proxmark3/fpga/lo_edge_detect.v

//-----------------------------------------------------------------------------
// The way that we connect things in low-frequency simulation mode. In this
// case just pass everything through to the ARM, which can bit-bang this
// (because it is so slow).
//
// Jonathan Westhues, April 2006
//-----------------------------------------------------------------------------

module lo_edge_detect(
    pck0, ck_1356meg, ck_1356megb,
    pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,
    adc_d, adc_clk,
    ssp_frame, ssp_din, ssp_dout, ssp_clk,
    cross_hi, cross_lo,
    dbg,
	  divisor,
		lf_field
);
    input pck0, ck_1356meg, ck_1356megb;
    output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;
    input [7:0] adc_d;
    output adc_clk;
    input ssp_dout;
    output ssp_frame, ssp_din, ssp_clk;
    input cross_hi, cross_lo;
    output dbg;
	  input [7:0] divisor;
		input lf_field;

// Divide the clock to be used for the ADC
reg [7:0] pck_divider;
reg clk_state;

wire tag_modulation; 
assign tag_modulation = ssp_dout & !lf_field;
wire reader_modulation; 
assign reader_modulation = !ssp_dout & lf_field & clk_state;

// No logic, straight through.
assign pwr_oe1 = 1'b0; // not used in LF mode
assign pwr_oe2 = tag_modulation;
assign pwr_oe3 = tag_modulation;
assign pwr_oe4 = tag_modulation;
assign ssp_clk = cross_lo;
assign pwr_lo = reader_modulation;
assign pwr_hi = 1'b0;
assign dbg = ssp_frame;

always @(posedge pck0)
begin
	if(pck_divider == divisor[7:0])
		begin
			pck_divider <= 8'd0;
			clk_state = !clk_state;
		end
	else
	begin
		pck_divider <= pck_divider + 1;
	end
end

assign adc_clk = ~clk_state;

// Toggle the output with hysteresis
//  Set to high if the ADC value is above 200
//  Set to low if the ADC value is below 64
reg is_high;
reg is_low;
reg output_state;

always @(posedge pck0)
begin
	if((pck_divider == 8'd7) && !clk_state) begin
		is_high = (adc_d >= 8'd190);
		is_low = (adc_d <= 8'd70);
	end
end

always @(posedge is_high or posedge is_low)
begin
	if(is_high)
		output_state <= 1'd1;
	else if(is_low)
		output_state <= 1'd0;
end

assign ssp_frame = output_state;

endmodule