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|
`default_nettype none
module sram_slave
(
/* Interface to memory */
output wire [17:0] sram_addr,
inout wire [15:0] sram_io,
output wire sram_cs_n,
output wire sram_oe_n,
output wire sram_we_n,
/* Wishbone slave interface */
output wire ACK_O,
input wire [17:0] ADR_I,
input wire CLK_I,
input wire [15:0] DAT_I,
output wire [15:0] DAT_O,
input wire RST_I,
input wire STB_I,
input wire WE_I,
output wire STALL_O
);
reg writing;
reg [15:0] write_data;
assign sram_io = sram_we_n ? 16'hZZZZ : write_data;
reg sram_cs_not;
reg sram_oe_not;
reg sram_we_not;
assign sram_cs_n = sram_cs_not;
assign sram_oe_n = sram_oe_not;
assign sram_we_n = sram_we_not;
reg [17:0] address;
assign sram_addr = address;
reg ack;
assign ACK_O = ack;
reg stall;
assign STALL_O = stall;
reg [15:0] output_data;
assign DAT_O = output_data;
reg [2:0] state;
always @ (posedge CLK_I) begin
output_data <= sram_io;
if (RST_I) begin
sram_cs_not <= 1;
sram_oe_not <= 1;
writing <= 0;
ack <= 0;
stall <= 0;
state <= 0;
end else begin
if (state == 0 || state == 2) begin /* possibly starting new job */
if (STB_I) begin
address <= ADR_I;
sram_cs_not <= 0;
sram_oe_not <= WE_I;
writing <= WE_I;
write_data <= DAT_I;
state <= WE_I ? 1 : 2;
stall <= WE_I ? 1 : 0;
end else begin // if (STB_I)
sram_cs_not <= 1;
sram_oe_not <= 1;
writing <= 0;
state <= 0;
end
end // if (state == 0 || state == 2)
if (state == 1) begin /* performing write; it takes 2 CLK_I cycles */
state <= 2;
stall <= 0;
end
if (state == 2) begin /* finishing job */
ack <= 1;
end else begin
ack <= 0;
end
end
end // always @ (posedge CLK_I)
/*
* avoid bus congestion by only driving sram_io
* during the middle part of write cycle
*/
always @ (negedge CLK_I) begin
if (writing)
sram_we_not <= !sram_we_not;
else
sram_we_not <= 1;
end
endmodule // sram_slave
|
