Bus bridge and data transfer method
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.
1. A bus bridge that is connected between a high-speed bus line and a low-speed bus line, wherein 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, comprising:
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 successively transfer the data from the low-speed device to the high-speed device a predetermined number times, wherein
the control includes releasing the high-speed bus line before completion of reading of the data from the low-speed device, storing the data read from the low-speed device 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, outputting the data from the buffer unit to the high-speed device and outputting dummy data to the high-speed device at a first access in the series of accesses to the high-speed device.
2. The bus bridge according to claim 1 , wherein:
the control logic unit performs, after completion of the predetermined number of accesses to the high-speed device, a control to output last data from the buffer unit to the high-speed device, wherein the last data is data that is read from the low-speed device in a last access to the low-speed device.
3. The bus bridge according to claim 2 , wherein:
the control logic unit performs a control to output, based on an event that occurs when the final access to the low-speed device is completed, final data from the buffer unit to the high-speed device, wherein the final data is data that remains in the buffer unit when the final access to the low-speed device is completed.
4. The bus bridge according to claim 3 , wherein:
the control logic unit performs the control to output the final data after confirming that the value stored in the register indicates that valid data is stored in the buffer unit, and releasing the high-speed bus line.
5. A data transfer method for successively transferring data from a low-speed device to a high-speed device via a bus bridge that is connected between a high-speed bus line and a low-speed bus line predetermined number times, the low-speed device being connected to the low-speed bus line, the high-speed device being connected to the high-speed bus line, comprising:
releasing the high-speed bus line before completion of reading of data from the low-speed device;
temporarily storing the data read from the low-speed device in the bus bridge;
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; and
outputting dummy data to the high-speed device at a first access in the series of accesses to the high-speed device.
6. The data transfer method according to claim 5 , further comprising, after completion of the predetermined number of accesses to the high-speed device, outputting last data from the buffer unit to the high-speed device, wherein the last data is data that is read from the low-speed device in a last access to the low-speed device.
7. The data transfer method according to claim 6 , further comprising outputting, based on an event that occurs when the final access is completed, final data from the buffer unit to the high-speed device, wherein the final data is data that remains in the buffer unit when the final access to the low-speed device is completed.
8. The data transfer method according to claim 7 , wherein the outputting the final data includes confirming;
that valid data is stored in the bus bridge; releasing the high-speed bus line; and
outputting the final data.