Add HACKING.txt with coding style guidelines

[proxmark3-svn] / fpga / lo_read.v

//-----------------------------------------------------------------------------\r
// The way that we connect things in low-frequency read mode. In this case\r
// we are generating the unmodulated low frequency carrier.\r
// The A/D samples at that same rate and the result is serialized.\r
//\r
// Jonathan Westhues, April 2006\r
//-----------------------------------------------------------------------------\r
\r
module lo_read(\r
    pck0, ck_1356meg, ck_1356megb,\r
    pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,\r
    adc_d, adc_clk,\r
    ssp_frame, ssp_din, ssp_dout, ssp_clk,\r
    cross_hi, cross_lo,\r
    dbg,\r
    lo_is_125khz, divisor\r
);\r
    input pck0, ck_1356meg, ck_1356megb;\r
    output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;\r
    input [7:0] adc_d;\r
    output adc_clk;\r
    input ssp_dout;\r
    output ssp_frame, ssp_din, ssp_clk;\r
    input cross_hi, cross_lo;\r
    output dbg;\r
    input lo_is_125khz; // redundant signal, no longer used anywhere\r
    input [7:0] divisor;\r
\r
reg [7:0] to_arm_shiftreg;\r
reg [7:0] pck_divider;\r
reg ant_lo;\r
\r
// this task runs on the rising egde of pck0 clock (24Mhz) and creates ant_lo\r
// which is high for (divisor+1) pck0 cycles and low for the same duration\r
// ant_lo is therefore a 50% duty cycle clock signal with a frequency of\r
// 12Mhz/(divisor+1) which drives the antenna as well as the ADC clock adc_clk\r
always @(posedge pck0)\r
begin\r
	if(pck_divider == divisor[7:0])\r
		begin\r
			pck_divider <= 8'd0;\r
			ant_lo = !ant_lo;\r
		end\r
	else\r
	begin\r
		pck_divider <= pck_divider + 1;\r
	end\r
end\r
\r
// this task also runs at pck0 frequency (24Mhz) and is used to serialize\r
// the ADC output which is then clocked into the ARM SSP.\r
\r
// because ant_lo always transitions when pck_divider = 0 we use the\r
// pck_divider counter to sync our other signals off it\r
// we read the ADC value when pck_divider=7 and shift it out on counts 8..15\r
always @(posedge pck0)\r
begin\r
	if((pck_divider == 8'd7) && !ant_lo)\r
        to_arm_shiftreg <= adc_d;\r
    else\r
	begin\r
        to_arm_shiftreg[7:1] <= to_arm_shiftreg[6:0];\r
		// simulation showed a glitch occuring due to the LSB of the shifter\r
		// not being set as we shift bits out\r
		// this ensures the ssp_din remains low after a transfer and suppresses\r
		// the glitch that would occur when the last data shifted out ended in\r
		// a 1 bit and the next data shifted out started with a 0 bit\r
        to_arm_shiftreg[0] <= 1'b0;\r
	end\r
end\r
\r
// ADC samples on falling edge of adc_clk, data available on the rising edge\r
\r
// example of ssp transfer of binary value 1100101\r
// start of transfer is indicated by the rise of the ssp_frame signal\r
// ssp_din changes on the rising edge of the ssp_clk clock and is clocked into\r
// the ARM by the falling edge of ssp_clk\r
//             _______________________________\r
// ssp_frame__|                               |__\r
//             _______         ___     ___\r
// ssp_din  __|       |_______|   |___|   |______\r
//         _   _   _   _   _   _   _   _   _   _\r
// ssp_clk  |_| |_| |_| |_| |_| |_| |_| |_| |_| |_\r
\r
// serialized SSP data is gated by ant_lo to suppress unwanted signal\r
assign ssp_din = to_arm_shiftreg[7] && !ant_lo;\r
// SSP clock always runs at 24Mhz\r
assign ssp_clk = pck0;\r
// SSP frame is gated by ant_lo and goes high when pck_divider=8..15\r
assign ssp_frame = (pck_divider[7:3] == 5'd1) && !ant_lo;\r
// unused signals tied low\r
assign pwr_hi = 1'b0;\r
assign pwr_oe1 = 1'b0;\r
assign pwr_oe2 = 1'b0;\r
assign pwr_oe3 = 1'b0;\r
assign pwr_oe4 = 1'b0;\r
// this is the antenna driver signal\r
assign pwr_lo = ant_lo;\r
// ADC clock out of phase with antenna driver\r
assign adc_clk = ~ant_lo;\r
// ADC clock also routed to debug pin\r
assign dbg = adc_clk;\r
endmodule\r