims. 
     Then, the components of the different elements are not shown to scale. Some dimensions of the related components are exaggerated and meaningless portions are not drawn to provide a more clear description and comprehension of the present invention. 
     One preferred embodiment of the present invention employs a base voltage generate circuit, e.g.: a base voltage is 1.2V in a DRAM design, as a stable voltage source such as a pair of voltage source for avoiding effects of temperature, etc,. as shown in FIG. 1A. A base voltage generate circuit  10  generates a base voltage V BGR . A compare circuit  12  compares a system voltage V DDQ  with the base voltage V BGR  to output a compare signal St. A reference voltage select circuit  14  receives a plurality of reference voltages and determines a reference voltage V REF  according to the compare signal St to output. One of the base voltage V BGR  and the system voltage V DDQ  can connects with a boost-divide voltage circuit  16  or both thereof connect with respective boost-divide voltage circuits  16  to change the base voltage V BGR  into V pBGR  or change the system voltage V DDQ  into V DDQ . The compare circuit  12  compares V pBGR  with V DDQ , V BGR  with V pDDQ , or V pBGR  with V pDD , and outputs a compare signal St, as shown in FIG. 1B, FIG.  1 C and FIG.  1 D. For example, a base voltage is 1.2V; first system voltage V DDQ  is 2.5V; second system voltage V DDQ  is 1.8V. The system voltages connect with a boost-divide voltage circuit  16  for diminishing the voltage to become 0.6 times of the original voltages. Therefore, the voltages of the first system and the second system change into 1.5V and 1.08V respectively. If the base voltage is lower than a system voltage, the compare signal St is 1; if the base voltage is higher than a system voltage, the compare signal St is 0. Hence, if the system voltage is 2.5V, the compare signal St of the compare circuit is 1, and the system voltage is 1.8V, the compare signal St of the compare circuit is 0. A base voltage is preferably a average of both system voltage by means of a boost-divide voltage circuits  16  with a suitable times and so the voltage differences between the base voltage and both system voltages are equal. The reference voltage select circuit receives both reference voltages V REF1 , and V REF2  coupling with both system voltages, i.e.: first reference voltage V REF1  is 1.25V and second reference voltage V REF2  is 0.9V. If the reference voltage select circuit  14  receives a compare signal St being 1, the reference voltage select circuit  14  selects the first reference voltage V REF1  as the reference voltage V REF . If the reference voltage select circuit  14  receives a compare signal St being 0, the reference voltage select circuit  14  selects the second reference voltage V REF2  as the reference voltage V REF , as shown in FIG.  2 A and FIG.  2 B. 
     For different logical electrical systems, a select reference voltage circuit for a logical electrical system of the present invention has different reference voltages coupling with the different logical electrical systems. The select reference circuit for a logical electrical system is assembled in a logical electrical system. When the system turns on, the select reference circuit determines a reference voltage suiting to the system according to the aforementioned process. Furthermore, if the system need a reference voltage changing according to an external condition when the system turns on every time, the select reference circuit of the present invention can provide a reference voltage suiting to the system every time. Moreover, a range of the reference voltage that provided by the select reference voltage circuit to a logical electrical system is between a maximum voltage of the select reference voltage circuit and a minimum voltage thereof. For example, the reference voltages that can be provided by the select reference voltage circuit are 1.2V, 1.8V and 2.5V. The range of the reference voltage is 1.2V to 2.5V. If a most preferable reference voltage for a system is 1.7V, the select reference voltage circuit selects a reference voltage nearest to most preferable reference voltage, i.e.: 1.8V, to output. Hence, the select reference voltage circuit can be assembled in a system that needs a reference voltage is between a maximum reference voltage of the select reference voltage circuit and a minimum voltage thereof. For example, a personal computer can provide system voltages of 0V, 1.75V, 2.5V, 3.3V, 5V, 12V, etc, and the system voltages is provided in a reference voltage select circuit of the select reference voltage circuit as a reference voltage source, therefore, the range of the reference voltage is 0V to 12V. Compared with a single reference voltage circuit or a reference voltage circuit with fuse in the conventional arts, the select reference voltage circuit can output a different reference voltage for suiting to a different reference voltage of demand. For example, a reference voltage circuit with a fuse, a output reference voltage is 1.25V when the fuse is unfused and a output voltage is 0.9V when the fuse is fused. Therefore the reference voltage circuit with a fuse only outputs 1.25V when the fuse is unfused and can not work in a system with a reference voltage of 0.9V, or only outputs 0.9V when the fuse is fused and can not work in a system with a reference voltage of 1.25V. However, the select reference voltage circuit of the present invention does not have the aforementioned problem. 
     Another preferred embodiment of the present invention employs a turn-on ready signal CHRDY generated when a system turned on, as shown in FIG. 3A, to select a reference voltage as following process. When a system voltage of the system reaches a stable voltage, the signal CHRDY is generated by the system, wherein the signal CHRDY is a signal from a high voltage to a low voltage, as show in the line  20  of FIG. 3B, or a signal from a low voltage to a high voltage, as show in the line  22  of FIG. 3B. A square wave generate circuit  18  provides a pulse signal  24  at the time t 1  that the signal CHRDY is generated, as shown in FIG.  3 C. The time width of high voltage of the pulse signal is (t 2 −t 1 ). Compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  work during t 1  to t 2  and a reference voltage V REF  is determined by the circuits. When the pulse  24  drop to a low voltage at the time t 2 , compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  turn off. Hence, a narrowest time width of the pulse signal  24  must be equal to or longer than a duration that compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage, circuit  16  can complete the process of determining a reference voltage V REF . Then, the system continues working and is not influenced any more with compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  due to these circuits have turned off. 
     If compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  turn off after pulse  24  drop to a low voltage at the time t 2 , the consumption power of the circuits can be efficiently economized and the reference voltage V REF  does not change after being determined. Therefore, the reference voltage V REF  is determined on an initially duration that system turns on for avoiding selecting a wrong reference voltage due to the system yet completely working and interference from system being least. 
     The aforementioned description is on the basis of the select reference voltage circuit with boost-divide voltage circuit  16 . If the select reference voltage circuit works without boost-divide circuit, the process about boost-divide voltage circuit  16  working and turning off leaves out. 
     Referring to FIG.  4 A and FIG. 4B, the relative position of the boost-divide voltage circuit  16  can be changed according to different conditions, wherein the complete components of the select reference voltage circuit are not drawn in FIG.  4 A and FIG.  4 B. The pulse signal  24  can respectively couples with compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  for shutting respective circuits after the pulse signal  24  drops to a low voltage. The pulse signal  24  may couple with at least one circuit thereof and the circuit is shut after the pulse signal  24  drops to a low voltage. The circuits may be sequentially shut with a mutual coupling signal, e.g.: the compare signal St between compare circuit  12  and reference voltage select circuit  14 ; the boost-divide voltage V pDDQ  or V pBGR  between compare circuit  12  and boost-divide voltage circuit  16 , etc. For example, when the pulse signal  24  drops to a low voltage, boost-divide voltage circuit  16  is shut and then the boost-divide voltage V pDDQ  or V pBGR  become 0V. Compare circuit  12  is shut when receiving the 0V of the boost-divide voltage V pDDQ  or V pBGR . Therefore, the compare signal St follows to become 0V and reference voltage select circuit  14  is shut when receiving the 0V of the compare signal St. Hence, compare circuit  12 , reference voltage select circuit  14  and boost-divide voltage circuit  16  are shut after the pulse signal  24  drops to a low voltage. 
     Accordingly, the another preferred embodiment not only reaches the advantages of the one preferred embodiment, but also reaches the stable and power-saving advantages. 
     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.