i2c_extender_code_example.v

verilog编写

module I2CslaveWith8bitsIO(SDA, SCL, IOout);
inout SDA;
input SCL;
output [7:0] IOout;

Then the 7-bits address that we want for our I2C slave. 
parameter I2C_ADR = 7'h27;

//Then the start and stop conditions detection logic.
//That's the "black magic" part of this design... 
// We use two wires with a combinatorial loop to detect the start and stop conditions
//  ... making sure these two wires don't get optimized away
wire SDA_shadow    /* synthesis keep = 1 */;
wire start_or_stop /* synthesis keep = 1 */;
assign SDA_shadow = (~SCL | start_or_stop) ? SDA : SDA_shadow;
assign start_or_stop = ~SCL ? 1'b0 : (SDA ^ SDA_shadow);

reg incycle;
always @(negedge SCL or posedge start_or_stop)
if(start_or_stop) incycle <= 1'b0; else if(~SDA) incycle <= 1'b1;

//Now we are ready to count the I2C bits coming in 
reg [3:0] bitcnt;  // counts the I2C bits from 7 downto 0, plus an ACK bit
wire bit_DATA = ~bitcnt[3];  // the DATA bits are the first 8 bits sent
wire bit_ACK = bitcnt[3];  // the ACK bit is the 9th bit sent
reg data_phase;

always @(negedge SCL or negedge incycle)
if(~incycle)
begin
    bitcnt <= 4'h7;  // the bit 7 is received first
    data_phase <= 0;
end
else
begin
    if(bit_ACK)
    begin
    	bitcnt <= 4'h7;
    	data_phase <= 1;
    end
    else
    	bitcnt <= bitcnt - 4'h1;
end

//and detect if the I2C address matches our own 
wire adr_phase = ~data_phase;
reg adr_match, op_read, got_ACK;
// sample SDA on posedge since the I2C spec specifies as low as 0μs hold-time on negedge
reg SDAr;  always @(posedge SCL) SDAr<=SDA;
reg [7:0] mem;
wire op_write = ~op_read;

always @(negedge SCL or negedge incycle)
if(~incycle)
begin
    got_ACK <= 0;
    adr_match <= 1;
    op_read <= 0;
end
else
begin
    if(adr_phase & bitcnt==7 & SDAr!=I2C_ADR[6]) adr_match<=0;
    if(adr_phase & bitcnt==6 & SDAr!=I2C_ADR[5]) adr_match<=0;
    if(adr_phase & bitcnt==5 & SDAr!=I2C_ADR[4]) adr_match<=0;
    if(adr_phase & bitcnt==4 & SDAr!=I2C_ADR[3]) adr_match<=0;
    if(adr_phase & bitcnt==3 & SDAr!=I2C_ADR[2]) adr_match<=0;
    if(adr_phase & bitcnt==2 & SDAr!=I2C_ADR[1]) adr_match<=0;
    if(adr_phase & bitcnt==1 & SDAr!=I2C_ADR[0]) adr_match<=0;
    if(adr_phase & bitcnt==0) op_read <= SDAr;
    // we monitor the ACK to be able to free the bus when the master doesn't ACK during a read operation
    if(bit_ACK) got_ACK <= ~SDAr;

    if(adr_match & bit_DATA & data_phase & op_write) mem[bitcnt] <= SDAr;  // memory write
end

//and drive the SDA line when necessary. 
wire mem_bit_low = ~mem[bitcnt[2:0]];
wire SDA_assert_low = adr_match & bit_DATA & data_phase & op_read & mem_bit_low & got_ACK;
wire SDA_assert_ACK = adr_match & bit_ACK & (adr_phase | op_write);
wire SDA_low = SDA_assert_low | SDA_assert_ACK;
assign SDA = SDA_low ? 1'b0 : 1'bz;

assign IOout = mem;
endmodule