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EXPERIMENT/xbeam: add probabilistic phosphor decay skip #164

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6588a4c
top/xbeam/persist: add probabilistic decay skip
vk2seb Apr 3, 2026
dc27776
top/xbeam: s/skip/rnd-skip
vk2seb Apr 3, 2026
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5 changes: 5 additions & 0 deletions gateware/src/rs/hal/src/persist.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
pub trait Persist {
fn set_persist(&mut self, value: u16);
fn set_decay(&mut self, value: u8);
fn set_skip(&mut self, value: u8);
}

#[macro_export]
Expand Down Expand Up @@ -29,6 +30,10 @@ macro_rules! impl_persist {
self.registers.decay().write(|w| unsafe { w.decay().bits(value) } );
}

fn set_skip(&mut self, value: u8) {
self.registers.skip().write(|w| unsafe { w.skip().bits(value) } );
}

}
)+
};
Expand Down
30 changes: 27 additions & 3 deletions gateware/src/tiliqua/raster/persist.py
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,7 @@ def __init__(self, *, bus_signature,
# Tweakables
"holdoff": In(16, init=holdoff_default),
"decay": In(4, init=1),
"skip": In(8, init=0),
# DMA bus / fb
"bus": Out(bus_signature),
"fbp": In(DMAFramebuffer.Properties()),
Expand Down Expand Up @@ -70,11 +71,21 @@ def elaborate(self, platform) -> Module:

# Latched version of decay speed control input
decay_latch = Signal.like(self.decay)
# Latched version of skip probability control input
skip_latch = Signal.like(self.skip)
# Track delay between read/write bursts
holdoff_count = Signal(32)
# Incoming pixel array (read from FIFO)
pixels_r = Signal(data.ArrayLayout(Pixel, 4))

# Free-running LFSR for probabilistic pixel skipping.
lfsr0 = Signal(unsigned(32), init=0x67452301)
lfsr1 = Signal(unsigned(32), init=0xefcdab89)
lfsr1_next = Signal(unsigned(32))
m.d.comb += lfsr1_next.eq(lfsr1 + lfsr0)
m.d.sync += lfsr1.eq(lfsr1_next)
m.d.sync += lfsr0.eq(lfsr0 ^ lfsr1_next)

m.d.comb += self.fifo.w_data.eq(bus.dat_r)

# Used for fastpath when all pixels are zero
Expand All @@ -101,6 +112,7 @@ def elaborate(self, platform) -> Module:

with m.State('BURST-IN'):
m.d.sync += decay_latch.eq(self.decay)
m.d.sync += skip_latch.eq(self.skip)
m.d.comb += [
bus.stb.eq(1),
bus.cyc.eq(1),
Expand Down Expand Up @@ -135,12 +147,17 @@ def elaborate(self, platform) -> Module:

with m.State('BURST-OUT'):
# The actual persistance calculation. 4 pixels at a time.
#
# Per-pixel LFSR comparison decides whether to decay or
# write back unchanged (probabilistic skip).
pixels_w = Signal(data.ArrayLayout(Pixel, 4))
for n in range(4):
# color
skip_this = Signal(name=f"skip_{n}")
m.d.comb += skip_this.eq(lfsr1[n*8:(n*8)+8] < skip_latch)
m.d.comb += pixels_w[n].color.eq(pixels_r[n].color)
# intensity
with m.If(pixels_r[n].intensity >= decay_latch):
with m.If(skip_this):
m.d.comb += pixels_w[n].intensity.eq(pixels_r[n].intensity)
with m.Elif(pixels_r[n].intensity >= decay_latch):
m.d.comb += pixels_w[n].intensity.eq(pixels_r[n].intensity - decay_latch)
with m.Else():
m.d.comb += pixels_w[n].intensity.eq(0)
Expand Down Expand Up @@ -184,6 +201,9 @@ class PersistReg(csr.Register, access="w"):
class DecayReg(csr.Register, access="w"):
decay: csr.Field(csr.action.W, unsigned(8))

class SkipReg(csr.Register, access="w"):
skip: csr.Field(csr.action.W, unsigned(8))

def __init__(self, bus_dma):
self.en = Signal()
self.persist = Persistance(bus_signature=bus_dma.bus.signature.flip())
Expand All @@ -193,6 +213,7 @@ def __init__(self, bus_dma):

self._persist = regs.add("persist", self.PersistReg(), offset=0x0)
self._decay = regs.add("decay", self.DecayReg(), offset=0x4)
self._skip = regs.add("skip", self.SkipReg(), offset=0x8)

self._bridge = csr.Bridge(regs.as_memory_map())

Expand All @@ -216,4 +237,7 @@ def elaborate(self, platform):
with m.If(self._decay.f.decay.w_stb):
m.d.sync += self.persist.decay.eq(self._decay.f.decay.w_data)

with m.If(self._skip.f.skip.w_stb):
m.d.sync += self.persist.skip.eq(self._skip.f.skip.w_data)

return m
13 changes: 8 additions & 5 deletions gateware/src/top/xbeam/fw/src/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -60,13 +60,14 @@ fn build_cc_mapper(opts: &Opts) -> MidiCcMapper {
m.add(31, global_index(opts, &opts.delay.delay_y), CcMapMode::Absolute);
m.add(32, global_index(opts, &opts.delay.delay_i), CcMapMode::Absolute);
m.add(33, global_index(opts, &opts.delay.delay_c), CcMapMode::Absolute);
// Beam page (CC 40-45)
// Beam page (CC 40-46)
m.add(40, global_index(opts, &opts.beam.persist), CcMapMode::Absolute);
m.add(41, global_index(opts, &opts.beam.decay), CcMapMode::Absolute);
m.add(42, global_index(opts, &opts.beam.ui_hue), CcMapMode::Absolute);
m.add(43, global_index(opts, &opts.beam.palette), CcMapMode::Absolute);
m.add(44, global_index(opts, &opts.beam.grid), CcMapMode::Absolute);
m.add(45, global_index(opts, &opts.beam.grid_i), CcMapMode::Absolute);
m.add(42, global_index(opts, &opts.beam.rnd_skip), CcMapMode::Absolute);
m.add(43, global_index(opts, &opts.beam.ui_hue), CcMapMode::Absolute);
m.add(44, global_index(opts, &opts.beam.palette), CcMapMode::Absolute);
m.add(45, global_index(opts, &opts.beam.grid), CcMapMode::Absolute);
m.add(46, global_index(opts, &opts.beam.grid_i), CcMapMode::Absolute);
// Misc page (CC 50-52)
m.add(50, global_index(opts, &opts.misc.plot_type), CcMapMode::Absolute);
m.add(51, global_index(opts, &opts.misc.plot_src), CcMapMode::Absolute);
Expand Down Expand Up @@ -283,9 +284,11 @@ fn main() -> ! {
opts.beam.ui_hue.value).ok();
persist.set_persist(128);
persist.set_decay(1);
persist.set_skip(0);
} else {
persist.set_persist(opts.beam.persist.value);
persist.set_decay(opts.beam.decay.value);
persist.set_skip(opts.beam.rnd_skip.value);
}


Expand Down
3 changes: 3 additions & 0 deletions gateware/src/top/xbeam/fw/src/options.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,7 @@ int_params!(DelayParams<u16> { step: 8, min: 0, max: 512, format: IntFormat
int_params!(PCScaleParams<u8> { step: 1, min: 0, max: 15 });
int_params!(PersistParams<u16> { step: 32, min: 32, max: 4096 });
int_params!(DecayParams<u8> { step: 1, min: 0, max: 15 });
int_params!(SkipParams<u8> { step: 16, min: 0, max: 240 });
int_params!(IntensityParams<u8> { step: 1, min: 0, max: 15 });
int_params!(HueParams<u8> { step: 1, min: 0, max: 15 });
int_params!(TriggerLvlParams<i16> { step: 500, min: -16000, max: 16000, format: IntFormat::Scaled { divisor: 4000, precision: 2, suffix: "V" } });
Expand Down Expand Up @@ -145,6 +146,8 @@ pub struct BeamOpts {
pub persist: IntOption<PersistParams>,
#[option(1)]
pub decay: IntOption<DecayParams>,
#[option(0)]
pub rnd_skip: IntOption<SkipParams>,
#[option(10)]
pub ui_hue: IntOption<HueParams>,
#[option]
Expand Down
9 changes: 5 additions & 4 deletions gateware/src/top/xbeam/top.py
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Original file line number Diff line number Diff line change
Expand Up @@ -106,10 +106,11 @@

BEAM persist 40 phosphor decay speed (high = slow)
BEAM decay 41 phosphor decay amount (low = slow)
BEAM ui-hue 42 menu and grid overlay hue
BEAM palette 43 color palette
BEAM grid 44 grid overlay style
BEAM grid-i 45 grid overlay intensity
BEAM rnd-skip 42 probabilistic decay skip (high = more)
BEAM ui-hue 43 menu and grid overlay hue
BEAM palette 44 color palette
BEAM grid 45 grid overlay style
BEAM grid-i 46 grid overlay intensity

MISC plot-type 50 vectorscope or oscilloscope
MISC plot-src 51 plot inputs or outputs
Expand Down
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