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fix FGPA hi_read_r_xcorr.v:

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* avoid overflows during accumulation and truncation
* explicitely cast unsigned ADC samples to signed
This commit is contained in:
pwpiwi 2018-08-13 21:24:42 +02:00
commit 290cc01525
2 changed files with 43 additions and 16 deletions
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fpga/hi_read_rx_xcorr.v
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@ -71,12 +71,15 @@ begin
corr_i_cnt <= corr_i_cnt + 1;
end
// And a couple of registers in which to accumulate the correlations.
// We would add at most 32 times the difference between unmodulated and modulated signal. It should
// And a couple of registers in which to accumulate the correlations. From the 64 samples
// we would add at most 32 times the difference between unmodulated and modulated signal. It should
// be safe to assume that a tag will not be able to modulate the carrier signal by more than 25%.
// 32 * 255 * 0,25 = 2040, which can be held in 11 bits. Add 1 bit for sign.
reg signed [11:0] corr_i_accum;
reg signed [11:0] corr_q_accum;
// Temporary we might need more bits. For the 212kHz subcarrier we could possible add 32 times the
// maximum signal value before a first subtraction would occur. 32 * 255 = 8160 can be held in 13 bits.
// Add one bit for sign -> need 14 bit registers but final result will fit into 12 bits.
reg signed [13:0] corr_i_accum;
reg signed [13:0] corr_q_accum;
// we will report maximum 8 significant bits
reg signed [7:0] corr_i_out;
reg signed [7:0] corr_q_out;
@ -104,7 +107,7 @@ begin
else
begin // 424 kHz
subcarrier_I = ~corr_i_cnt[4];
subcarrier_Q = ~(corr_i_cnt[4]^corr_i_cnt[3]);
subcarrier_Q = ~(corr_i_cnt[4] ^ corr_i_cnt[3]);
end
end
@ -119,32 +122,56 @@ begin
if(snoop)
begin
// Send 7 most significant bits of tag signal (signed), plus 1 bit reader signal
corr_i_out <= {corr_i_accum[11:5], after_hysteresis_prev_prev};
corr_q_out <= {corr_q_accum[11:5], after_hysteresis_prev};
if (corr_i_accum[13:11] == 3'b000 || corr_i_accum[13:11] == 3'b111)
corr_i_out <= {corr_i_accum[11:5], after_hysteresis_prev_prev};
else // truncate to maximum value
if (corr_i_accum[13] == 1'b0)
corr_i_out <= {7'b0111111, after_hysteresis_prev_prev};
else
corr_i_out <= {7'b1000000, after_hysteresis_prev_prev};
if (corr_q_accum[13:11] == 3'b000 || corr_q_accum[13:11] == 3'b111)
corr_q_out <= {corr_q_accum[11:5], after_hysteresis_prev};
else // truncate to maximum value
if (corr_q_accum[13] == 1'b0)
corr_q_out <= {7'b0111111, after_hysteresis_prev};
else
corr_q_out <= {7'b1000000, after_hysteresis_prev};
after_hysteresis_prev_prev <= after_hysteresis;
end
else
begin
// Send 8 most significant bits of tag signal
corr_i_out <= corr_i_accum[11:4];
corr_q_out <= corr_q_accum[11:4];
// Send 8 bits of tag signal
if (corr_i_accum[13:11] == 3'b000 || corr_i_accum[13:11] == 3'b111)
corr_i_out <= corr_i_accum[11:4];
else // truncate to maximum value
if (corr_i_accum[13] == 1'b0)
corr_i_out <= 8'b01111111;
else
corr_i_out <= 8'b10000000;
if (corr_q_accum[13:11] == 3'b000 || corr_q_accum[13:11] == 3'b111)
corr_q_out <= corr_q_accum[11:4];
else // truncate to maximum value
if (corr_q_accum[13] == 1'b0)
corr_q_out <= 8'b01111111;
else
corr_q_out <= 8'b10000000;
end
// Initialize next correlation.
// Both I and Q reference signals are high when corr_i_nct == 0. Therefore need to accumulate.
corr_i_accum <= adc_d;
corr_q_accum <= adc_d;
corr_i_accum <= $signed({1'b0,adc_d});
corr_q_accum <= $signed({1'b0,adc_d});
end
else
begin
if (subcarrier_I)
corr_i_accum <= corr_i_accum + adc_d;
corr_i_accum <= corr_i_accum + $signed({1'b0,adc_d});
else
corr_i_accum <= corr_i_accum - adc_d;
corr_i_accum <= corr_i_accum - $signed({1'b0,adc_d});
if (subcarrier_Q)
corr_q_accum <= corr_q_accum + adc_d;
corr_q_accum <= corr_q_accum + $signed({1'b0,adc_d});
else
corr_q_accum <= corr_q_accum - adc_d;
corr_q_accum <= corr_q_accum - $signed({1'b0,adc_d});
end