Abstract:
A circuit and a circuit design method are provided. The circuit operates between a first power source voltage and a ground voltage. The circuit comprises at least one low speed circuit path and at least one high speed circuit path. The low speed circuit path adjusts voltage level at the first power source voltage or the ground voltage. The low speed circuit path provides a first return path and isolates unwanted noise signals for a signal on the high speed circuit path.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a circuit, and in particular relates to a circuit board with specific circuit paths operating at power voltage or ground voltage providing more signal return paths. 
         [0003]    2. Description of the Related Art 
         [0004]    With the continuing development of advanced technology, circuit functions have become more and more. Thus, circuits need more and more pins to transmit/receive signals to/from other circuits or terminals. 
         [0005]    A circuit comprises mainly three types of pins. Some pins are signal pins for transmitting or receiving signals, some pins are GPIO (general purpose I/O) pins for a user to program pins for specific functions and some pins are power pins, such as power pins or ground pins. Since functions of circuits are increasing, corresponding pin requirements are also increasing. However, with regard to a fixed circuit package size, the circuit has a fixed number of pins. In some situations for design engineering considerations, such as requirements for more signal pins or less total number of pins for a circuit package, ground pins of a circuit will be not enough for signal return paths. If the circuit does not have enough signal return paths, the resistance of the signal return path increases and some unwanted noise signals and EM (Electric-Magnetic) problems occur accordingly. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0007]    An embodiment of a circuit is provided. The circuit operates between a first power source voltage and a ground voltage. The circuit comprises at least one low speed circuit path and at least one high speed circuit path. The low speed circuit path adjusts voltage level at the first power source voltage or the ground voltage. The low speed circuit path provides a first return path and isolates unwanted noise signals for a signal on the high speed circuit path. 
         [0008]    Another embodiment of a circuit design method is provided. A circuit operates between a first power source voltage and a ground voltage and comprises at least one low speed circuit path and at least one high speed circuit path. The method comprises adjusting the low speed circuit path at the first power source voltage or the ground voltage and adjusting the high speed circuit path between the first power source voltage and the ground voltage. The low speed circuit path provides a first return path and isolates unwanted noise signals for a signal on the high speed circuit path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0010]      FIG. 1  is a schematic diagram of an interconnection between a circuit and corresponding terminals according to an embodiment of the invention; 
           [0011]      FIG. 2  is a schematic diagram of a printed circuit board connecting a flexible printed circuit with low speed circuit paths, high speed circuit paths and quasi-state circuit paths according to another embodiment of the invention; 
           [0012]      FIG. 3  is a CMOS inverter for low speed circuit paths and quasi-state circuit paths adjusting voltage levels at the power source voltage or the ground voltage according to another embodiment of the invention; 
           [0013]      FIG. 4  is an E-field plot of a flexible printed circuit according to another embodiment of the invention; and 
           [0014]      FIG. 5  is a signal plot diagram according to the above embodiment of  FIG. 4 . 
           [0015]      FIG. 6  is an E-field plot of a flexible printed circuit according to another embodiment of the invention; and 
           [0016]      FIG. 7  is a signal plot diagram according to the above embodiment of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0018]      FIG. 1  is a schematic diagram of an interconnection between a chip  110  and corresponding terminals according to an embodiment of the invention. The chip  110  operates between a power source voltage VCC and a ground voltage and comprises pins  111 ,  112 ,  113 ,  114  and so on . . . for transmitting or receiving signals. Pin  111  receives or transmits signals through low speed circuit path  121  to terminal I/O  1 . As shown in  FIG. 1 , the signals on low speed circuit path  121  are at low voltage level (low state) and do not often adjust voltage levels. It is noted that the low voltage level is the ground voltage level. If the signal on low speed circuit path  121  adjusts to the high voltage level (high state), the high voltage level is the power source voltage VCC. Pin  112  receives or transmits signals through high speed circuit path  122  to terminal I/O  2 . As shown in  FIG. 1 , the signals on high speed circuit path  122  are high speed signals that adjust voltage levels at a high frequency. 
         [0019]    Pin  113  receives or transmits signals through low speed circuit path  123  to terminal I/O  3 . As shown in  FIG. 1 , the signals on low speed circuit path  123  are at high voltage level (high state) and do not often adjust voltage levels. It is noted that the high voltage level is the power source voltage VCC. If the signal on low speed circuit path  123  adjusts to the low voltage level (low state), the low voltage level is the ground voltage GND. Pin  114  receives or transmits signals through quasi-state circuit path  124  to terminal I/O  4 . As shown in  FIG. 1 , the signals on quasi-state circuit path adjust voltage level at a high voltage level (high state) or a low voltage level (low state) at a very low frequency. The high voltage level is the power source voltage VCC and the low voltage level is the ground voltage GND. Since the low speed circuit path  121  is at the ground voltage GND or power source Voltage VCC, the low speed circuit path  121  can provide a return path for the high speed circuit path  122  and isolate unwanted noise signals (shielding effect function). The low speed circuit path  123  has the same function as the low speed circuit path  121 . With regard to the quasi-state circuit path  124 , since the quasi-state circuit path  124  adjusts voltage level at power voltage VCC or ground voltage GND at very low frequency, the quasi-state circuit path  124  can also provide a return path for the high speed circuit path  122  and isolate unwanted noise signals (shielding effect function). 
         [0020]    It is not limited that the chip only comprises two low speed circuit paths, one quasi-state circuit path and one high speed circuit path. Chip  110  can comprises more than two low speed circuit paths, one quasi-state circuit path and one high speed circuit path. In addition, it is preferred that the high speed circuit paths are disposed beside or close to the low speed circuit paths or quasi-state circuit paths. 
         [0021]      FIG. 2  is a schematic diagram of a printed circuit board  210  connecting a flexible printed circuit  220  with low speed circuit paths, high speed circuit paths and quasi-state circuit paths according to another embodiment of the invention. The low speed circuit paths  121  and  123 , high speed circuit path  122  and quasi-state circuit path  124  are connected to circuit  110  through the printed circuit board  210  and to corresponding terminal I/Os through the flexible printed circuit  220 . According to an embodiment of the invention, at least one low speed circuit path or quasi-state speed circuit path is disposed beside or close to one high speed circuit path on the printed circuit board  210  and the flexible printed circuit  220  for isolating unwanted noise signals and providing a signal return path that the low speed circuit path or quasi-state speed circuit path adjusts voltage level at the power source voltage VCC or the ground voltage GND. However, it is not limited to use both the printed circuit board  210  and the flexible printed circuit  220 . One of the printed circuit board  210  and the flexible printed circuit  220  between chip  110  and corresponding terminal I/Os can be used. In addition, some unused GPIO pins can connect to low speed circuit paths and operate at the power source voltage or the ground voltage for providing a signal return path for the high speed circuit path and preventing unwanted noise signals from affecting the signal on the high speed circuit path. 
         [0022]      FIG. 3  is a CMOS inverter  310  for low speed circuit paths and quasi-state circuit paths adjusting voltage levels according to another embodiment of the invention. According to an embodiment, low speed circuit paths  121  and  123  and quasi-state circuit path  124  respectively pass through one CMOS inverter  310  to connect to inside circuits of the chip  110 . The CMOS inverter  310  comprises PMOS transistor  312  and NMOS transistor  314 . The source, gate and drain of the PMOS transistor  312  are respectively coupled to power source voltage VCC, inside circuits of the chip  110  and low speed circuit paths or quasi-state circuit paths. The source, gate and drain of the NMOS transistor  314  are respectively coupled to the ground voltage GND, inside circuits of the chip  110  and low speed circuit paths or quasi-state circuit paths. 
         [0023]      FIG. 4  is an electric field (E-field) plot of a flexible printed circuit according to another embodiment of the invention. There are six circuit paths in the flexible printed circuit. Circuit path b 0  is at ground voltage level GND. Other circuit paths b 1 , b 2 , b 3 , b 4  and b 5  transmit signals which adjust voltage levels between 1.2V to 0V.  FIG. 4  only shows the E-field plot between the circuit path b 1  and circuit paths b 0  and b 2 . 
         [0024]      FIG. 5  is a signal eye diagram according to the above embodiment of  FIG. 4 . The circuit path b 0  is at ground voltage level GND. Other circuit paths b 1 , b 2 , b 3 , b 4  and b 5  transmit signals which adjust voltage levels between 1.2V to 0V. As shown in  FIG. 5 , the signal degradation of circuit path b 5  is the worst because circuit path b 5  is the farthest circuit path to the circuit path b 0  (GND). 
         [0025]      FIG. 6  is an electric field (E-field) plot of a flexible printed circuit according to another embodiment of the invention. There are six circuit paths in the flexible printed circuit. Circuit paths b 6 , b 8  and b 10  are at ground voltage level GND. Other circuit paths b 7 , b 9  and b 11  transmit signals which adjust voltage levels between 1.2V to 0V. 
         [0026]      FIG. 7  is a signal eye diagram according to the above embodiment of  FIG. 6 . The circuit paths b 6 , b 8  and b 10  are at ground voltage level GND. Other circuit paths b 7 , b 9  and b 11  transmit signals which adjust voltage levels between 1.2V to 0V. As shown in  FIG. 7 , the signal of circuit path b 11  is worse than signals of circuit paths b 7  and b 9  because circuit path b 11  has only one ground side compared to circuit paths b 7  and b 9  having two ground sides. Comparing  FIGS. 4 and 5  to  FIGS. 6 and 7 , it is therefore obvious that signal quality is improved with more ground circuit paths. According to the above embodiments, using low speed circuit paths and quasi-state circuit paths as ground circuit paths beside the high speed circuit path can improve the signal quality of the high speed circuit path. 
         [0027]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited to thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.