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wb_switch - A Wishbone Bus Interconnect for FPGAs

This is the project homepage for wb_switch, a generic, multiplexer based interconnect component for the Wishbone bus, written in VHDL. It includes support for bus arbitration for multiple masters as well as address translation for multiple slaves.

Last updated: $Date: 2013/01/19 21:33:14 $.

Overview

"Wishbone" is a specification for a parallel bus system, primarily to be used as an on-chip bus to connect several independent logic blocks within one chip design. The Wishbone Bus uses a Master/Slave Concept where only "Master" Components may drive the bus whereas "Slave" components may only use the bus when instructed by a Master component. Also, there may be multiple Master and/or Slave devices present on the bus.

One problem with multiple bus participants is that only one participant may drive the bus at any time. For slave participants, this is relatively easy since slaves may not use the bus when not addressed anyways (but care must be taken to not place two devices on the same bus address). However, multiple bus masters require arbitration so that only the one master to which the bus is granted drives the bus.

There is another problem when designing a shared bus as outlined above inside a single chip, particularily an FPGA: Bus participants which may not drive the bus must neither drive a low, nor a high signal on the bus. Typically, this is done by setting the signal drivers to a "High-Z" state. However, there typically aren't any Tri-State buffers inside an FPGA, but only the drivers for the I/O Pins are Tri-State capable. Hence, when using this technique on an FPGA, the design tools are required to route the bus signals through the I/O Buffers, which is usually a pessimization with regards to the performance of the design.

wb_switch solves those problems by introducing a switch logic based on multiplexers, a resource that is easily available in FPGAs. That way, each bus participant can be designed as if directly connected to a peer component.

Because the performance of FPGA designs suffers from long unbuffered signal paths, the wb_switch logic organizes its internal components in a pipelined logic. This does not increas the FPGA logic usage, because the required registers are present in the FPGA anyways (it would be a waste not to use them). However, this increases the latency between Wishbone masters and slaves. This doesn't impact the Wishbone throughput because pipelined transfers can be used.

Architecture

wb_switch consists of four basic blocks: An arbiter, an address translator and two multiplexer components for masters and slaves. Except for the arbiter, the individual blocks implement filters, i.e. each component implements a Wishbone slave interface as well as a Wishbone master interface. Wishbone transfers seen on the slave interface are processed and then re-issued on the master interface. Similar to that, slave responses are forwarded from the master interface implementation to the slave interface implementation. The filters are daisy-chained together, forming a pipeline. Figure 1 below illustrates the architecture.

Illustration of the Architecture Fig. 1: Architecture

It should be noted that the filters do their processing within one clock cycle. Therefore, both Wishbone classic read/write cycles as well as classic pipelined read/write cycles are supported. As required by the Wishbone specification, the stall signal is not buffered.

Multiplexers

The multiplexer components place a master or slave on the wishbone bus, depending on the select signals driven by the arbiter or the address translator, respectively.

All signals driven by all connected Wishbone Master components are routed into the master multiplexer, and reduced to one set of master-to-slave signals. Respectively, all slave driven signals are reduced to a single set of slave-to-master signals by the slave multiplexer.

Arbiter

The arbiter grants the bus to a master by driving the select signals of the master multiplexer. The current design uses a counter which steps unless the current master has taken the bus, i.e. drives cyc. Hence, once a master has placed a transaction on the bus, the arbiter will not advance to the next master unless the active master releases the bus.

On the other hand, this also means that the master must claim the bus by asserting the cyc until the slave has acknowledged the cycle by asserting either ack, rty or err.

Address Translator

The address translator component is responsible for routing the slave-to-master signals from the currently selected slave back to the active master. Also, it needs to take care that the stb signal is only routed to the targeted slave. Furthermore, the address translator makes sure that no two slaves can respond to the same address on the wishbone bus.

The current design uses the upper bits of the wishbone address to select the slave that is being addressed. While this reduces the address space available for a single slave, it allows all slaves to decode their addresses starting at zero.

Usage

Using the wb_switch component in your VHDL design is as easy as declaring and instantiating any other component. Below is the VHDL entity declaration of the component.

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The design is customizable through generics, which are described in the following table.

Generic Description
dat_sz Width of Wishbone data bus.
nib_sz Width of the Wishbone sel signal.
addr_sz Width of the Wishbone address bus.
mstr_bits Determines the number of Wishbone masters that can be connected to the switch. The number of supported Wishbone masters is (2^mstr_bits).
slv_bits Determines the number of Wishbone slaves that can be connected to the switch. The number of supported Wishbone slaves is (2^slv_bits).

Registers

The module does not have any user accessible registers.

Master Ports

The wb_switch module provides several Wishbone master ports through the use of vectors. The table below shows what Wishbone master signals are to be connected to what vector index, based on the assumption that master #n is to be connected.

Master Signal Mux Master Port #n
cyc_o wb_mstr_cyc_i(n)
lock_o wb_mstr_lock_i(n)
stb_o wb_mstr_stb_i(n)
adr_o wb_mstr_adr_o((((n+1)*adr_sz)-1) downto (n*adr_sz))
we_o wb_mstr_we_i(n)
dat_o wb_mstr_dat_i((((n+1)*dat_sz)-1) downto (n*dat_sz))
sel_o wb_mstr_sel_i((((n+1)*nib_sz)-1) downto (n*nib_sz))
dat_i wb_mstr_dat_o((((n+1)*dat_sz)-1) downto (n*dat_sz))
ack_i wb_mstr_ack_o
rty_i wb_mstr_rty_o
err_i wb_mstr_err_o
stall_i wb_mstr_stall_o

Slave Ports

The wb_switch module provides several Wishbone slave ports through the use of vectors. The table below shows what Wishbone slave signals are to be connected to what vector index, based on the assumption that slave #n is to be connected.

Slave Signal Mux Slave Port #n
cyc_i wb_slv_cyc_o
lock_i wb_slv_lock_o
stb_i wb_slv_stb_o(n)
adr_i wb_slv_adr_o
we_i wb_slv_we_o
dat_i wb_slv_dat_o
sel_i wb_slv_sel_o
dat_o wb_slv_dat_i((((n+1)*dat_sz)-1) downto (n*dat_sz))
ack_o wb_slv_ack_i(n)
rty_o wb_slv_rty_i(n)
err_o wb_slv_err_i(n)
stall_o wb_slv_stall_i(n)

Download

Below are the links to the VHDL files of the four individual components, as well as the top component which integrates the four.

Links

Here are a few links related to this project.

Contact

Contact the author Philip Schulz by Email.

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