This version code now reads a TI tag properly.

fpga/lo_passthru.v

@@ -9,7 +9,7 @@ module lo_passthru(
     adc_d, adc_clk,
     ssp_frame, ssp_din, ssp_dout, ssp_clk,
     cross_hi, cross_lo,
-    dbg
+    dbg, divisor
 );
     input pck0, ck_1356meg, ck_1356megb;
     output pwr_lo, pwr_hi, pwr_oe1, pwr_oe2, pwr_oe3, pwr_oe4;
@@ -19,14 +19,35 @@ module lo_passthru(
     output ssp_frame, ssp_din, ssp_clk;
     input cross_hi, cross_lo;
     output dbg;
+    input [7:0] divisor;
 
-// No logic, straight through.
+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
+
+// the antenna is modulated when ssp_dout = 1, when 0 the
+// antenna drivers stop modulating and go into listen mode
 assign pwr_oe3 = 1'b0;
-assign pwr_oe1 = 1'b1;
-assign pwr_oe2 = 1'b1;
-assign pwr_oe4 = 1'b1;
-assign pwr_lo = 1'b0;
+assign pwr_oe1 = ssp_dout;
+assign pwr_oe2 = ssp_dout;
+assign pwr_oe4 = ssp_dout;
+assign pwr_lo = ant_lo && ssp_dout;
 assign pwr_hi = 1'b0;
 assign adc_clk = 1'b0;
 assign ssp_din = cross_lo;