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Xilinx OPB Usage
Ideal FPGA Implementation of OPB-based System
The ideal FPGA implementation of an OPB-based system has the following features:
•
Requires no conversion cycles
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Uses only the byte-enable architecture as specified in the OPB specification
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Does not require masters to mirror write data
These characteristics help determine how Xilinx-developed OPB devices are implemented.
The detailed specifications that describe how the OPB is used in Xilinx intellectual
property are provided in the next section.
Specifications for OPB Usage in Xilinx-developed OPB Devices
Xilinx-developed OPB devices adhere to the following OPB usage rules:
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The width of the OPB data buses and address buses is 32 bits. Note that some
peripherals may parameterize these widths, but currently only 32-bit buses are
supported. Peripherals that are smaller than 32-bits can be attached to the OPB with a
corresponding restriction in addressing. For example, an 8-bit peripheral at base
address A can be attached to byte lane 0, but can only be addressed at A, A+4, A+8,
and so on.
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All OPB devices (masters and slaves) are byte-enable devices. These devices do not
support the legacy data transfer signals and therefore do not support dynamic bus
sizing. OPB masters do not mirror data to unused byte lanes. See Figure 1-1 for the
byte lane usage for aligned transfers.
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All OPB devices (masters and slaves) are required to output logic zero when they are
inactive. This eliminates the need for the Mn_DBusEn and Sln_DBusEn signals
external to the master or slave. The enable function is still implemented within the
device.
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To obtain better timing in the FPGA implementation of the OPB, the OPB_timeout
signal is registered. This means that all slaves must assert Sl_xferAck or Sl_retry on or
before the rising edge of the 16th clock cycle after the assertion of OPB_select. If an
OPB slave wishes to assert Sl_toutSup, Sl_toutSup must be asserted on or before the
rising edge of the 15th clock after the assertion of OPB_select.
Processor IP Reference Guide
August 2004
www.xilinx.com
1-800-255-7778
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