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Corrected noise LFSR writeback
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@daglem
daglem committed May 28, 2023
1 parent 442d7d1 commit 0b5d34b
Showing 1 changed file with 59 additions and 35 deletions.
94 changes: 59 additions & 35 deletions gateware/sid_waveform.sv
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,7 @@ module sid_waveform #(
logic [5:0] osc19_prev = '0;
logic [5:0] osc19_prev_up = '0;
logic [5:0] pulse = '0;
logic [5:0] pulse_prev = '0;
logic pulse_next;

always_comb begin
Expand Down Expand Up @@ -135,6 +136,7 @@ module sid_waveform #(
osc19_prev <= { osc19_prev[4:0], osc19_prev[5] };
osc19_prev_up <= { osc19_prev_up[4:0], osc19_prev_up[5] };
pulse <= { pulse[4:0], pulse[5] };
pulse_prev <= { pulse_prev[4:0], pulse_prev[5] };

if (cycle >= 2 && cycle <= 7) begin
// Update oscillators.
Expand All @@ -151,7 +153,8 @@ module sid_waveform #(
osc19_prev_up[0] <= ~osc19_prev[5] & o5[19];

// Pulse.
pulse[0] <= pulse_next;
pulse[0] <= pulse_next;
pulse_prev[0] <= pulse[5];
end

if (cycle == 4 || cycle == 7) begin
Expand Down Expand Up @@ -210,6 +213,8 @@ module sid_waveform #(
logic nclk_prev = 0;
logic nset;
sid::reg8_t noise;
sid::reg8_t noise_lsl;
sid::reg8_t noise_lsl_writeback;

// Noise.
always_comb begin
Expand All @@ -221,8 +226,10 @@ module sid_waveform #(
NOISE_TTL_8580));

// Noise bits are taken out from the noise LFSR.
noise = { n0.lfsr[20], n0.lfsr[18], n0.lfsr[14], n0.lfsr[11],
n0.lfsr[9], n0.lfsr[5], n0.lfsr[2], n0.lfsr[0] };
noise = { n0.lfsr[20], n0.lfsr[18], n0.lfsr[14], n0.lfsr[11],
n0.lfsr[9], n0.lfsr[5], n0.lfsr[2], n0.lfsr[0] };
noise_lsl = { n0.lfsr[21], n0.lfsr[19], n0.lfsr[15], n0.lfsr[12],
n0.lfsr[10], n0.lfsr[6], n0.lfsr[3], n0.lfsr[1] };
end

always_ff @(posedge clk) begin
Expand All @@ -238,7 +245,7 @@ module sid_waveform #(
// The LFSR stays at 'h7ffffe after reset (clocked on reset release).
n0.nres_prev <= nres;
n0.nclk_prev <= nclk;
nclk_prev <= n0.nclk_prev;
nclk_prev <= n5.nclk_prev;

if (~nclk || !primed) begin
// LFSR shift phase 1.
Expand Down Expand Up @@ -269,10 +276,11 @@ module sid_waveform #(
// Shift phase != 1 - writeback to current bits.
if (waveform_5 > 'h8) begin
{ n1.lfsr[20], n1.lfsr[18], n1.lfsr[14], n1.lfsr[11],
n1.lfsr[9], n1.lfsr[5], n1.lfsr[2], n1.lfsr[0] } <= wav[11-:8];
n1.lfsr[9], n1.lfsr[5], n1.lfsr[2], n1.lfsr[0] } <=
wav[11-:8];
end

// Shift phase 2 - writeback to shifted bits.
// Shift phase 1 -> 2: Writeback to shifted bits.
// The transition from shift phase 1 to phase 2 coincides
// with the transition of waveforms and waveform selectors
// at the rising edge of phi1. This implies that in theory,
Expand All @@ -284,18 +292,19 @@ module sid_waveform #(
// constant during the transition. The sawtooth and
// triangle waveform selectors can always transition,
// however.
// FIXME: The check for the pulse waveform is adapted from
// VICE in order to pass more tests, and should probably be
// replaced with a generic model for bit zeroing (see other
// FIXME below).
if (~nclk_prev &&
wav_prev.waveform[5] > 'h8 && // Combined waveforms in the previous cycle.
waveform_5 > 'h8) // Combined waveforms in the current cycle.
wav_prev.waveform[5] > 'h8 && // Combined waveforms in the previous cycle.
!(wav_prev.waveform[5] == 'hc && // Not only noise + pulse high in the
pulse_prev[3]) && // previous cycle.
waveform_5 > 'h8) // Combined waveforms in the current cycle.
begin
// For now, we assume that the corresponding previous
// and current waveform bits must both be zero for a
// latched noise bit to be pulled down.
// FIXME: Come up with a model which better matches
// reality.
{ n1.lfsr[21], n1.lfsr[19], n1.lfsr[15], n1.lfsr[12],
n1.lfsr[10], n1.lfsr[6], n1.lfsr[3], n1.lfsr[1] } <=
noise & (wav_prev.wav[5][11-:8] | wav[11-:8]);
noise_lsl_writeback;
end
end
end
Expand Down Expand Up @@ -330,8 +339,8 @@ module sid_waveform #(

always_ff @(posedge clk) begin
// Rotation on cycles 5 - 10 for update of 8580 sawtooth / triangle.
// Rotation on six additional cycles for final mix-in of 8580 sawtooth /
// triangle at phi2.
// Freewheeling rotation on six additional cycles for final mix-in of
// 8580 sawtooth / triangle at phi2.
if (cycle >= 5 || cycle_16) begin
{ st5, st4, st3, st2, st1, st0 } <= { st4, st3, st2, st1, st0, st5 };

Expand Down Expand Up @@ -373,11 +382,12 @@ module sid_waveform #(
sid::reg8_t sid_waveform_P_T_8580[2048];

always_ff @(posedge clk) begin
// Cycles 5 - 10 for OSC3 output and 6581 waveforms, cycles 11 - 16 for
// final 8580 waveforms.
// Cycles 5 - 10 for OSC3 and audio output. Cycles 11 - 16 for final
// mix-in of 8580 sawtooth / triangle, used in 8580 noise LFSR
// writeback and 8580 waveform 0 output in the next SID cycle.
if (cycle >= 5 || cycle_16) begin
// Waveform candidates: Combined waveforms from BRAM and
// combinational logic, and sawtooth / triangle.
// combinational logic, plus sawtooth / triangle.
// Noise and pulse are mixed in below.
pst <= sid_waveform_PST(model_4, saw_tri);
ps__6581 <= sid_waveform_PS__6581[saw_tri];
Expand All @@ -390,22 +400,9 @@ module sid_waveform #(
end

sid::reg8_t noise_mask;
sid::reg8_t noise_lsl_mask;

always_comb begin
// Zero noise outputs always zero the corresponding waveform bits, and can
// also zero neighboring bits when pulse is selected.
if (waveform_5[2]) begin
// The four lowermost bits of the noise waveform are grounded, and
// always pull the neighboring two bits down via the pulse line.
// The bit mask below is adapted from reSID in VICE. The same bit
// mask is used for the 6581 as well as for the 8580 here, since at
// least one 6581 shows the same behavior.
noise_mask = (noise < 'hfc) ? { noise[7:1] & noise[6:0], 1'b0 } : 'hfc;
end else begin
// Pulse not selected.
noise_mask = noise;
end

// Final waveform selection / mixing, excluding noise.
unique case (waveform_5[2:0])
'b111: wav = { pst & { 8{pulse[3]} }, 4'b0 };
Expand All @@ -418,13 +415,40 @@ module sid_waveform #(
'b000: wav = wav_prev.wav[5]; // Waveform 0
endcase

// Zero noise outputs always zero the corresponding waveform bits, and
// can also zero neighboring bits when pulse is selected.
// FIXME: All combinations of noise and waveforms should be
// investigated for bit zeroing. For now, we adapt the 8580 pulse bit
// mask from VICE.
// We perform writeback to both shifted and current bits after any LFSR
// shift, in order to avoid a further delay of the waveform
// output. Note note that in theory, the zeroing of bits may be
// different for shifted (latched) and current (SRAM) bits.
if (waveform_5[2]) begin
// The four lowermost bits of the noise waveform are grounded, and
// always pull the neighboring two bits down via the pulse line.
noise_mask = (noise < 'hfc) ? { noise[7:1] & noise[6:0], 1'b0 } : 'hfc;
noise_lsl_mask = (noise_lsl < 'hfc) ? { noise_lsl[7:1] & noise_lsl[6:0], 1'b0 } : 'hfc;
end else begin
// Pulse not selected.
noise_mask = noise;
noise_lsl_mask = noise_lsl;
end

// Noise LFSR writeback to shifted bits at shift phase 1 -> 2.
// We assume that the corresponding previous and current waveform bits
// must both be zero for a latched noise bit to be pulled down.
noise_lsl_writeback =
(wav_prev.wav[5][11-:8] | (wav[11-:8] & noise_lsl_mask));

// Mix in noise.
if (waveform_5[3]) begin
if (waveform_5[2:0] == '0) begin
// Only noise is selected.
wav = { noise, 4'b0 };
end else begin
// Noise outputs can zero waveform output bits.
// Noise outputs can zero waveform output bits. The result is
// also written back to noise LFSR bits at shift phase != 1.
wav = { wav[11-:8] & noise_mask, 4'b0 };
end
end
Expand All @@ -439,7 +463,7 @@ module sid_waveform #(
// Update of previous waveform selector and waveform output, for
// noise writeback and waveform 0 output.
if (cycle >= 6 || cycle_16 || cycle_17) begin
wav_prev.waveform <= { wav_prev.waveform[4:0], wav_prev.waveform[5] };
wav_prev.waveform <= { wav_prev.waveform[4:0], waveform_5 };
wav_prev.wav <= { wav_prev.wav[4:0], wav_prev.wav[5] };
wav_prev.age <= { wav_prev.age[4:0], wav_prev.age[5] };

Expand Down

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