proxmark3/fpga/lo_read.v

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//-----------------------------------------------------------------------------
// The way that we connect things in low-frequency read mode. In this case
// we are generating the unmodulated low frequency carrier.
// The A/D samples at that same rate and the result is serialized.
//
// Jonathan Westhues, April 2006
//-----------------------------------------------------------------------------
module lo_read(
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,
lo_is_125khz, divisor
);
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 lo_is_125khz; // redundant signal, no longer used anywhere
input [7:0] divisor;
reg [7:0] to_arm_shiftreg;
reg [7:0] pck_divider;
reg ant_lo;
// this task runs on the rising egde of pck0 clock (24Mhz) and creates ant_lo
// which is high for (divisor+1) pck0 cycles and low for the same duration
// ant_lo is therefore a 50% duty cycle clock signal with a frequency of
// 12Mhz/(divisor+1) which drives the antenna as well as the ADC clock adc_clk
always @(posedge pck0)
begin
if(pck_divider == divisor[7:0])
begin
pck_divider <= 8'd0;
ant_lo = !ant_lo;
end
else
begin
pck_divider <= pck_divider + 1;
end
end
// this task also runs at pck0 frequency (24Mhz) and is used to serialize
// the ADC output which is then clocked into the ARM SSP.
// because ant_lo always transitions when pck_divider = 0 we use the
// pck_divider counter to sync our other signals off it
// we read the ADC value when pck_divider=7 and shift it out on counts 8..15
always @(posedge pck0)
begin
if((pck_divider == 8'd7) && !ant_lo)
to_arm_shiftreg <= adc_d;
else
begin
to_arm_shiftreg[7:1] <= to_arm_shiftreg[6:0];
// simulation showed a glitch occuring due to the LSB of the shifter
// not being set as we shift bits out
// this ensures the ssp_din remains low after a transfer and suppresses
// the glitch that would occur when the last data shifted out ended in
// a 1 bit and the next data shifted out started with a 0 bit
to_arm_shiftreg[0] <= 1'b0;
end
end
// ADC samples on falling edge of adc_clk, data available on the rising edge
// example of ssp transfer of binary value 1100101
// start of transfer is indicated by the rise of the ssp_frame signal
// ssp_din changes on the rising edge of the ssp_clk clock and is clocked into
// the ARM by the falling edge of ssp_clk
// _______________________________
// ssp_frame__| |__
// _______ ___ ___
// ssp_din __| |_______| |___| |______
// _ _ _ _ _ _ _ _ _ _
// ssp_clk |_| |_| |_| |_| |_| |_| |_| |_| |_| |_
// serialized SSP data is gated by ant_lo to suppress unwanted signal
assign ssp_din = to_arm_shiftreg[7] && !ant_lo;
// SSP clock always runs at 24Mhz
assign ssp_clk = pck0;
// SSP frame is gated by ant_lo and goes high when pck_divider=8..15
assign ssp_frame = (pck_divider[7:3] == 5'd1) && !ant_lo;
// unused signals tied low
assign pwr_hi = 1'b0;
assign pwr_oe1 = 1'b0;
assign pwr_oe2 = 1'b0;
assign pwr_oe3 = 1'b0;
assign pwr_oe4 = 1'b0;
// this is the antenna driver signal
assign pwr_lo = ant_lo;
// ADC clock out of phase with antenna driver
assign adc_clk = ~ant_lo;
// ADC clock also routed to debug pin
assign dbg = adc_clk;
endmodule