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// Filename: wbarbiter.v
// Project: Zip CPU -- a small, lightweight, RISC CPU soft core
// Purpose: This is a priority bus arbiter. It allows two separate wishbone
// masters to connect to the same bus, while also guaranteeing
// that the last master can have the bus with no delay any time it is
// idle. The goal is to minimize the combinatorial logic required in this
// process, while still minimizing access time.
// The core logic works like this:
// 1. If 'A' or 'B' asserts the o_cyc line, a bus cycle will begin,
// with acccess granted to whomever requested it.
// 2. If both 'A' and 'B' assert o_cyc at the same time, only 'A'
// will be granted the bus. (If the alternating parameter
// is set, A and B will alternate who gets the bus in
// this case.)
// 3. The bus will remain owned by whomever the bus was granted to
// until they deassert the o_cyc line.
// 4. At the end of a bus cycle, o_cyc is guaranteed to be
// deasserted (low) for one clock.
// 5. On the next clock, bus arbitration takes place again. If
// 'A' requests the bus, no matter how long 'B' was
// waiting, 'A' will then be granted the bus. (Unless
// again the alternating parameter is set, then the
// access is guaranteed to switch to B.)
// Creator: Dan Gisselquist, Ph.D.
// Gisselquist Technology, LLC
// Copyright (C) 2015-2018, Gisselquist Technology, LLC
// This program is free software (firmware): you can redistribute it and/or
// modify it under the terms of the GNU General Public License as published
// by the Free Software Foundation, either version 3 of the License, or (at
// your option) any later version.
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
// for more details.
// You should have received a copy of the GNU General Public License along
// with this program. (It's in the $(ROOT)/doc directory. Run make with no
// target there if the PDF file isn't present.) If not, see
// <> for a copy.
// License: GPL, v3, as defined and found on,
`default_nettype none
module wbarbiter(i_clk, i_rst,
// Bus A -- the priority bus
i_a_cyc, i_a_stb, i_a_we, i_a_adr, i_a_dat, i_a_sel,
o_a_ack, o_a_stall, o_a_err,
// Bus B
i_b_cyc, i_b_stb, i_b_we, i_b_adr, i_b_dat, i_b_sel,
o_b_ack, o_b_stall, o_b_err,
// Combined/arbitrated bus
o_cyc, o_stb, o_we, o_adr, o_dat, o_sel, i_ack, i_stall, i_err);
// 18 bits will address one GB, 4 bytes at a time.
// 19 bits will allow the ability to address things other than just
// the 1GB of memory we are expecting.
parameter DW=32, AW=19;
// Wishbone doesn't use an i_ce signal. While it could, they dislike
// what it would (might) do to the synchronous reset signal, i_rst.
input wire i_clk, i_rst;
// Bus A
input wire i_a_cyc, i_a_stb, i_a_we;
input wire [(AW-1):0] i_a_adr;
input wire [(DW-1):0] i_a_dat;
input wire [(DW/8-1):0] i_a_sel;
output wire o_a_ack, o_a_stall, o_a_err;
// Bus B
input wire i_b_cyc, i_b_stb, i_b_we;
input wire [(AW-1):0] i_b_adr;
input wire [(DW-1):0] i_b_dat;
input wire [(DW/8-1):0] i_b_sel;
output wire o_b_ack, o_b_stall, o_b_err;
output wire o_cyc, o_stb, o_we;
output wire [(AW-1):0] o_adr;
output wire [(DW-1):0] o_dat;
output wire [(DW/8-1):0] o_sel;
input wire i_ack, i_stall, i_err;
// All the fancy stuff here is done with the three primary signals:
// o_cyc
// w_a_owner
// w_b_owner
// These signals are helped by r_cyc, r_a_owner, and r_b_owner.
// If you understand these signals, all else will fall into place.
// r_cyc just keeps track of the last o_cyc value. That way, on
// the next clock we can tell if we've had one non-cycle before
// starting another cycle. Specifically, no new cycles will be
// allowed to begin unless r_cyc=0.
reg r_cyc;
initial r_cyc = 1'b0;
always @(posedge i_clk)
if (i_rst)
r_cyc <= 1'b0;
r_cyc <= o_cyc;
// Go high immediately (new cycle) if ...
// Previous cycle was low and *someone* is requesting a bus cycle
// Go low immadiately if ...
// We were just high and the owner no longer wants the bus
// WISHBONE Spec recommends no logic between a FF and the o_cyc
// This violates that spec. (Rec 3.15, p35)
assign o_cyc = ((!r_cyc)&&((i_a_cyc)||(i_b_cyc)))
// Register keeping track of the last owner, wire keeping track of the
// current owner allowing us to not lose a clock in arbitrating the
// first clock of the bus cycle
reg r_a_owner, r_b_owner;
wire w_a_owner, w_b_owner;
reg r_a_last_owner;
initial r_a_owner = 1'b0;
initial r_b_owner = 1'b0;
always @(posedge i_clk)
if (i_rst)
r_a_owner <= 1'b0;
r_b_owner <= 1'b0;
end else begin
r_a_owner <= w_a_owner;
r_b_owner <= w_b_owner;
if (w_a_owner)
r_a_last_owner <= 1'b1;
else if (w_b_owner)
r_a_last_owner <= 1'b0;
// If you are the owner, retain ownership until i_x_cyc is no
// longer asserted. Likewise, you cannot become owner until o_cyc
// is de-asserted for one cycle.
// 'A' is given arbitrary priority over 'B'
// 'A' may own the bus only if he wants it. When 'A' drops i_a_cyc,
// o_cyc must drop and so must w_a_owner on the same cycle.
// However, when 'A' asserts i_a_cyc, he can only capture the bus if
// it's had an idle cycle.
// The same is true for 'B' with one exception: if both contend for the
// bus on the same cycle, 'A' arbitrarily wins.
assign w_a_owner = (i_a_cyc) // if A requests ownership, and either
&& ((r_a_owner) // A has already been recognized or
|| ((!r_cyc) // the bus is free and
&&((!i_b_cyc) // B has not requested, or if he
||(!r_a_last_owner)) )); // has, it's A's turn
assign w_b_owner = (i_b_cyc)&& ((r_b_owner) || ((!r_cyc)&&((!i_a_cyc)||(r_a_last_owner)) ));
assign w_a_owner = (i_a_cyc)&& ((r_a_owner) || (!r_cyc) );
assign w_b_owner = (i_b_cyc)&& ((r_b_owner) || ((!r_cyc)&&(!i_a_cyc)) );
// Realistically, if neither master owns the bus, the output is a
// don't care. Thus we trigger off whether or not 'A' owns the bus.
// If 'B' owns it all we care is that 'A' does not. Likewise, if
// neither owns the bus than the values on the various lines are
// irrelevant. (This allows us to get two outputs per Xilinx 6-LUT)
assign o_stb = (o_cyc) && ((w_a_owner) ? i_a_stb : i_b_stb);
assign o_we = (w_a_owner) ? i_a_we : i_b_we;
assign o_adr = (w_a_owner) ? i_a_adr : i_b_adr;
assign o_dat = (w_a_owner) ? i_a_dat : i_b_dat;
assign o_sel = (w_a_owner) ? i_a_sel : i_b_sel;
// We cannot allow the return acknowledgement to ever go high if
// the master in question does not own the bus. Hence we force it
// low if the particular master doesn't own the bus.
assign o_a_ack = (w_a_owner) ? i_ack : 1'b0;
assign o_b_ack = (w_b_owner) ? i_ack : 1'b0;
// Stall must be asserted on the same cycle the input master asserts
// the bus, if the bus isn't granted to him.
assign o_a_stall = (w_a_owner) ? i_stall : 1'b1;
assign o_b_stall = (w_b_owner) ? i_stall : 1'b1;
assign o_a_err = (w_a_owner) ? i_err : 1'b0;
assign o_b_err = (w_b_owner) ? i_err : 1'b0;
`ifdef FORMAL
reg f_last_clk;
initial assume(!i_clk);
always @($global_clock)
assume(i_clk != f_last_clk);
f_last_clk <= i_clk;
`define `ASSUME assume
`define `ASSUME assert
reg f_past_valid;
initial f_past_valid = 1'b0;
always @($global_clock)
f_past_valid <= 1'b1;
initial assume(!i_a_cyc);
initial assume(!i_a_stb);
initial assume(!i_b_cyc);
initial assume(!i_b_stb);
initial assume(!i_ack);
initial assume(!i_err);
always @(posedge i_clk)
if (o_cyc)
if ((f_past_valid)&&($past(o_cyc))&&(o_cyc))
assert($past(w_a_owner) == w_a_owner);
assert($past(w_b_owner) == w_b_owner);
if ((f_past_valid)&&($past(!o_cyc))&&($past(i_a_cyc)))
reg f_reset;
initial f_reset = 1'b1;
always @(posedge i_clk)
f_reset <= 1'b0;
parameter F_LGDEPTH=3;
wire [(F_LGDEPTH-1):0] f_nreqs, f_nacks, f_outstanding,
f_a_nreqs, f_a_nacks, f_a_outstanding,
f_b_nreqs, f_b_nacks, f_b_outstanding;
fwb_master #(.DW(DW), .AW(AW),
f_wbm(i_clk, f_reset,
o_cyc, o_stb, o_we, o_adr, o_dat, o_sel,
i_ack, i_stall, 32'h0, i_err,
f_nreqs, f_nacks, f_outstanding);
fwb_slave #(.DW(DW), .AW(AW),
f_wba(i_clk, f_reset,
i_a_cyc, i_a_stb, i_a_we, i_a_adr, i_a_dat, i_a_sel,
o_a_ack, o_a_stall, 32'h0, o_a_err,
f_a_nreqs, f_a_nacks, f_a_outstanding);
fwb_slave #(.DW(DW), .AW(AW),
f_wbb(i_clk, f_reset,
i_b_cyc, i_b_stb, i_b_we, i_b_adr, i_b_dat, i_b_sel,
o_b_ack, o_b_stall, 32'h0, o_b_err,
f_b_nreqs, f_b_nacks, f_b_outstanding);
always @(posedge i_clk)
if (w_a_owner)
assert(f_b_nreqs == 0);
assert(f_b_nacks == 0);
assert(f_a_outstanding == f_outstanding);
end else if (w_b_owner) begin
assert(f_a_nreqs == 0);
assert(f_a_nacks == 0);
assert(f_b_outstanding == f_outstanding);
always @(posedge i_clk)
if ((w_a_owner)&&(i_b_cyc))
always @(posedge i_clk)
if ((w_b_owner)&&(i_a_cyc))
always @(posedge i_clk)
always @(posedge i_clk)
always @(posedge i_clk)
if ((f_past_valid)&&($past(o_cyc))&&(!$past(i_rst)))
always @(posedge i_clk)
if (!f_past_valid)
always @(posedge i_clk)
if ((!f_past_valid)||((f_past_valid)&&($past(i_rst))))