Subversion Repositories xulalx25soc

///////////////////////////////////////////////////////////////////////////

//

// Filename:    spiarbiter.v

//

// Project:     XuLA2 board

//

// Purpose:     The XuLA2 offers SPI access to both a FLASH and an SD Card.

//              This simple script arbitrates between the two of those to

//      determine who has access.  Drivers for both chips may interact with

//      this arbiter as though it were a SPI device with one additional piece

//      of functionality: the clock line may not be brought low until access

//      has been granted to the chip.  Thus, the controller wishing to access

//      its device should pull the CS line low, and then wait to read that

//      its grant line is high.  Once CS is low and grant is high, it may

//      then bring CK low and start its transaction.

//

//      When two or more controllers request access at the same time,

//      access will be given in priority order.  Further, access is always

//      granted to device 'A' without request as long as device 'B' isn't

//      busy.

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

///////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015, 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.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

///////////////////////////////////////////////////////////////////////////

//

//

module  spiarbiter(i_clk,

                i_cs_a_n, i_ck_a, i_mosi_a,

                i_cs_b_n, i_ck_b, i_mosi_b,

                o_cs_a_n, o_cs_b_n, o_ck, o_mosi,

                o_grant);

        input           i_clk;

        input           i_cs_a_n, i_ck_a, i_mosi_a;

        // output       wire    o_grant_a;

        input           i_cs_b_n, i_ck_b, i_mosi_b;

        // output       wire    o_grant_b;

        output  wire    o_cs_a_n, o_cs_b_n, o_ck, o_mosi;

        //

        output  wire    o_grant; // == o_grant_a = ~o_grant_b

        reg     a_owner;

        initial a_owner = 1'b1;

        always @(posedge i_clk)

                if ((i_cs_a_n)&&(i_cs_b_n))

                        a_owner <= 1'b1; // Keep control

                else if ((i_cs_a_n)&&(~i_cs_b_n))

                        a_owner <= 1'b0; // Give up control

                else if ((~i_cs_a_n)&&(i_cs_b_n))

                        a_owner <= 1'b1; // Take control

        // assign       o_grant_a = a_owner;

        // assign       o_grant_b = (~a_owner);

        assign  o_cs_a_n = (~a_owner)||(i_cs_a_n);

        assign  o_cs_b_n = ( a_owner)||(i_cs_b_n);

        assign  o_ck     = ( a_owner)?i_ck_a   : i_ck_b;

        assign  o_mosi   = ( a_owner)?i_mosi_a : i_mosi_b;

        assign  o_grant = ~a_owner;

endmodule