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1 //-----------------------------------------------------------------------------
2 // The way that we connect things in low-frequency read mode. In this case
3 // we are generating the unmodulated low frequency carrier.
4 // The A/D samples at that same rate and the result is serialized.
5 //
6 // Jonathan Westhues, April 2006
7 // iZsh <izsh at fail0verflow.com>, June 2014
8 //-----------------------------------------------------------------------------
9
10 module lo_read(
11 input pck0, input [7:0] pck_cnt, input pck_divclk,
12 output pwr_lo, output pwr_hi,
13 output pwr_oe1, output pwr_oe2, output pwr_oe3, output pwr_oe4,
14 input [7:0] adc_d, output adc_clk,
15 output ssp_frame, output ssp_din, output ssp_clk,
16 output dbg,
17 input lf_field
18 );
19
20 reg [7:0] to_arm_shiftreg;
21
22 // this task also runs at pck0 frequency (24Mhz) and is used to serialize
23 // the ADC output which is then clocked into the ARM SSP.
24
25 // because pck_divclk always transitions when pck_cnt = 0 we use the
26 // pck_div counter to sync our other signals off it
27 // we read the ADC value when pck_cnt=7 and shift it out on counts 8..15
28 always @(posedge pck0)
29 begin
30 if((pck_cnt == 8'd7) && !pck_divclk)
31 to_arm_shiftreg <= adc_d;
32 else begin
33 to_arm_shiftreg[7:1] <= to_arm_shiftreg[6:0];
34 // simulation showed a glitch occuring due to the LSB of the shifter
35 // not being set as we shift bits out
36 // this ensures the ssp_din remains low after a transfer and suppresses
37 // the glitch that would occur when the last data shifted out ended in
38 // a 1 bit and the next data shifted out started with a 0 bit
39 to_arm_shiftreg[0] <= 1'b0;
40 end
41 end
42
43 // ADC samples on falling edge of adc_clk, data available on the rising edge
44
45 // example of ssp transfer of binary value 1100101
46 // start of transfer is indicated by the rise of the ssp_frame signal
47 // ssp_din changes on the rising edge of the ssp_clk clock and is clocked into
48 // the ARM by the falling edge of ssp_clk
49 // _______________________________
50 // ssp_frame__| |__
51 // _______ ___ ___
52 // ssp_din __| |_______| |___| |______
53 // _ _ _ _ _ _ _ _ _ _
54 // ssp_clk |_| |_| |_| |_| |_| |_| |_| |_| |_| |_
55
56 // serialized SSP data is gated by ant_lo to suppress unwanted signal
57 assign ssp_din = to_arm_shiftreg[7] && !pck_divclk;
58 // SSP clock always runs at 24Mhz
59 assign ssp_clk = pck0;
60 // SSP frame is gated by ant_lo and goes high when pck_divider=8..15
61 assign ssp_frame = (pck_cnt[7:3] == 5'd1) && !pck_divclk;
62 // unused signals tied low
63 assign pwr_hi = 1'b0;
64 assign pwr_oe1 = 1'b0;
65 assign pwr_oe2 = 1'b0;
66 assign pwr_oe3 = 1'b0;
67 assign pwr_oe4 = 1'b0;
68 // this is the antenna driver signal
69 assign pwr_lo = lf_field & pck_divclk;
70 // ADC clock out of phase with antenna driver
71 assign adc_clk = ~pck_divclk;
72 // ADC clock also routed to debug pin
73 assign dbg = adc_clk;
74 endmodule
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