| 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 |