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[/] [openrisc/] [trunk/] [orpsocv2/] [boards/] [actel/] [ordb1a3pe1500/] [bench/] [verilog/] [spi_slave.v] - Rev 408
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/* Simple SPI slave code from http://www.fpga4fun.com/SPI2.html */ /* Reading the module increments a counter. */ /* LSbit of written data shows on the LED port */ module spi_slave(clk, SCK, MOSI, MISO, SSEL, LED); input clk; input SCK, SSEL, MOSI; output MISO; output LED; // sync SCK to the FPGA clock using a 3-bits shift register reg [2:0] SCKr = 0; always @(posedge clk) SCKr <= {SCKr[1:0], SCK}; // now we can detect SCK rising edges wire SCK_risingedge = (SCKr[2:1]==2'b01); // and falling edges wire SCK_fallingedge = (SCKr[2:1]==2'b10); // same thing for SSEL reg [2:0] SSELr = 3'b111; always @(posedge clk) SSELr <= {SSELr[1:0], SSEL}; wire SSEL_active = ~SSELr[1]; // SSEL is active low // message starts at falling edge wire SSEL_startmessage = (SSELr[2:1]==2'b10); // message stops at rising edge wire SSEL_endmessage = (SSELr[2:1]==2'b01); // and for MOSI reg [1:0] MOSIr = 0; always @(posedge clk) MOSIr <= {MOSIr[0], MOSI}; wire MOSI_data = MOSIr[1]; // we handle SPI in 8-bits format, so we need a 3 bits counter to // count the bits as they come in reg [2:0] bitcnt = 0; reg byte_received= 0; // high when a byte has been received reg [7:0] byte_data_received = 0; always @(posedge clk) begin if(~SSEL_active) bitcnt <= 3'b000; else if(SCK_risingedge) begin bitcnt <= bitcnt + 3'b001; // implement a shift-left register (since we receive the data // MSB first) byte_data_received <= {byte_data_received[6:0], MOSI_data}; end end always @(posedge clk) byte_received <= SSEL_active && SCK_risingedge && (bitcnt==3'b111); // we use the LSB of the data received to control an LED reg LED = 0; always @(posedge clk) if(byte_received) LED <= byte_data_received[0]; reg [7:0] byte_data_sent = 0; reg [7:0] cnt = 0; always @(posedge clk) if(SSEL_startmessage) cnt<=cnt+8'h1; // count the messages always @(posedge clk) if(SSEL_active) begin if(SSEL_startmessage) // first byte sent in a message is the message count byte_data_sent <= cnt; else if(SCK_fallingedge) begin if(bitcnt==3'b000) byte_data_sent <= 8'h00; // after that, we send 0s else byte_data_sent <= {byte_data_sent[6:0], 1'b0}; end end assign MISO = (!SSEL) ? byte_data_sent[7] : 1'bZ; // send MSB first // we assume that there is only one slave on the SPI bus // so we don't bother with a tri-state buffer for MISO // otherwise we would need to tri-state MISO when SSEL is inactive endmodule