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Combined all sigmadelta things to one input block

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Joppe Blondel
2025-10-19 20:03:51 +02:00
parent 165faefa59
commit 49b8a77480
5 changed files with 137 additions and 66 deletions
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101
rtl/util/rc_alpha_q15.vh
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@@ -7,61 +7,82 @@
`define RC_ALPHA_Q15_VH
function integer alpha_q15_from_rc;
input integer R_OHM; // resistance in ohms
input integer C_PF; // capacitance in picofarads
input integer FS_HZ; // sampling frequency in Hz
input integer R_OHM; // ohms
input integer C_PF; // picofarads
input integer FS_HZ; // Hz
integer N; // fractional bits for x (QN)
reg [127:0] num_1e12_sllN;
reg [127:0] denom_u;
reg [127:0] x_qN; // x in QN
reg [255:0] x2; // x^2 in Q(2N)
reg [383:0] x3; // x^3 in Q(3N)
integer term1_q15; // x -> Q1.15
integer term2_q15; // x^2/2 -> Q1.15
integer term3_q15; // x^3/6 -> Q1.15
integer acc; // accumulator for result
begin
// Choose QN for x. N=24 is a good balance for accuracy/width.
N = 24;
integer N;
// x = 1 / (Fs * R * C) with C in pF ==> x = 1e12 / (Fs * R * C_PF)
// We'll keep everything as unsigned vectors; inputs copied into vectors first.
reg [63:0] R_u, C_u, FS_u;
// x = 1 / (Fs * R * C) with C in pF -> x = 1e12 / (Fs*R*C_pf)
// x_qN = round( x * 2^N ) = round( (1e12 << N) / denom )
num_1e12_sllN = 128'd1000000000000 << N;
// denom = Fs * R * C_PF (fits in 64..96 bits for typical values)
denom_u = 0;
denom_u = denom_u + FS_HZ[127:0];
denom_u = denom_u * R_OHM[127:0];
denom_u = denom_u * C_PF[127:0];
// rounded divide for x_qN
x_qN = (num_1e12_sllN + (denom_u >> 1)) / denom_u;
reg [127:0] NUM_1E12_SLLN; // big enough for 1e12 << N
reg [127:0] DENOM; // Fs*R*C
reg [127:0] X_qN; // x in QN
// Powers
x2 = x_qN * x_qN; // 128x128 -> 256
x3 = x2 * x_qN; // 256x128 -> 384
reg [255:0] X2; // x^2 in Q(2N)
reg [383:0] X3; // x^3 in Q(3N)
// term1 = x -> shift from QN to Q15
term1_q15 = (x_qN >> (N - 15)) & 16'hFFFF;
integer term1_q15;
integer term2_q15;
integer term3_q15;
integer acc;
// term2 = x^2 / 2 -> shift from Q(2N) to Q15 and divide by 2
term2_q15 = (x2 >> (2*N - 15 + 1)) & 16'hFFFF;
begin
N = 24;
// term3 = x^3 / 6 -> shift from Q(3N) to Q15, then divide by 6 (rounded)
begin : gen_term3
// Copy integer inputs into 64-bit vectors (no bit-slicing of integers)
R_u = R_OHM[31:0];
C_u = C_PF[31:0];
FS_u = FS_HZ[31:0];
// Denominator = Fs * R * C_pf (fits in < 2^64 for typical values)
DENOM = 128'd0;
DENOM = FS_u;
DENOM = DENOM * R_u;
DENOM = DENOM * C_u;
// // Guard: avoid divide by zero
// if (DENOM == 0) begin
// alpha_q15_from_rc = 0;
// disable alpha_q15_from_rc;
// end
// Numerator = (1e12 << N). 1e12 * 2^24 ≈ 1.6777e19 (fits in 2^64..2^65),
// so use 128 bits to be safe.
NUM_1E12_SLLN = 128'd1000000000000 << N;
// x_qN = rounded division
X_qN = (NUM_1E12_SLLN + (DENOM >> 1)) / DENOM;
// Powers
X2 = X_qN * X_qN;
X3 = X2 * X_qN;
// Convert terms to Q1.15:
// term1 = x -> shift from QN to Q15
term1_q15 = (X_qN >> (N - 15)) & 16'hFFFF;
// term2 = x^2 / 2 -> Q(2N) to Q15 and /2
term2_q15 = (X2 >> (2*N - 15 + 1)) & 16'hFFFF;
// term3 = x^3 / 6 -> Q(3N) to Q15, then /6 with rounding
begin : gen_t3
reg [383:0] tmp_q15_wide;
reg [383:0] tmp_div6;
tmp_q15_wide = (x3 >> (3*N - 15));
tmp_div6 = (tmp_q15_wide + 6'd3) / 6; // +3 for rounding
tmp_q15_wide = (X3 >> (3*N - 15));
tmp_div6 = (tmp_q15_wide + 6'd3) / 6;
term3_q15 = tmp_div6[15:0];
end
// Combine: alpha_q15 = x - x^2/2 + x^3/6 ; clamp to [0, 0x7FFF]
// Combine and clamp
acc = term1_q15 - term2_q15 + term3_q15;
if (acc < 0) acc = 0;
else if (acc > 16'h7FFF) acc = 16'h7FFF;
if (acc < 0) acc = 0;
else if (acc > 16'h7FFF) acc = 16'h7FFF;
alpha_q15_from_rc = acc;
end