Abstract:
An output circuit applied in the double data rate (DDR) system for generating sampling clocks. Assume that the sampling clocks are initially set as a first logic level. The output circuit comprises an output transistor unit for outputting the sampling clock and a pre-pulling unit that connects to an output terminal of the output transistor unit and a second logic level and receives a control signal. The control signal has a pulse before the first time the sampling clock changes from the initial first logic level at the output terminal of the output transistor unit. This pulse can be used to control the pre-pulling unit, so that the output terminal of the output circuit shifts a voltage difference in advance from the first logic level toward the second logic level, thereby preventing initial oscillation of the sampling clock and maintaining the completeness of the data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a processing method for high-frequency transmission applications, more particularly, to an output circuit for high-frequency transmission applications, which can avoid the errors of a first set of transmitted data caused by the circuitry factor. 
     2. Description of the Related Art 
     Due to the high-speed requirements of electronic systems, the data transmission speed must be raised to meet the needs of the system. There are many techniques for increasing the transmission speed. For example, the double data rate technique, namely, DDR, can be used in synchronized dynamic random access memories (SDRAM) to speed up the original transmission speed. This technique utilizes both the rising edge and the falling edge within a clock signal to acquire double the original data rate under the same timing specification. 
     FIG. 1 (PRIOR ART) is a sampling timing diagram illustrating the operating principle of the DDR technique. As shown in the figure, sampling clocks STROB and STROB# are complementary pulse signals with a fixed frequency. In addition, the edges of the sampling clocks STROB/STROB# are also opposite, that is, one is the falling edge and the other is the rising edge. For example, at time points S 1  and S 3 , the sampling clock STROB is on the falling edge and the sampling clock STROB# is on the rising edge. At time point S 2 , the sampling clock STROB is on the rising edge and the sampling pulse STROB# is on the falling edge. Therefore, data on the data signal DATA can be sampled based on these time points S 1 , S 2  and S 3 . Accordingly, the practical data rate over a transmission line can be doubled by using the same sampling clocks. 
     FIG. 2 (PRIOR ART) is a circuit diagram of the conventional output circuit for the data signal DATA. As shown in the figure, output data A is sent to buffers  10  and  12 , and further to the gates of PMOS transistor  14  and NMOS transistor  16  serially connected, respectively. Thus, the data signal DATA is pulled out from the connected drain electrodes of the PMOS transistor  14  and the NMOS transistor  15 . When the output data A to be output is at logic HIGH, the NMOS transistor  16  turns on and the PMOS transistor  14  turns off. In this case, the data signal DATA is 0V (the ground voltage), which represents logic “0”. When the output data A to be output is at logic LOW, the PMOS transistor  14  turns on and the NMOS transistor  16  turns off, the data signal DATA is the system voltage VPP, which represents logic “1”. 
     FIG. 3 (PRIOR ART) is a circuit diagram of the conventional output circuit for the sampling clocks STROB/STROB#. Notice that FIG. 3 only demonstrates an output circuit for one of the sampling clocks STROB/STROB# and the output circuits for both of the sampling clocks STROB/STROB# are similar. As shown in the figure, control signals S/S# are sent to buffers  20  and  22 , and further to the gates of PMOS transistor  24  and NMOS transistor  26  serially connectedly, respectively. The corresponding sampling clocks STROB/STROB# are pulled out from the connected drain electrodes of the PMOS transistor  24  and the NMOS transistor  26 . The signal variations of the sampling clocks STROB/STROB# depend on the control signals S/S#. 
     However, since the high-frequency data will be affected by the delay effect or other factors during the transmission over the transmission line, the sampling clocks STROB/STROB# usually cannot achieve full swing at the first sampling point. If the sampling clocks STROB/STROB# are going to 1.5V and 0V at the first sampling point, respectively, they cannot achieve these predetermined voltages within the timing specification in practical operation. In this case, since the sampling operation is also affected by other factors, for example, ground/power bounce, IR drop or signal coupling, the extraction of the data signal DATA is probably incorrect. 
     FIG. 4 (PRIOR ART) is a waveform diagram showing the waveforms of the sampling clocks STROB/STROB# at the first set of data. As shown in the figure, the waveforms of the sampling clocks STROB/STROB# cannot achieve full swing. This could cause the drifting of setup/hold time between the data signal DATA and the sampling clacks STROB/STROB# and the deterioration of the signal eye patttern. As described above, the DDR system extracts data on the data signal DATA based on the opposite crossing point of the sampling clocks STROB/STROB#. Therefore, in the prior art, the extraction of the first set of transmitted data is prone to error. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an output circuit, which can prevent the first set of the transmitted data from being erroneous in high-frequency transmission applications and is especially suitable for transmission systems with the DDR function. 
     According to the above object, the present invention provides an output circuit, which can output a sampling clock signal, for example, one of the sampling clock signals in the double data rate system. The output terminal of the output circuit is initially set as a first logic level (such as logic “1”). The output circuit includes an output transistor unit having an input terminal and an output terminal serving as the output terminal of the output circuit. In addition, there is a pre-pulled unit, which is connected between the output terminal of the output circuit and a second logic level and receives a control signal. There is a pulse in the control signal before the first time the logic level starts to change at the output terminal of the output transistor unit. This pulse can be used to control the pre-pulled unit, so that the output terminal of the output circuit shifts by a voltage difference toward the second logic level from the initial first logic level. For example, the first logic level corresponds to logic “1”. Before the sampling clock signal falls from logic “1” to logic “0” for the first time, the control signal can control the pre-pulled unit to lower the voltage of the original logic “1”, thereby guaranteeing the pulse signal correctly falling down to logic “0” at the first conversion time. The above operation also can be applied to the case where the logic “0” rises to logic “1”. 
     In addition, the input terminal of the output transistor unit can be connected to a transmission gate unit, which is used to transmit an input signal to the input terminal of the output transistor unit. The transmission gate unit can be controlled by the control signal. Normally, the input signal can be sent to the input terminal of the output transistor unit via a first channel in the transmission gate unit. During the period of the pulse of the control signal, the input signal can be sent to the input terminal of the output transistor unit via the second channel in the transmission gate unit. The second channel involves a delay time with respect to the first channel to adjust the timing variation introduced by the previously pulling operation. 
     Furthermore, the present invention also provides an output apparatus of double data rate system, which includes a plurality of data output circuits for the data signals, a first sampling clock output circuit and a second sampling clock output circuit for outputting the first sampling clock and the second sampling clock, respectively. The data signal can be sampled at the intersections of rising edges and falling edges of the first sampling clock and the second sampling clock, thereby doubling the sampling rate. The first sampling clock output circuit and the second sampling clock output circuit can be implemented by the above-described output circuit. It is noted that the logic value of the first sampling clock is opposite to the logic value of the second sampling clock for the sampling purpose of the double data rate system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 (PRIOR ART) is a sampling timing diagram illustrating the operating principle of the DDR technique; 
     FIG. 2 (PRIOR ART) is a circuit diagram of the conventional output circuit for the output data signal DATA; 
     FIG. 3 (PRIOR ART) is a circuit diagram of the conventional output circuit for the sampling clocks STROB/STROB#; 
     FIG. 4 (PRIOR ART) is a waveform diagram showing the waveforms of the sampling clocks STROB/STROB# at the first set of data; 
     FIG. 5 is a timing diagram of the data signal, the sampling clock signal and the preset signal in the preferred embodiment of the present invention; 
     FIG. 6 is a circuit diagram of an output circuit for the output data signal DATA in accordance with the double data rate system of the present invention; 
     FIG. 7 is a circuit diagram of an output circuit for the sampling clock signal STROB in accordance with the double data rate system of the present invention; and 
     FIG. 8 is a circuit diagram of an output circuit for the sampling clock signal STROB# in accordance with the double data rate system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention deals with the waveforms of the sampling clocks for sampling a first set of the transmitted data by using a previously pulling operation. For example, the previously pulling operation is to pull down a predetermined voltage from the HIGH level for the sampling clock with an initial logic state “1” and to pull up a predetermined voltage from the LOW level for the sampling clock with an initial logic state “0”. Accordingly, the previously pulling operation can lower the oscillation of the waveforms of the sampling clocks induced by the inertia effect, thereby allowing the sampling clocks in the double data rate system to achieve full swing and preventing from erroneous sampling in the first sampling operation. Next, the embodiments will be described in details with reference to figures. 
     FIG. 5 is a signal timing diagram of various signals in the double data rate system in accordance with the preferred embodiment, where PRAM is a pre-pulled control signal, STROB and STROB# are a pair of sampling clock signals and DATA is a data signal. The functions of these signals will be described later. As described above, when a rising edge of one sampling clock (such as STROB) intersects a falling edges of another sampling clock (such as STROB#), the data on the data signal DATA can be fetched. 
     The main difference between the timing diagram of FIG.  5  and the prior art is the pre-pulled control signal PRAM. Pre-pulled control signal PRAM has a pulse (between time T 1  and T 2 ) before data signal DATA (i.e. time T 3 ) is sampled for the first time. As shown in FIG. 5, before time T 1 , the pre-pulled control signal PRAM is in the LOW voltage level, and the sampling clocks STROB/STROB# are in logic HIGH and logic LOW, respectively. At time T 1 , the pulse of the pre-pulled control signal PRAM appears and starts to rise to a high voltage. At this time, the sampling clocks STROB/STROB# are activated by pre-pulling and change their voltage levels, where the voltage of the sampling clock STROB decreases from the initial high level by a voltage difference V and the voltage of the sampling clock STROB# increases from the initial low level by the voltage difference V. Note that the previous pulling operation should not change the logic states of the sampling clocks STROB/STROB# and only the actual voltages of the sampling clocks STROB/STROB# are changed to be closer to the logic level to be converted. At time T 2 , the pulse of the pre-pulled control signal PRAM ends. On the other hand, sampling clocks STROB/STROB# enter into a period of falling/rising edges, respectively, where the sampling clock STROB continues to fall toward the logic LOW level and the sampling clock STROB# continues to raise toward the logic HIGH level. At time T 3 , the falling edge of the sampling clock STROB intersects the rising edge of the sampling clock STROB#, so the information of the data signal DATA is extracted. Subsequently, the remaining waveforms of the sampling clocks STROB/STROB# can follow the original specifications and the pre-pulling operation activated by the pre-pulled control signal PRAM is no longer needed. In the present embodiment, the pre-pulled operation is used to change the initial voltages of the sampling clocks in advance before the first set of the data is extracted (i.e. two sampling clocks exchange their logic values for the first time). Therefore, the initial oscillating phenomenon can be suppressed to secure the integrity of the first set of transmitted data. 
     FIG. 6 is a circuit diagram of the output circuit for the data signal DATA in the preferred embodiment. As shown in the figure, the output circuit includes a transmission gate  30 , buffers  31  and  32 , a PMOS transistor  35  and an NMOS transistor  37 . Data A to be output are sent to buffers  31  and  32  respectively via transmission gate  30 , thus to the gate connected electrodes of the PMOS transistor  35  and the NMOS transistor  37 . Finally, the data signal DATA is generated from the connected drain electrodes of the PMOS transistor  35  and NMOS transistor  37 . 
     Since one of control terminals of the transmission gate  30  is connected to the voltage VDD and another one is connected to the ground, the transmission gate  30  remains conducting. When the data A to be output is in logic “1”, the NMOS transistor  37  turns on and the PMOS transistor  35  turns off. Thus, the data signal DATA outputs  0 V ground voltage that represents logic “0”. When the data A to be output is in logic “0”, the PMOS transistor  35  turns on and the NMOS transistor  37  turns off. Data signal DATA then outputs a voltage VPP, that represents logic “1”. In this embodiment, the transmission gate  30  is mainly used to adjust the output path delay for the data signal DATA to synchronize with the sampling clocks STROB/STROB# described later. 
     FIG. 7 shows a circuit diagram of an output circuit for the sampling clock STROB in this embodiment, and FIG. 8 is a circuit diagram of an output circuit for the sampling clock STROB# in this embodiment. 
     As shown in FIG. 7, the output circuit of the sampling clock STROB includes a transmission gate stage  40 , an output transistor stage  50  and a pre-pulled unit  60 . The output transistor stage  50  includes buffers  52  and  54 , a PMOS transistor  56  and an NMOS transistor  58 . The operation of this output transistor stage  50  is basically the same as those described above. Pre-pulled unit  60  includes a PMOS transistor  62  with a source and a gate connected together and a NMOS transistor  66  controlled by the pre-pulled control signal PRAM. Buffer  64  is positioned at the gate of the NMOS transistor  66 . Assume that the logic state of the sampling clock STROB is “1”. When there is a pulse on the pre-pulled control signal PRAM, the NMOS transistor  66  turns on. Therefore, a conduction route is established between the terminal of the pre-pulled control signal PRAM and the ground terminal. In other words, the voltage of the sampling clock STROB previously starts to fall toward logic “0” by means of the pre-pulling operation. Pre-pulled control signal PRAM only appears before the first set of the transmitted data, so this conducting route will not consume too much power. On the other hand, in the process of the normal data transmission (i.e. not the first set of the transmitted data), the PMOS transistor  62  and NMOS transistor  66  can also be used for electrostatic discharge (ESD). 
     Furthermore, the transmission gate stage  40  is positioned between a terminal receiving the control signal S and an input terminal of the output transistor stage  50 . Accordingly, transmission gate  42  constitutes a first transmission channel, and transmission gate  44  and buffer  46  constitutes a second transmission channel. Transmission gates  42  and  44  are controlled by the pre-pulled control signal PRAM and its inverting signal PRAM#. However, the control effects of both the control signals PRAM/PRAM# are opposite. That is, when the transmission gate  42  turns on, the transmission gate  44  turns off. When the transmission gate  44  turns on, the transmission gate  42  turns off. According to FIG. 7, during the pulse of the pre-pulled control signal PRAM, the control signal S is sent to the output transistor stage  50  via the second transmission channel formed by the transmission gate  44  and the buffer  46 . Under other situations, however, the control signal S is sent to the output transistor stage  50  via the first transmission channel formed by the transmission gate  42 . Apparently, there is a component delay difference induced by the buffer  46  between the two transmission channels. Since the driving capacity can be enhanced by the pre-pulled operation and thus the original delay time for the second transmission channel will be shortened, the buffer  46  is inserted in the second transmission channel to compensate the timing variations of the sampling clocks and the data signal, such as setup/hold time, introduced by the pre-pulled operation. Similar to the situation illustrated in FIG. 7, the output circuit of the sampling clock STROB# in FIG. 8 includes a transmission gate stage  70 , an output transistor stage  80  and a pre-pulled unit  90 . In the transmission gate stage  70 , a transmission gate  72  constitutes a first transmission channel, which is used to send the control signal S# to the output transistor stage  80  under normal operational conditions. A transmission gate  74  and a buffer  76  constitute a second transmission channel, which is used to send the control signal S# to the output transistor stage  80  during the pulse of the pre-pulling control signal PRAM. Similarly, the buffer  76  is used to compensate the timing variations introduced by the pre-pulling operation. 
     Output transistor stage  80 , including buffers  82  and  84 , a PMOS transistor  86  and an NMOS transistor  88 , is used to generate the sampling clock STROB# according to the received control signal S#. Pre-pulling unit  90  includes an NMOS transistor  92  with coupled source and gate electrodes and a PMOS transistor  96  controlled by the pre-pulled control signal PRAM. The buffer  94  is positioned at the gate electrode of the PMOS transistor  96 . Assume that the sampling clock STROB# is in logic “0”. When there is a pulse on the pre-pulled control signal PRAM, the PMOS transistor  96  turns on and thus a conduction route is established between terminals of the pre-pulled control signal PRAM and the voltage VPP. In other words, the voltage of the sampling clock STROB# previously starts to rise toward logic “1” by means of the pre-pulling operation. In addition, during the period for transmitting data rather than the first set, the PMOS transistor  96  and the NMOS transistor  92  can also be used for electrostatic discharge (ESD). 
     According to the discussion above, a pre-pulling unit coupled to the output terminals of the sampling clocks STROB/STROB# can be used to slightly adjust the voltages of the sampling clocks STROB/STROB# in advance before the first sampling action, thereby preventing the oscillation phenomenon of the sampling clocks and securing the completeness of the first set of the transmitted data. In addition, the additional pre-pulling transistors can be used for electrostatic charging in the remaining transmission process. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. ON 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.