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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 //-----------------------------------------------------------------------------
8
9 module lo_read(
10 pck0, ck_1356meg, ck_1356megb,
11 pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4,
12 adc_d, adc_clk,
13 ssp_frame, ssp_din, ssp_dout, ssp_clk,
14 cross_hi, cross_lo,
15 dbg,
16 lo_is_125khz, divisor
17 );
18 input pck0, ck_1356meg, ck_1356megb;
19 output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;
20 input [7:0] adc_d;
21 output adc_clk;
22 input ssp_dout;
23 output ssp_frame, ssp_din, ssp_clk;
24 input cross_hi, cross_lo;
25 output dbg;
26 input lo_is_125khz; // redundant signal, no longer used anywhere
27 input [7:0] divisor;
28
29 reg [7:0] to_arm_shiftreg;
30 reg [7:0] pck_divider;
31 reg ant_lo;
32
33 // this task runs on the rising egde of pck0 clock (24Mhz) and creates ant_lo
34 // which is high for (divisor+1) pck0 cycles and low for the same duration
35 // ant_lo is therefore a 50% duty cycle clock signal with a frequency of
36 // 12Mhz/(divisor+1) which drives the antenna as well as the ADC clock adc_clk
37 always @(posedge pck0)
38 begin
39 if(pck_divider == divisor[7:0])
40 begin
41 pck_divider <= 8'd0;
42 ant_lo = !ant_lo;
43 end
44 else
45 begin
46 pck_divider <= pck_divider + 1;
47 end
48 end
49
50 // this task also runs at pck0 frequency (24Mhz) and is used to serialize
51 // the ADC output which is then clocked into the ARM SSP.
52
53 // because ant_lo always transitions when pck_divider = 0 we use the
54 // pck_divider counter to sync our other signals off it
55 // we read the ADC value when pck_divider=7 and shift it out on counts 8..15
56 always @(posedge pck0)
57 begin
58 if((pck_divider == 8'd7) && !ant_lo)
59 to_arm_shiftreg <= adc_d;
60 else
61 begin
62 to_arm_shiftreg[7:1] <= to_arm_shiftreg[6:0];
63 // simulation showed a glitch occuring due to the LSB of the shifter
64 // not being set as we shift bits out
65 // this ensures the ssp_din remains low after a transfer and suppresses
66 // the glitch that would occur when the last data shifted out ended in
67 // a 1 bit and the next data shifted out started with a 0 bit
68 to_arm_shiftreg[0] <= 1'b0;
69 end
70 end
71
72 // ADC samples on falling edge of adc_clk, data available on the rising edge
73
74 // example of ssp transfer of binary value 1100101
75 // start of transfer is indicated by the rise of the ssp_frame signal
76 // ssp_din changes on the rising edge of the ssp_clk clock and is clocked into
77 // the ARM by the falling edge of ssp_clk
78 // _______________________________
79 // ssp_frame__| |__
80 // _______ ___ ___
81 // ssp_din __| |_______| |___| |______
82 // _ _ _ _ _ _ _ _ _ _
83 // ssp_clk |_| |_| |_| |_| |_| |_| |_| |_| |_| |_
84
85 // serialized SSP data is gated by ant_lo to suppress unwanted signal
86 assign ssp_din = to_arm_shiftreg[7] && !ant_lo;
87 // SSP clock always runs at 24Mhz
88 assign ssp_clk = pck0;
89 // SSP frame is gated by ant_lo and goes high when pck_divider=8..15
90 assign ssp_frame = (pck_divider[7:3] == 5'd1) && !ant_lo;
91 // unused signals tied low
92 assign pwr_hi = 1'b0;
93 assign pwr_oe1 = 1'b0;
94 assign pwr_oe2 = 1'b0;
95 assign pwr_oe3 = 1'b0;
96 assign pwr_oe4 = 1'b0;
97 // this is the antenna driver signal
98 assign pwr_lo = ant_lo;
99 // ADC clock out of phase with antenna driver
100 assign adc_clk = ~ant_lo;
101 // ADC clock also routed to debug pin
102 assign dbg = adc_clk;
103 endmodule
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