Abstract:
In response to a direct memory access (DMA) request, a direct memory access controller (DMAC) performs reading at a host side at which a high-speed bus master is arranged. A bus bridge sends a dummy data to the DMAC, and performs reading at an input/output (I/O) side at which a low-speed slave device is arranged. In response to a following DMA request, the DMAC performs reading at the host side. The bus bridge sends a data read for a previous DMA request at the I/O side to the DMAC, and performs reading at the I/O side. Data that is read in response to a final DMA request at the I/O side is stored in a buffer inside the bus bridge. A central processing unit (CPU) reads a last read data from the buffer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-298166, filed on Oct. 12, 2004, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to a bus bridge and a data transfer method. 
     2) Description of the Related Art 
     Recently, an operation frequency of a central processing unit (CPU) on a bus line has been improved, and a data transfer speed between high-speed input/output (I/O) devices has also been improved. However, to ensure compatibility with older computer systems or for other purposes, it is necessary to connect a conventional low-speed I/O device to a computer system that includes a high-speed bus line. 
     Conventionally, a master device that is connected to a high-speed bus is connected to a bus bridge to access to a low-speed device. Through the bus bridge, the master device accesses to a low-speed device that is connected to a low-speed bus. Such a technology is disclosed in, for example, Japanese Patent Laid-Open Publication No. H10-4420. Moreover, in a conventional technology, a structure that includes a bus bridge to connect a Peripheral Component Interconnect (PCI) bus, which is a high-speed bus, and a low-speed bus is applied. The PCI bus is connected to a high-speed device and the low-speed bus is connected to a low-speed device, and the bus bridge is arranged therebetween. Such technology is disclosed in, for example, Japanese Patent Laid-Open Publication No. H11-110342. Furthermore, a technology that includes a structure in which a direct memory access (DMA) controller is arranged is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2001-109706. The structure includes a bus bridge, the DMA controller, a CPU (microprocessor), a high-speed I/O device, a low-speed I/O device, a high-speed bus, and a low-speed bus. 
       FIG. 7  is a timing chart of a conventional method to perform a read access to an I/O device on a low-speed bus line (hereinafter, a low-speed slave device) in a conventional system in which a computer system includes a high-speed bus line, and is connected a low-speed I/O device. In the conventional system, a bus master is arranged on a high-speed bus line (hereinafter, a high-speed bus master), and the bus master accesses to the low-speed slave device. In a following description, a side of the high-speed bus master is referred to as a host side, while a side of the low-speed slave device is referred to as an I/O side. 
     As shown in  FIG. 7 , the low-speed slave device generates a DMA request  1  toward a DMA controller (DMAC). Upon receiving the DMA request, the DMAC performs the read bus cycle  1  at the host side, which is shown with a reference numeral  2  in  FIG. 7 . A bus bridge performs the read bus cycle at the I/O side, which is shown with a reference numeral  3  in  FIG. 7 . When reading at the I/O side is completed, and the bus bridge sends a read data to the DMAC, the low-speed slave device generates a new DMA request  4  toward the DMAC. 
     The DMAC performs the read bus cycle  4  at the host side, which is shown with reference numeral  5  in  FIG. 7 . Also, the bus bridge performs the read bus cycle  4  at the I/O side, which is shown with a reference numeral  6  in  FIG. 7 . Then, the bus bridge sends a read data to the DMAC. Such a process is repeated to transfer a data. When the low-speed slave device generates a final DMA request  7 , the DMAC and the bus bridge performs the final read bus cycle at the host side and the I/O side respectively, which are shown with reference numerals  8  and  9 . When a final read data is sent from the bus bridge to the DMAC, the transfer of the data is finished. 
     However, if a low-speed I/O device is connected to a high-speed bus line, a large number of wait cycles are inserted when the low-speed I/O device is accessed. This may deteriorate the performance of the system. Moreover, if the high-speed I/O device and the low-speed I/O device are arranged on a same bus line, even a high-speed operation of the high-speed I/O device is inhibited because of the wait cycles that are inserted when the low-speed I/O device is accessed on the same bus line. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve at least the above problems in the conventional technology. 
     A bus bridge according to one aspect of the present invention is connected between a high-speed bus line and a low-speed bus line. A high-speed device is connected to the high-speed bus line and a low-speed device is connected to the low-speed bus line. The bus bridge includes a buffer unit that temporarily stores data that is read from the low-speed device via the low-speed bus line; a register that stores a value of a valid flag that indicates that valid data is stored in the buffer unit; and a control logic unit that performs a control to output the data from the buffer unit to the high-speed device. The control includes releasing the high-speed bus line before completion of reading of the data from the low-speed device, writing the data read in the buffer unit, setting a value in the register to indicate that valid data is stored in the buffer unit, confirming, at a following access in a series of accesses to the high-speed device, that a value stored in the register indicates that valid data is stored in the buffer unit, and outputting the data from the buffer unit to the high-speed device. 
     A data transfer method for successively transferring data from a low-speed device to a high-speed device via a bus bridge according to another aspect of the present invention is connected between a high-speed bus line and a low-speed bus line. The low-speed device is connected to the low-speed bus line, and the high-speed device is connected to the high-speed bus line. The data transfer method includes releasing the high-speed bus line before completion of reading of data from the low-speed device; writing the data read in the bus bridge; temporarily storing the data in the bus bridge; and outputting the data from the bus bridge to the high-speed device at a following access in a series of accesses to the high-speed device. 
     The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a timing chart for explaining a read access method according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of a system to which a bus bridge according to the embodiment is applied; 
         FIG. 3  is a block diagram of a bus bridge according to the embodiment; 
         FIG. 4  is a timing chart for explaining a read access method according to the embodiment; 
         FIG. 5  is another timing chart for explaining the read access method; 
         FIG. 6  is a block diagram of a bus bridge according to an embodiment of the present invention; and 
         FIG. 7  is a timing chart for explaining a conventional read access method. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of a bus bridge and a data transfer method according to the present invention will be explained below in detail with reference to the accompanying drawings. 
       FIG. 2  is a block diagram of a system to which a bus bridge according to an embodiment of the present invention is applied. As shown in  FIG. 2 , a bus bridge  31  includes a high-speed host-side bus  32  having connected thereto a DMAC  34  and a CPU  35  that together serve as a high-speed bus master. The bus bridge  31  also includes a low-speed I/O-side bus  33  having connected thereto a low-speed slave device  36 . The low-speed slave device  36  produces and sends DMA requests (DMA transfer requests) to the DMAC  34 . Also, the low-speed slave device  36  produces and sends interrupt requests  38  to the CPU  35 . 
       FIG. 3  is a block diagram of the bus bridge  31 . In  FIG. 3 , the DMAC  34  and the CPU  35  are connected to the host side (at left in the drawing), while the low-speed slave device  36  is connected to the I/O side (at right in the drawing). As shown in  FIG. 3 , the bus bridge  31  includes a control logic unit  41 , an address buffer unit  42 , a write data buffer unit  43 , and a read data buffer unit  44 . 
     The control logic unit  41  is a logic circuit that controls the operation of the bus bridge  31 , and incorporates a valid flag  45 . The valid flag  45  is a register for storing a value indicating that valid data is retained in the read data buffer unit  44 . The value of the valid flag  45  is read by the host side in association with an RS_IN signal, which will be described further below, and is then output to the host-side data bus (Data_A) via a buffer  46 , and is then read by the CPU. At this time, the buffer  46  is controlled based on a control signal output as an internal signal from the control logic unit  41 . 
     The address buffer unit  42  is a buffer register for retaining an address, to be accessed by the host side to the I/O side, while a bus cycle continues at the I/O side. The write data buffer unit  43  is a buffer register for retaining data to be written from the host side to the low-speed slave device  36  while the bus cycle continues at the I/O side. The read data buffer unit  44  is a buffer register that temporarily retains data read from the I/O side. 
     The control logic unit  41  receives inputs of an RD_IN signal and a WR_IN signal from the host side. The RD_IN signal is a signal indicating that data reading is being performed in a bus cycle at the host side. The WR_IN signal is a signal indicating that data writing is being performed in a bus cycle at the host side. The control logic unit  41  also receives inputs of a CS_IN signal and the RS_IN signal generated at an address decoder  51 . The CS_IN signal is a signal indicating that the I/O side is being accessed in the bus cycle at the host side. 
     The RS_IN signal is a selection signal for use in accessing the read data buffer unit  44  from the host side. In the present embodiment, which is not particularly restrictive, the address decoder  51  is provided at the host side to receive an input of an address of the host side from a host-side address bus (Address_In). The control logic unit  41  outputs an RDY_OUT signal to the host side. The RDY_OUT signal is a signal that notifies the high-speed bus master at the host side of the completion of the bus cycle at the host side. 
     Also, the control logic unit  41  outputs an RD_OUT signal, a WR_OUT signal, and a CS_OUT signal to the I/O side. The RD_OUT signal is a signal indicating that data reading is being performed in the bus cycle being executed by the bus bridge  31  at the I/O side. The WR_OUT signal is a signal indicating that data writing is being performed in the bus cycle being executed by the bus bridge  31  at the I/O side. 
     The CS_OUT signal is a signal indicating that the bus bridge  31  is executing a bus cycle at the I/O side. The control logic unit  41  also receives an input of an RDY_IN signal from the I/O side. The RDY_IN signal is a signal that notifies the bus bridge  31  of the completion of the bus cycle at the I/O side, and is output from the low-speed slave device  36 . 
     Furthermore, the control logic unit  41  outputs an RDLE signal, a WDOE signal, a WDLE signal, an RDOE signal, and an ALE signal as internal signals of the bus bridge  31 . The RDLE signal is a signal for instructing the read data buffer unit  44  to capture data from an I/O-side data bus (Data_B). The WDOE signal is a signal for instructing the outputting of data from the write data buffer unit  43  to the I/O-side data bus (Data_B) via a buffer  47 . The WDLE signal is a signal for instructing the write data buffer unit  43  to capture data from the host-side data bus (Data_A). 
     The RDOE signal is a signal for instructing the outputting of data from the read data buffer unit  44  to the host-side data bus (Data_A) via a buffer  48 . The ALE signal is a signal for instructing the capturing of an address at the host side from the host-side address bus (Address_In) to the address buffer unit  42 . The contents of the address buffer unit  42  are output to an I/O-side address bus (Address_Out). 
     The bus bridge  31  that has the structure described above performs a following operation on a combination of input signals from the host side. When the CS_IN signal and the RS_IN signal are both at a relatively low potential level (such a level is hereinafter denoted as “L”), or when the RD_IN signal and the WR_IN signal are both at “L”, the operation of the bus bridge  31  enters a “no operation” state. When the CS_IN signal and the RS_IN signal are both at a relatively high potential level (such a level is hereinafter denoted as “H”), or when the RD_IN signal and the WR_IN signal are both at “H”, the operation of the bus bridge  31  prohibits any input. 
     When the CS_IN signal, the RS_IN signal, the RD_IN signal, and the WR_IN signal are at “L”, “H”, “H”, and “L”, respectively, a read access is performed on the register in the bus bridge  31  according to the value of the least significant bit of the address on the host-side address bus (Address_In). If the value of the least significant bit of the address is “0”, the value of the valid flag  45  is to be read. If the value of the least significant bit is “1”, data in the read data buffer unit  44  is to be read. When the CS_IN signal, the RS_IN signal, the RD_IN signal, and the WR_IN signal are at “L”, “H”, “L”, and “H”, respectively, a write access is performed on a register in the bus bridge  31 . In the example shown in the drawing, no writable register is present in the bus bridge  31 , and therefore the operation of the bus bridge  31  enters a “no operation” state. 
     When the CS_IN signal, the RS_IN signal, the RD_IN signal, and the WR_IN signal are at “H”, “L”, “H”, and “L”, respectively, the bus bridge  31  starts a read access to the I/O side. The bus bridge  31  then returns the data of the read data buffer unit  44 , without waiting for the completion of the access to the I/O side, to complete the operation of the host side. When the CS_IN signal, the RS_IN signal, the RD_IN signal, and the WR_IN signal are at “H”, “L”, “L”, and “H”, respectively, the bus bridge  31  starts a write access to the I/O side. Next, the bus bridge  31  captures data into the write data buffer unit  43 , and then completes the operation of the host side without waiting for the completion of the operation at the I/O side. 
       FIG. 4  is a timing chart for explaining an operation when a read access is performed at the I/O side in response to the read access from the host side. As shown in  FIG. 4 , an initial DMA request is produced. With this, either one of the CPU and the DMAC (hereinafter collectively referred to as a host-side bus master) for controlling transfer at the host-side bus asserts the CS_IN signal (in the example shown in the drawing, drives the signal to “H”) and simultaneously outputs an address to the host-side address bus (Address_In) for starting an access. Slightly after the start of the access, the host-side bus master asserts the RD_IN signal to indicate that the access performed by the host-side bus is to read data. 
     Upon assertion of the RD_IN signal, the bus bridge  31  outputs the contents of the read data buffer unit  44  to the host-side data bus (Data_A) and simultaneously asserts the RDY_OUT signal to notify the host-side bus master of the completion of the host-side bus cycle. At this time, the valid flag  45  is negated, and no valid data is present in the read data buffer unit  44 . Therefore, invalid data, that is, dummy data, is output to the host-side data bus (Data_A). Then, the host-side bus master negates the RD_IN signal (in the example shown in the drawing, drives the signal to “L”), the bus bridge  31  negates the RDY_OUT signal, and the host-side bus master negates the CS_IN signal. 
     On the other hand, upon detection of the start of the read access at the host-side bus based on the assertion of the RD_IN signal, the bus bridge  31  relays the access to the I/O side, and asserts the CS_OUT signal. Simultaneously, the bus bridge  31  outputs the contents of the address buffer unit  42  to the I/O-side address bus (Address-Out). Also, upon assertion of the CS_OUT signal, the bus bridge  31  asserts the RD_OUT signal. 
     In this state, upon detection of the assertion of the RDY_IN signal, the bus bridge  31  captures data on the I/O data bus (Data B) into the read data buffer unit  44 . The bus bridge  31  then negates the RD_OUT signal, the CS_OUT signal, the CS_OUT signal, and the RDY_IN signal to complete the read bus cycle at the I/O side. At this time, valid data is present in the read data buffer unit  44 , and therefore the valid flag  45  is asserted. A period up until the end of this process is referred to as a period A. 
     When a second DMA request is produced, the host-side bus master asserts again the CS_IN signal and simultaneously outputs an address to the host-side address bus (Address_In) for starting an access. Thereafter, the procedure until the contents of the read data buffer unit  44  are output to the host-side data bus (Data_A) to complete the host-side bus cycle is the same as the procedure for the period A described above. However, in the present access, the valid flag  45  is asserted to indicate that valid data is present in the read data buffer unit  44 . Therefore, the bus bridge  31  outputs the data of the read data buffer unit  44  to the host-side data bus (Data_A), and then negates the valid flag  45  after completion of the access. 
     Thereafter, as described above, in the read bus cycle at the I/O side, the data on the I/O-side data bus (Data_B) is captured into the read data buffer unit  44 . Therefore, the valid flag  45  is again asserted. A period starting from after the period A until this assertion is referred to as a period B. Thereafter, when the CS_IN signal is again asserted due to the occurrence of a DMA request, the operation during the period B is repeated. In the state where the I/O side is being continuously accessed, data is always sent to the host side with a delay of one cycle. Therefore, after the host-side bus master has completed DMA transfer for a programmed number of times, the data read from the I/O side remains in the read data buffer unit  44 . This remaining data is drawn to the host side by an operation shown in  FIG. 5 . 
       FIG. 5  is a timing chart for explaining the operation to draw the data remaining in the read data buffer unit  44  to the host side after completion of the DMA transfer, the timing chart of  FIG. 5  following the timing chart shown in  FIG. 4 . As shown in  FIG. 5 , the host-side bus master asserts the CS_IN signal and outputs an address for starting an access. Thereafter, until valid data is stored in the read data buffer unit  44  in the last read bus cycle at the I/O side to assert the valid flag  45  (during a period C in the drawing), the operation is the same as that shown in  FIG. 4  during the period B. 
     Upon completion of the last read bus cycle at the I/O side, an interrupt request is produced. With this, to check that valid data is present in the read data buffer unit  44 , the host-side bus master asserts the RS_IN signal and simultaneously outputs an address to the host-side address bus (Address_In) for starting a read access to the valid flag  45 . Slightly after the start of the access, the host-side bus master asserts the RD_IN signal to indicate that the access performed at the host-side bus is to read data. 
     Upon assertion of the RD_IN signal, the bus bridge  31  outputs the value of the valid flag  45  to the host-side data bus (Data_A) and simultaneously asserts the RDY_OUT signal to notify the host-side bus master of the completion of the host-side bus cycle. After the host-side bus master negates the RD_IN signal, the bus bridge  31  negates the RDY_OUT signal. Also, after negating the RD_IN signal, the host-side bus master negates the RS_IN signal. Unlike the case of an access to the I/O side, when the register in the bus bridge  31  is accessed, no access occurs at the I/O side. A period after the period C until the negation of the RS_IN signal is referred to as a period D. 
     When the valid flag  45  is negated, valid data has not yet been present in the read data buffer unit  44 . Therefore, the value of the valid flag  45  is again checked. Then, the operation of the period D is repeated until the value of the valid flag  45  is asserted. When the valid flag  45  is asserted and valid data is present in the read data buffer unit  44 , the host-side bus master asserts the RS_IN signal and simultaneously outputs an address to the host-side address bus (Address_In) for starting a read access to the read data buffer unit  44 . 
     Slightly after the start of the access to the read data buffer unit  44 , the host-side bus master asserts the RD_IN signal to indicate the access performed at the host-side bus is to read data. Upon assertion of the RD_IN signal, the bus bridge  31  outputs the data of the read data buffer unit  44  to the host-side data bus (Data_A) and simultaneously asserts the RDY_OUT signal to notify the host-side bus master of the completion of the host-side bus cycle. 
     After the host-side bus master negates the RD_IN signal, the bus bridge  31  negates the RDY_OUT signal. Also, after negating the RD_IN signal, the host-side bus master negates the RS_IN signal. Since an access during a period E immediately after the period D is also to access the register in the bus bridge  31 , no access to the I/O side occurs. Finally, since the valid data present in the read data buffer unit  44  has been output to the host-side data bus (Data_A), the bus bridge  31  negates the valid flag  45  after completion of the access. 
     Here, instead of reading the register of the valid flag  45  in the bus bridge  31  from the host side to check the value of the valid flag  45  based on the value of the register, the value of the valid flag  45  may be lead as a signal line to the outside of the bus bridge  31 , thereby allowing the value of the valid flag  45  to be checked. In this case, a CPU including a general-purpose I/O port is used for control, and the signal line for the bus bridge  31  to output the value of the valid flag  45  is connected to the general-purpose I/O port. With this, by reading the general-purpose I/O port having connected thereto the signal line, the value of the valid flag  45  can be checked. Also, as shown in  FIG. 6 , the bus bridge  31  may include the address decoder  51 . 
     As described above, according to the present embodiment, a waiting state of the high-speed host-side bus  32  can be reduced, thereby causing the host-side bus  32  to be released quickly. Therefore, an effect can be achieved such that a read access to the low-speed I/O-side bus  33  can be performed without occupying the high-speed host-side bus  32  for a long time. Also, the bus bridge  31  can be used with bus standards without bus protocols, such as “disconnect” or “retry”, and therefore can be applied as an interface between the low-speed slave device  36  having a low-speed, old-style bus and the high-speed CPU  35 . 
     Particularly, as for DMA transfer, information indicating that data to be transferred is ready is reported from the I/O side as a DMA request. Therefore, polling to the valid flag only at one last time is enough. This can efficiently reduce a bus occupying time. The present invention described above is not restricted to the embodiment described above, but may be variously modified. 
     According to the present invention, it is possible to carry out a read access to a bus on a side of a low-speed I/O without occupying a bus on a side of a high-speed host for a long time. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.