Abstract:
A parallel in serial out (PISO) circuit for converting inputted parallel data bits into a corresponding serial data is disclosed. In the case where the inputted parallel data bits is four, the PISO circuit includes a first latch group, which is provided with four flip-flops, for latching respectively the four data bits at the four flip-flops in synchronism with a clock of 50 MHz. A first selector group is further provided which includes two selectors each of which selectively receives two different data bits latched at the first latch group and each of which outputs sequentially the received two different data bits in synchronism with the clock of 50 MHz. A second latch group, which follows the first selector group, is provided with two flip-flops for latching the two data bits outputted form the two selectors of the first selector group in synchronism with a clock of 100 MHz. The PISO circuit further has a single selector selectively receiving the two data bits latched at the second latch group in synchronism with the clock of 100 MHz. A flip-flop, which is preceded by the single selector, latches the data bit outputted for the single selector in synchronism with a clock of 200 MHx. Thus, the four data bits inputted in parallel to the PISO circuit is converted into the corresponding serial data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates generally to a parallel in serial out (PISO) circuit for use in a digital data communication system, and more specifically to a PISO circuit which is suitable for use in a physical layer defined by IEEE (Institute of Electrical and Electronic Engineers) 1394 standard. 
     2. Description of Related Art 
     In order to comply with the demand for large volume transfer of digital data at a high speed between a personal computer and a peripheral(s) byway of example, the IEEE 1394-1995 Standard for a High Performance Serial Bus (hereinafter sometimes referred to as 1394 serial bus) was developed. As is known in the art, the 1394 serial bus protocols are described as a set of three stacked layers: transaction layer, link layer and physical layer. Explaining these layers in brief, the transaction layer defines a complete request-response control to perform the bus transactions. The link layer provides a one-way (half-duplex) data transfer with confirmation of request service to the transaction layer, and further provides addressing, data checking, and date framing for packet transmission and reception. On the other hand, the physical layer translates the logical symbols used by the link layer into electrical signals, and defines the mechanical interfaces for the serial bus. More specifically, the physical layer has three primary functions: transmission and reception of data bits, arbitration, and provision for the electrical and mechanical interface. 
     Before turning to the present invention, it is deemed preferable to briefly describe, with reference to FIGS.  1 - 6 (C), prior art to which the present invention is applicable. 
     Referring to FIG. 1, a conventional data transfer circuit (denoted by  10 ) is schematically shown in block diagram form, which circuit is provided in the physical layer. As shown, the data transfer circuit  10  generally comprises a parallel in serial out (PISO) circuits  12  and  14 , a clock frequency divider  16 , a timing controller  18 , and a data&#39;strobe encoder  20 , all of which are coupled as illustrated. 
     According to the IEEE 1394 protocol, the data transfer speed is selected among 400, 200 and 100 Mbps (Mega-bit per second). The cock frequency divider  16 , which takes the form of 3-stage counter using three flip-flops (FFs), receives a 400 MHz clock, and divides the same so as to output three different clocks that have respectively clock rates of 200 MHz, 100 MHz, and 50 MHz. In addition, the clock frequency divider  16  allows the inputted 400 MHz clock to pass therethrough. That is to say, the clock frequency divider  16  issues the four different clocks of 400 MHz, 200 MHz, 100 MHz and 50 MHz, all of which are applied to the timing controller  18 . These clocks are respectively denoted by CLK 400 , CLK 200 , CLK 100 , and CLK 50  in the instant disclosure. 
     The data/strobe encoder  20  is supplied with a parallel data DATA_Parallel of 8 bits (in the instant case) to be transmitted in serial, and outputs two parallel data: one is a parallel data DATA_Parallel which is exactly the same as the inputted parallel data; and the other is a parallel strobe signal STRB_Parallel. These parallel data DATA_Parallel and STRB_Parallel are applied to each of the PISO circuits  12  and  14 . Although not shown in FIG. 1, the parallel data (8 bits) are respectively numbered  0 ,  1 ,  2 , . . . ,  7  in order of sequential transmission from each of the PISO circuits  12  and  14 . The parallel strobe signal STRB_Parallel is produced by simply reversing a logic level of each of the odd number data bits (viz., data bits respectively numbered  1 ,  3 ,  5 , and  7 ) of the parallel data of DATA_Parallel. 
     As mentioned above, according to the IEEE 1394 standard, two serial data DATA_Serial (Tx) and STRB_Serial (Tx), which are respectively outputted from the PISO circuits  12  and  14 , are transmitted at the transfer speed of 400, 200, or 100 Mbps using a lock rate of 400 MHz, 200 MHz, or 100 MHz. In order to select one of these clock rates, the timing controller  18  is supplied with data transfer speed control signals SPD 0  and SPD 1 , and also receives the above-mentioned four clocks CLK 400 , CLK 200 , CLK 100 , and CLD 50 . The speed control signals SPD 0  and SOD 1  will further be described later. 
     Referring to FIG. 2, the timing controller  18  of FIG. 1 is shown in detail in block diagram form. As shown in FIG. 2, the timing controller  18  comprises decoders  22   a - 22   c , flip-flops (FFs)  24   a - 24   e , and selectors  26   a - 26   c , all of which are coupled as shown. As mentioned above, the controller  18  is supplied with the four locks CLK 400 , CLK 200 , CLK 100 , and CLK 50 , and further receives the data transfer speed control signals SPD 0  and SPD 1  from logic circuitry (not shown) provided in the physical layer. The timing controller  18  per se is not directly concerned with the present invention, and hence, only the brief description thereof will be given for the sake of simplifying the instant disclosure. The decoder  22   c  receives the data transfer speed control signals SPD 0  and SPD 1 , and issues data transfer speed control signals C 1 -C 3 . The control signal C 1  is applied to the selectors  26   a  and  26   c , while the control signals C 2  and C 3  are both applied to each of the selectors  26   a  and  26   b.    
     The timing controller  18  can be divided into two sections in terms of function thereof. That is, the first section includes the flip-flop  24   e  and the selectors  26   b - 26   c , while the second section includes the decodes  22   a - 22   b , the flip-flops  24   a - 24   d , and the selector  26   a . When both of the data transfer speed signals SPD 0  and SPD 1  is non-active, the output of the selector  26   c  of the first section becomes CLK 400  (400 MHz), and the output of the flip-flop  24   d  is 50 MHz (viz., one eight of 400 MHz). Further, when only SPD 1  becomes active, the output of the selector  26   c  is CLK 200  (200 MHz) and at the same time, the output of the flip-flop  24   d  is 25 MHz. Still further, when only SPD 0  becomes active, the output of the selector  26   c  becomes CLK 100 , and at the same time, the flip-flop  24   d  becomes 12.5 MHz. The clock and the data acquisition signal thus generated are both applied to the PISO circuits  12  and  14 . 
     FIG. 3 is a diagram showing the PISO circuit  12  in detail. It is to be noted that the other PISO circuit  14  is configured in exactly the same manner as the PISO circuit  12 . The PISO circuit  12  is a conventional type 8-bit shift register which is configured by eight flip-flops  30   a - 30   h , selectors  32   a - 32   g , and an AND gate  34 . A clock signal (400 MHz, 200 MHz, or 100 MHz) is applied t the clock terminals of the flip-fops  30   a - 30   h . Data bits D 0 -D 6  are respectively applied to the selectors  32   a - 32   g  by way of input terminals T 0 -T 6 , and on the other hand, a data bit D 7  is applied to the AND gate  34  via an input terminal T 7 . When the data acquisition signal DAS is applied to the PISO circuit  12 , the flip-flops  30   a - 30   h  acquire respectively the outputs of the preceding circuits. The contents of the flip-flops  30   a - 30   h  are then shifted in synchronism with the clock until the data bit D 7  is outputted from the flip-flop  30   h , and thereafter, these operations are repeated on the following eight parallel data (viz., D 8 -D 15 ). 
     FIG. 4 is a timing chart graphically showing the above-mentioned operation of the PISO circuit  12 . It is clearly understood that the first 8-bit parallel data D 0 -D 7  are acquired at the flip-flops  30   a - 30   h  at a time in response to the data acquisition signal DAS, after which the PISO circuit  12  outputs in series D 0 -D 7  in synchronism with the clock as DATA_Serial (Tx), and thereafter, the same operation is carried out on the following 8-bit parallel data D 8 -D 15 . 
     FIG. 5 is a timing chart showing the operation of the other PISO circuit  14 . As mentioned above, the parallel strobe data STRB_Parallel differs from the corresponding parallel data DATA_Parallel only with respect to the fact that the odd number data bits of DATA_Parallel (viz., data bits respectively number  1 ,  3 ,  5 , . . . ) are reversed in terms of logic level. In FIG. 5, the character “X” attached to each of the data bits such as D 1 , D 3 , etc. denotes that the logic level of each of the data bits D 1 , D 3 , . . . etc. of DATA_Parallel is reversed. Accordingly, the timing chart of FIG. 5 is readily understood when referring to that of FIG. 4, and hence, further description of the timing chart of FIG. 5 will be omitted for the sake of simplifying the instant disclosure. 
     FIG.  6 (A) is a timing chart schematically showing DATA_Serial (Tx) and STRB_Serial (Tx) which are respectively outputted from the. PISO circuits  12  and  14 . As shown, the logic level of each of the data bits at the odd bit numbers  1 ,  3 ,  5 , and  7  of STRB_Serial (Tx) are reversed with respect to the corresponding data bits of DATA_Serial (Tx). 
     FIG.  6 (B) is a timing chart schematically showing a dock which is reproduced by performing exclusive-OR operation of DATA_Serial (Rx) and STRB_Serial (Rx) received at a given node (e.g., a certain periphery coupled to a personal computer (for example)). The reproduction of the clock using the received DATA_Serial (Rx) and STRB_Serial (Rx) is to improve the skew tolerance of data bits transferred across the serial bus. 
     The prior art thus far described has encountered the difficulty that the power consumption of the data transfer circuit  10  is undesirably high in that a large number of the flip-flops operable at the high frequency of 400 MHz are provided. In more specific terms, the clock frequency divider  16  is provided with three flip-flops operating at the 400 MHz clock, and further, the timing controller  18  requires the five flip-flops which are configured such as to operate at 400 MHz. Still further, each of the PISO circuits  12  and  14  contains eight flip-flops operable at 400 MHz. Summing up, there are 24 (twenty four) flip-flops in total in the data transfer circuit  10 , each of which is operable at 400 MHz. Accordingly, if the circuit  10  operates such as to output DATA_Serial and STRB_Serial at 400 MHz, then the power consumption becomes objectionable when the circuit  10  is fabricated on a semiconductor chip. 
     One approach to overcoming the above-mentioned problem is disclosed in Japanese Laid-open Patent Application No 10-22837. According to this prior art, two selectors are provided for respectively converting 8-bit parallel data and 8-bit parallel strobe into the corresponding two 8-bit serial data. This prior art utilizes eight switches in place of eight flip-flops in a shift-register, wherein each switch is designed so as to operate at a high frequency, and as such, it is expected to reduce the overall power consumption of the data transfer circuit. However, the switches disclosed in the prior art in question are directly coupled at the outputs thereof, and thus die signals generated therefrom tend to be adversely affected by the existence of the output wire load and the output capacitance of each of the switches. Consequently, the rising and falling edges of each of the output pulses is apt to be deteriorated, leading to the fact that jitters are liable to occur in each of the received DATA_Serial (Rx) and STRB_Serial (Rx). The jitters in turn cause skews between DATA_Serial (Rx) and STRB_Serial (Rx), which renders it difficult to correctly reproduce the clock because the duty cycles of the pulses may be considerably different with each other as shown in FIG.  6 (C). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a PISO circuit via which the power consumption can effectively be realized while correctly reproducing a clock at a data receiving side (node). 
     In brief, the object is achieved by a parallel in serial out (PISO) circuit for converting inputted parallel data bits into a corresponding serial data is disclosed. In the case where the inputted parallel data bits is four, the PISO circuit comprises a first latch group, which is provided with four flip-flops, for latching respectively the four data bits at the four flip-fops in synchronism with a clock of 50 MHz. A first selector group is further provided which comprises two selectors each of which selectively receives two different data bits latched at the first latch group and each of which outputs sequentially the received two different data bits in synchronism with the clock of 50 MHz. A second latch group, which follows the first selector group, is provided with two flip-flops for latching the two data bits outputted from the two selectors of the first selector group in synchronism with a clock of 100 MHz. The PISO circuit further comprises a single selector for selectively receiving the two data bits latched at the second latch group in synchronism with the clock of 100 MHz. A flip-flop, which is preceded by the single selector, latches the data bit outputted from the single selector in synchronism with a clock of 200 MHz. Thus, the four data bits inputted in parallel to the PISO circuit is converted into the corresponding serial data. 
     One aspect of the present invention resides in a parallel in serial out circuit for converting 2 r  data bits (r is a natural number) applied thereto in parallel into a corresponding serial data, comprising: first to (r+1)-th latch means wherein the s-th (1≦s≦r+1) latch means comprises 2 r+1−s  flip-flops; first to r-th selector means respectively provided between adjacent two of the first to (r+1)-th latch means; wherein the t-th latch means (1≦t≦r) is provided such as to latch 2 r+1−t  data bits in synchronism with a clock signal of 2 t−2 ×100 MHz, and the (r+1)-th latch means is provided to latch a single data bit in synchronism with a clock signal of 2 t+1−2 ×100 MHz, and wherein the u-th selector means (1≦u≦r) is provided with 2 r−u  selectors each of which receives two different data bits outputted from the u-th latch means and generates sequentially the received two different data bits in synchronism with a clock signal of 2 u−2 ×100 MHz. 
     Another aspect of the present invention resides in a parallel in serial out circuit for converting four data bits applied thereto in parallel into a corresponding serial data, comprising: first latch means, which is provided with four flip-flops, for latching respectively the four data bits at the four flip-flops in synchronism with a clock of 50 MHz: first selector means comprising two selectors each of which selectively receives two different data bits latched at the first latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 50 MHz: second latch means, which is provided with two flip-flops, for latching the two data bits outputted from the two selectors of the first selector means in synchronism with a clock of 100 MHz; second selector means comprising a single selector for selectively receiving the two data bits latched at the second latch means in synchronism with the clock of 100 MHz; and a flip-flop for latching the data bit outputted from the single selector in synchronism with a clock of 200 MHz. 
     Still another aspect of the present invention resides in a parallel in serial out circuit for converting eight data bits applied thereto in parallel into a corresponding serial data, comprising: first latch means, which is provided with eight flip-flops, for latching respectively the eight data bits at the eight flip-flops in synchronism with a clock of 50 MHz; first selector means comprising four selectors each of which selectively receives two different data bits latched at the first latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 50 MHz; second latch means, which is provided with four flip-flops for latching respectively the four data bits outputted from the four selectors of the first selector means at the four flip-flops in synchronism with a clock of 100 MHz; second selector means comprising two selectors each of which selectively receives two different data bits latched at the second latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 100 MHz; third latch means, which is provided with two flip-flops, for latching respectively the two data bits outputted from the two selectors of the second selector means in synchronism with a clock of 200 MHz; third selector means comprising a single selector for selectively receiving two different data bits latched at the third latch means in synchronism with the clock of 200 MHz and a flip-flop for latching the data bit outputted from the single selector in synchronism with a clock of 400 MHz. 
     Still another aspect of the present invention resides in a parallel in serial out circuit for converting sixteen data bits applied thereto in parallel into a corresponding serial data, comprising: first latch means, which is provided with sixteen flip-flops, for latching respectively the sixteen data bits at the sixteen flip-flops in synchronism with a clock of 50 MHz; first selector means comprising eight selectors each of which selectively receives two different data bits latched at the first latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 50 MHz second latch means, which is provided with eight flip-flops, for respectively latching the eight data bits outputted from the eight selectors of the first selector means at the eight flip-flops in synchronism with a clock of 100 MHz; second selector means comprising four selectors each of which selectively receives two different data bits latched at the second latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 100 MHz; third latch means, which is provided with four flip-flops, for latching respectively the four data bits outputted from the four selectors of the second selector means at the four flip-flops in synchronism with a clock of 200 MHz third selector means comprising two selectors each of which selectively receives two different data bits latched at the third latch means and each of which outputs sequentially the received two different data bits in synchronism with the clock of 200 MHz; fourth latch means, which is provided with two flip-flops for latching respectively the two data bits outputted from the two selectors of the third selector means in synchronism with a clock of 400 MHz; fourth selector means comprising a single selector for selectively receiving two different data bits latched at the fourth latch means in synchronism with the clock of 400 MHz; and a flip-flop for latching the data bit outputted from the single selector in synchronism with a clock of 800 MHz 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like elements or portions are denoted by like reference numerals and in which: 
     FIG. 1 is a block diagram schematically showing a conventional data transfer circuit provided in the physical layer defined by the IEEE 1394 standard, having been referred to in the opening paragraphs in the instant disclosure; 
     FIG. 2 is a block diagram showing one block of FIG. 1 in detail; 
     FIG. 3 is a block diagram showing a PISO circuit of FIG. 1; 
     FIG. 4 is a timing chart for describing the operation of the PISO circuit of FIG. 3; 
     FIG. 5 is a timing chart for describing the operation of another PISO of FIG. 1; 
     FIG.  6 (A) is a timing chart showing outputs of two PISO circuits of FIG. 1; 
     FIG.  6 (B) is a timing chart for describing clock reproduction at a data receiving side; 
     FIG.  6 (C) is a timing chart for describing the problem that tends to occur with the prior art; 
     FIG. 7 is a diagram schematically showing a data transfer circuit including PISO circuits according to the preferred embodiment of the present invention; 
     FIG. 8 is a diagram showing in detail one of the PISO circuits of FIG. 7; 
     FIG. 9 is a timing chart for describing the operation of the PISO circuit of FIG. 8; 
     FIG. 10 is a timing chart for describing the operation of another PISO of FIG. 8; 
     FIG. 11 is a diagram showing a modification of the embodiment of the present invention shown in FIG. 8; and 
     FIG. 12 is diagram showing another modification of the embodiment of the present invention shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention and the modifications thereof will be described with reference to FIGS. 7 to  12 . 
     Referring first to FIG. 7, a data transfer circuit (denoted by  40 ) is schematically shown in block diagram form, which circuit  40  comprises PISO circuits  42  and  44  embodying the present invention. The blocks other than this are substantially the same as the counterparts shown in FIG. 1, and thus, the blocks already referred to in the opening paragraphs are represented by like reference numerals, and the description thereof will be omitted except for becoming necessary in context in order to simplify the instant disclosure. 
     One advantage derived from the first embodiment is to render it unnecessary to provide the timing controller  18  of FIG. 1, via which the number of flip-flops operating at the high frequency such as 400 MHz in the data transfer circuit  40  can effectively reduced. The other advantage of the preferred embodiment will be referred to latter. 
     FIG. 8 is a diagram showing the detail of the PISO circuit  42  of FIG. 7, which is supplied with the 8-bit parallel data DATA_Parallel (D 0 -D 7 ) and converts the same into the corresponding 8-bit serial data. As shown in FIG. 8, the PISO circuit  42  comprises a first latch group (or means) consisting of eight flip-flops  46   a - 48   h , a second latch group consisting of four flip-flops  48   a - 48   d , a third latch group consisting of two flip-flops  50   a - 50   b , and a latch (fourth latch means) or flip-flop  52 . Further, the PISO circuit  42  comprises a first selector group (or means) consisting of four selectors  54   a - 54   d , a second selector group consisting of two selectors  56   a - 56   b , and a selector  58 . The PISO circuit  42  is supplied with the clocks CLK 50 , CLK 100  CLK 200  and CLK 400 , whose clock rates are respectively 50 MHz, 100 MHz, 200 MHz and 400 MHz as mentioned in the opening paragraphs. Further, the PISO circuit  42  receives or latches the eight (8) parallel data bits D 0 -D 7  at the first latch group (flip-flops  46   a - 4   h ) applied thereto via input terminals T 0 -T 7 . 
     The numbers 0 to 7 attached to “Ds” of parallel data bits D 0 -D 7  denote the serial data bits outputted in this order from the PISO circuit  42 . That is, the parallel data D 0 -D 7  are sequentially outputted in this order from the PISO circuit  42  as DATA_Serial (Tx). It is to be noted that, in order to serially output the parallel data D 0 -D 7  in this order, the order of the parallel data bits D 0 -D 7  applied to the first latch group are arranged such that D 0 , D 4 , D 2 , D 6 , D 1 , D 5 , D 3 , and D 7  of the parallel data bits are respectively applied to the flip-flops  46   a - 46   h  in the case shown in FIG.  8 . 
     Each of the selectors  54   a - 54   d ,  56   a - 56   b , and  58  is provided with two inputs denoted by “1” and “0”. When the clock applied to the selector assumes a high level, the selector, in response to the high level, selects data bit applied to the input “1” and relays the selected data bit to the following latch (viz., flip-flop). Contrarily, when the clock applied to the selector assumes a low level, the selector, in response to the low level, selects data bit applied to the input “0” and relays the selected data bit to the following latch. 
     The operation of the PISO circuit  42  will be described with reference to FIG.  9 . The flip-flops  46   a - 46   h  latch respectively the data bits D 0 , D 4 , D 2 , D 6 , D 1 , D 5 , D 3  and D 7  at a time in synchronism of the clock CLK  50  (50 MHz) and hold them during one cycle thereof. In other words, the data bits D 0 -D 7  are acquired into the flip-flops  46   a - 46   h  at a rising (leading) edge R 1  of the clock CLK 50 . On the other hand, the selectors  54   a - 54   d  select respectively the data bits applied to the input “1”s thereof while the clock CLK  50  takes a high level and then outputs the selected data bits. Thereafter, the selectors  54   a - 54   d  select respectively the data bits applied to the input “0”s thereof while the clock CLK  50  takes a low level and then outputs the selected data bits. 
     The flip-flops  48   a - 48   d , which are respectively preceded by the selectors  54   a - 54   d , latch respectively the data bits D 0 -D 3  at a rising edge R 2  of the clock CLK 100  at a time, and hold them during one cycle. Subsequently, the flip-flops  48   a - 48   d  latch respectively the data bits D 4 -D 7  at a rising edge R 3  of the clock CLK 100 , and hold them during one cycle. 
     In a similar manner, the selectors  56   a - 56   d  select respectively the data bits applied to the input “1”s thereof while the clock CLK  100  takes a high level and then outputs the selected data bits. Subsequently, the selectors  56   a - 56   b  select respectively the data bits applied to the input sons thereof while the clock CLK  100  takes a low level and then outputs the selected data bits. 
     The flip-flops  50   a - 50   b , which are respectively preceded by the selectors  56   a - 56   b , latch respectively the data bits D 0 -D 1  at a time at a rising edge R 4  of the clock CLK 200 , and hold them during one cycle. Subsequently, the flip-flops  50   a - 50   b  latch respectively the data bits D 2 -D 3  at a rising edge R 5  of the clock CLK 200 , and hold them during one cycle. Likewise, the other data bits D 4 -D 5  and D 6 -D 7  are respectively acquired into the flip-flops  55   a - 50   b  at rising edges R 6  and R 7 . 
     The selector  58  selects the data bit applied to the input “1” thereof while the clock CLK  200  takes a high level, and then outputs the selected data bit. Subsequently, the selector  58  selects the data bit applied to the input “0” thereof while the clock CLK  200  takes a low level, and then outputs the selected data bit. 
     Lastly, the flip-flop  52  latches subsequently the outputs of the preceding selector  58  at the rising edges of the clock CLK 400 , and thus, generates the serial DATA_Serial (Tx) in the order of D 0 -D 7 . The above-mentioned operations are performed on the following 8-bit parallel data D 8 -D 15 . 
     FIG. 10 is a timing chart showing the operation of the other PISO circuit  44  which is configured in exactly the same manner as the PISO  42  and which generates a serial strobe signal STRB_Serial (Tx). Reference symbols  46   h ′,  46   g ′, . . .  52 ′ appearing at the leftmost portions indicate flip-flops (although not shown) corresponding to the flip-flops  46   h ,  46   g , . . . ,  52  shown in FIG.  8 . As mentioned above, the parallel strobe data STRB_Parallel applied to the PISO circuit  44  differs from the corresponding parallel data DATA_Parallel only with respect to the fact that the odd number data bits of DATA_Parallel (viz., data bits respectively number  1 ,  3 ,  5 , . . . ) are reversed in terms of logic level. In FIG. 10, the character “X” attached to each of the data bits such as D 1 , D 3 , etc. denotes that the logic level of each of the data bits D 1 , D 3  . . . etc. of DATA_Parallel is reversed. Accordingly, the timing chart of FIG. 10 is readily understandable when referring to that of FIG. 9, and hence, further description of the timing chart of FIG. 10 will be omitted for the sake of simplifying the instant disclosure. 
     It is understood from the foregoing that the number of flip-flops operating at the high frequency of 400 MHz can drastically be reduced with each of the PISO circuits  42  and  44 . That is, each of the PISO circuit  42  and  44  is provided with only one flip-flop  52  operating at 400 MHz, two flip-flops  50   a - 50   b  operating at 200 MHz, four flip-flops  48   a - 48   d  operating at 100 MHz, and eight flip-flops  46   a - 46   h  operating at 50 MHz. If the above-mentioned flip-flops are converted into the flip-flops operating at the clock of 400 MHz, the number of such flip-flops becomes four. Although the number of the flip-flops provided in the clock frequency divider  16  and the data/strobe encoder  20  remains unchanged between the arrangements of FIGS. 1 and 7, the experiment conducted by the inventor reveals that the power consumption of the data transfer circuit  40  of FIG. 7 can be reduced by approximately 26% relative to that of the data transfer circuit  10  shown in FIG.  1 . 
     Further, the PISO circuit  42  is provided with the flip-flop  52  at the final stage. Therefore, even if skew occurs between DATA_Serial and STRB_Serial at a data receiving side, the skew can be removed without difficulty by adjusting the timing of the clock CLK 400 . 
     FIG. 11 is a diagram showing a variation of the above-mentioned embodiment of the present invention. As shown, a PISO (parallel in serial out) circuit  60  is to convert a parallel data of four bits (D 0 -D 3 ) into a 4-bit serial data, and comprises flip-flops  62   a - 62   d , selectors  64   a - 64   b , flip-flops  66   a    66   b , a selector  68 , and a flip-flop  70 . Further, the PISO circuit  60  is supplied with three kinds of clocks CLK 200 , CLK 100 , and CLK 50  in that the inputted parallel data is four. The flip-flops  62   a - 62   d  are respectively supplied with inputted parallel data bits D 0 , D 2 , D 1 , and D 3 , by way of input terminals T 0 -T 3 , in order to serially output the data bits in order of DO. D 1 , D 2 , and D 3 . The operation of the PISO  60  can readily be understood from the forgoing, and thus, the further description thereof will be omitted for brevity. 
     FIG. 12 is a diagram showing another variation of the above-mentioned embodiment of the present invention. As shown, a PISO (parallel in serial out) circuit  80  of FIG. 12 is to convert a parallel data of 16 bits (D 0 -D 15 ) into a 16-bit serial data, and comprises flip-flops  82   a - 82   o , selectors  84   a - 84   h , flip-flops  86   a - 86   h , selector  88   a - 88   d , flip-flops  90   a - 90   d , selectors  92   a - 92   b , flip-flops  94   a - 94   b , a selector  96 , and a flip-flop  98 . In FIG. 12, some of the reference symbols are omitted merely for the sake of simplifying the drawing. Further, the PISO circuit  80  is supplied with five kinds of clocks CLK 800  (800 MHz), CLK  400 , CLK 200 , CLK 100 , and CLK 50  in that the number of inputted parallel data is  16 . The flip-flops  82   a - 82   o  are respectively supplied with inputted parallel data bits D 0 , D 8 , D 4  . . . , D 11 , D 7 , D 15 , by way of input terminals T 0 -T 15 , in order to serially output the data bits in order of D 0 -D 15 . The operation of the PISO  80  can readily be understood from the forgoing, and thus, the further description thereof will be omitted for brevity. 
     In the above, in order to convert the 4-bit parallel data into the corresponding serial data, the PISO circuit  60  of FIG. 11 Is presented, wherein the flip-flops  46   a - 46  and the selectors  54   a - 54   d  of the PISO circuit  42  of FIG. 8 are omitted and wherein the application of the clock CLK 400  is omitted. However, a PISO circuit for converting 4-bit parallel data into the corresponding serial data can be realized without changing the configuration of FIG.  8 . In such a case, the first bit data D 0  of the 4-bit parallel data is applied to both the input terminals T 0  and T 4 , the second bit data D 1  is applied to both the input terminals T 2  and T 6 , the third bit data D 2  is applied to both the input terminals T 1  and T 5 , and the last (viz., fourth) bit data D 3  is applied to both the input terminals T 3  and T 7 . Therefore, the serial out DATA_Serial (Tx) becomes D 1 , D 1 , D 2 , D 2 , D 3 , D 3 , D 4 , D 4 , . . . at the data speed of 400 Mbps. Although the two identical data bits are sequentially-transmitted at the rate of 400 Mbps, the PISO circuit  42  can be used to implement parallel-to-serial conversion of 4-bit data bits without any hardware change thereof. This can be applied to the PISO circuit  80  of FIG. 12, which has been described for implementing parallel-to-serial conversion of 16-bit parallel data. Namely, the PISO circuit  80  can be used to perform the parallel-to-serial conversion of 8 or 4 bit data without any hardware change thereof. 
     The foregoing descriptions show one preferred embodiment and some modifications thereof. However, other various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiments and modification shown and described are only illustrated, not restrictive.