Roflin/rebbarb
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

spi_mode3_slave: document the engine, and fix three stale comments

Browse Source 
418 -> 439 lines, 31% -> 14% comment-only.  The reasoning that matters here is
kept in full, because the edge conventions and the TX launch phase are the two
things in this design that cost silicon bugs; what went was the history around
them and the console measurement logs, which CLAUDE.md already holds.

Three comments described mechanisms that no longer exist:

  * `miso_next` was documented as feeding "an SB_IO output register clocked on
    the FALLING edge".  It is clocked on the RISING edge (NEG_TRIGGER=0) --
    that is the whole point of the full-cycle-early launch.  The file
    contradicted itself, since the comment on the assignment 100 lines later
    said "after the next RISING edge" correctly.
  * `id_arm` was documented as "the register file's id_phase, synchronised
    into this domain by the parent".  The one-shot was removed; id_arm is tied
    high and the register file has no id_phase.  It is now driven low only by
    the PHASE_METER build.
  * T8's rationale said the gate exists so that "once the GC's first NCRA
    reset clears id_phase, address 0 reads back NCRA".  That justification was
    not merely stale, it was WRONG -- NCRA is register space and the fast path
    requires command space, so they were never ambiguous.  T8's real job is to
    prove a disarmed build passes the FIFO byte through, which is what
    PHASE_METER depends on.

Structure: elaborate() now delegates to _rx_bytes, _device_id and _tx_bits,
each carrying the reasoning for its own block -- why rx_byte/rx_valid must be
combinational (a registered pulse loses the last byte of every transaction),
why the device ID is answered here rather than through the FIFOs, and the
output-path budget that forces the early launch.

VERIFIED EQUIVALENT: generated RTLIL is identical for BOTH build variants
(phase_meter False and True), 395 lines each, after stripping \src attributes.
Executable testbench code is unchanged apart from one trailing comment.  The
suite still has teeth: mutating the TX load back to rx_bit == 0 (a cycle late)
fails 8 of the 12 checks.  Full module suite passes and bba_top T9 is 6/6 at
EXI_HALF=1, 2 and 8.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Dennis Brentjes
2026-10-10 19:56:51 +02:00
parent b5961d0731
commit 41ba716759
1 changed files with 186 additions and 165 deletions
Download Patch File Download Diff File Expand all files Collapse all files
exi_bba/spi_mode3_slave.py
+184 -163
View File
@@ -1,38 +1,30 @@
"""SPI Mode 3 byte-oriented slave for the EXI bus — CLOCKED BY EXI CLK.
"""SPI Mode 3 byte-oriented slave for the EXI bus, clocked by the EXI clock.
CPOL=1, CPHA=1 (real Mode 3): CLK idles HIGH, the master CHANGES MOSI on the
falling edge and both ends SAMPLE on the rising edge.
Mode 3 is CPOL=1, CPHA=1: CLK idles HIGH, the master changes MOSI on the
FALLING edge, and both ends sample on the RISING edge.
Architecture
------------
The engine is clocked by the EXI clock itself rather than oversampling it from
a faster domain:
The engine is clocked by the EXI clock pin itself rather than oversampled from
a faster domain, so there is exactly one clock edge and one phase reference in
the whole module:
exiclk posedge of EXI CLK — sample MOSI, assemble RX bytes, AND
launch MISO one bit early (see the TX
note in elaborate)
CS_N asynchronous reset
exiclk posedge of EXI CLK. Samples MOSI, assembles RX bytes, and
launches MISO -- see `_tx_bits` for why MISO goes out on this
edge rather than the falling one.
spi_cs_n asynchronous reset of that domain.
Why, and what it fixes (hardware 2026-10-03)
--------------------------------------------
The previous version oversampled EXI CLK from a 54 MHz `capture` domain, which
put BIT-level timing across a clock-domain boundary: the engine's view of the
clock lagged the pin by 2 stages of `FFSynchronizer` plus a register, ~3 ticks
≈ 55 ns. One EXI bit at 27 MHz is 37 ns, so MISO came out ~2 bits late and the
master read `0x17` as `05 C0` (= `0x17 >> 2`). Register reads were correct up
to 4 MHz and failed from 8 MHz — the sweep read `ok=[1,1,1,0,0,0]`.
Oversampling cannot work here, which is why it is not an option to fall back
to: it puts BIT-level timing across a clock-domain boundary, and the engine's
view of the clock then lags the pin by two `FFSynchronizer` stages plus a
register -- about 55 ns against a 37 ns bit time at 27 MHz. Clocking from the
pin makes MISO clock-to-Q and leaves the byte-level `AsyncFIFO`s as the only
crossing, where a whole byte time (~296 ns) of slack is available instead of a
fraction of a bit.
No amount of oversampling fixes that: at 54 MHz there are only 2 ticks per bit,
and the design cannot close the 81 MHz that 3x would need. Clocking the engine
from the EXI pin makes MISO clock-to-Q instead, and moves the ONLY remaining
crossing to the byte-level AsyncFIFOs, which have a whole byte-time (~296 ns at
27 MHz) of slack instead of a fraction of a bit.
EXI CLK is on pin 44, which sits at die (6,0) on the 5k-sg48 = global buffer
input net 6, so it drives a real clock network with no board change.
EXI CLK (pin 44) sits at die (6,0) on the 5k-sg48, which is global buffer input
net 6 — so it can drive a real clock network with no board change.
The domains are created by the PARENT (`ExiCapture`) so the byte FIFOs can name
them; this module only uses `m.d.exiclk`.
The clock domain is created by the PARENT (`ExiCapture`) so the byte FIFOs can
name it; this module only uses `m.d.exiclk`.
"""
from amaranth import *
@@ -45,27 +37,40 @@ class SPIMode3Slave(Elaboratable):
Ports
-----
spi_clk / spi_mosi / spi_cs_n : raw inputs from the GC. spi_clk drives the
exiclk domain and spi_cs_n its async reset (both wired by
the parent); they are still listed here so the module is self-describing
and so spi_cs_n can gate MISO combinationally.
spi_miso : output to the GC; idles HIGH while CS is deasserted.
rx_byte : last complete received byte (valid while rx_valid is high)
rx_valid : 1-cycle pulse in `exiclk` when rx_byte holds a new byte
tx_byte : byte to transmit; sampled at each byte boundary
tx_load : 1-cycle pulse asking upstream for the next byte, one per byte
boundary. No header-length or slot-threshold logic: see the TX comment
in elaborate() for why unconditional pops are both sufficient and
necessary.
spi_clk, spi_mosi, spi_cs_n
Raw inputs from the GC. `spi_clk` drives the `exiclk` domain and
`spi_cs_n` is its async reset, both wired by the parent; they are
listed here so the module is self-describing.
spi_miso
Serial output to the GC. Idles HIGH while CS is deasserted.
miso_next
The same bit, but combinational from the pre-edge state, so it can
feed an SB_IO output register clocked on the RISING edge. The
synthesised build drives the pad from that register; see `synth.py`.
id_arm
Enables the device-ID fast path. Normally tied high -- real hardware
answers EXI_GetID whenever it is asked -- and driven low only by the
PHASE_METER diagnostic build, so the constant test pattern is not
replaced by ID bytes.
rx_byte
Last complete received byte, valid while `rx_valid` is high.
rx_valid
One-cycle pulse in `exiclk` when `rx_byte` holds a new byte.
tx_byte
Byte to transmit, sampled at each byte boundary.
tx_load
One-cycle pulse per byte boundary asking upstream for the next byte.
Unconditional: there is no header-length or slot-threshold logic, and
`_tx_bits` explains why that is both sufficient and necessary.
"""
def __init__(self, phase_meter=False):
# DIAGNOSTIC: when set, every byte the engine transmits is the constant
# 0x80 instead of the FIFO head. The load/shift/mux/output path is
# otherwise untouched, so the register file and both byte FIFOs drop
# out of the picture and what the host reads back is purely the
# engine's byte PHASE: 0x80 = aligned, 0x40 = 1 bit late, 0x20 = 2 bits
# late, and so on. Not a functional build.
# DIAGNOSTIC build: every transmitted byte becomes the constant 0x80
# instead of the FIFO head, leaving the load/shift/output path
# untouched. The register file and both byte FIFOs therefore drop out
# of the MISO path, and whatever the host reads back is purely the
# engine's byte PHASE -- 0x80 aligned, 0x40 one bit late, 0x20 two
# bits late. Not a functional bitstream.
self._phase_meter = phase_meter
self.spi_clk = Signal(init=1) # idles HIGH
@@ -73,15 +78,8 @@ class SPIMode3Slave(Elaboratable):
self.spi_cs_n = Signal(init=1) # active LOW
self.spi_miso = Signal(init=1)
# The SAME bit as spi_miso, but combinational from the PRE-edge state,
# so it can feed an SB_IO output register clocked on the falling edge.
# Driving MISO from a flop inside the IO block removes the fabric
# routing between the shift register and the pad, which is what the
# 27 MHz GameCube read fails on -- see the note in synth.py.
self.miso_next = Signal(init=1)
# Asserted while the device-ID query is still answerable (the register
# file's id_phase, synchronised into this domain by the parent).
self.id_arm = Signal()
self.rx_byte = Signal(8)
@@ -91,40 +89,59 @@ class SPIMode3Slave(Elaboratable):
def elaborate(self, platform):
m = Module()
rx_bit, nxt = self._rx_bytes(m)
id_sel, id_byte = self._device_id(m, rx_bit, nxt)
self._tx_bits(m, rx_bit, id_sel, id_byte)
return m
# ── RX: sample MOSI on the RISING edge (Mode 3 samples on edge 2) ──
def _rx_bytes(self, m):
"""Sample MOSI on the rising edge and assemble bytes, MSB first.
Returns `(rx_bit, nxt)`, where `nxt` is the shift register's next
value -- i.e. the complete byte during the final bit.
`rx_byte`/`rx_valid` are COMBINATIONAL rather than registered. The
clock stops and CS rises immediately after a transaction's final bit,
so a registered pulse would need one more clock edge that never
arrives, and the LAST byte of every transaction would be silently
lost: a 4-byte write landed as DE AD BE 00, and a 34-byte TX stream as
15 bytes. Driving them combinationally makes the parent's FIFO
capture the byte on the very edge that completes it.
"""
rx_shift = Signal(8)
rx_bit = Signal(3)
nxt = Cat(self.spi_mosi, rx_shift[:-1]) # MSB-first: shift left, new bit at LSB
m.d.exiclk += rx_shift.eq(nxt)
m.d.exiclk += rx_bit.eq(rx_bit + 1)
nxt = Cat(self.spi_mosi, rx_shift[:-1]) # shift left, new bit at LSB
m.d.exiclk += [rx_shift.eq(nxt),
rx_bit.eq(rx_bit + 1)]
# rx_byte/rx_valid are COMBINATIONAL, not registered. The clock stops
# and CS rises immediately after a transaction's final bit, so a
# registered pulse would need one more clock edge that never arrives
# and the LAST byte of every transaction would be silently lost
# (symptom: a 4-byte write landed as DE AD BE 00, and a 34-byte TX
# stream as 15 bytes). Driving them combinationally makes the FIFO
# capture the byte on the very edge that completes it.
m.d.comb += self.rx_byte .eq(nxt)
m.d.comb += self.rx_valid.eq(rx_bit == 7)
m.d.comb += [self.rx_byte .eq(nxt),
self.rx_valid.eq(rx_bit == 7)]
return rx_bit, nxt
# ── Device-ID fast path, answered HERE and not by the register file ──
# A command-space header is only TWO bytes, so a reply that travels
# rx AsyncFIFO -> sync FSM -> tx AsyncFIFO has one byte time (296 ns
# at 27 MHz) to make it back. That round trip costs ~280-320 ns, so
# it is marginal by construction, and measured on a console it LOST:
# the ID came back one byte late (00 04 02 02). The inter-transfer
# gap cannot rescue it either -- the reply must be in hand by the last
# rising edge of the header transfer, and the clock is paused after
# that edge, so nothing can update during the gap.
#
# The device ID is a CONSTANT and belongs to the physical interface
# rather than to the register file, so answer it from this domain and
# skip both crossings entirely. byte 0 == 0x00 identifies the query
# exactly: bit 7 clear = command space, bit 6 clear = read, reg = 0.
byte_idx = Signal(3) # bytes completed this transaction (saturating)
def _device_id(self, m, rx_bit, nxt):
"""Answer the device-ID query from this domain, not the register file.
Returns `(id_sel, id_byte)` for the TX mux.
A command-space header is only TWO bytes, so a reply that travels
rx `AsyncFIFO` → `sync` FSM → tx `AsyncFIFO` has one byte time to make
it back -- 296 ns at 27 MHz -- against a round trip costing roughly
280-320 ns. That is marginal by construction, and on a console it
lost: the ID came back one byte late.
The inter-transfer gap cannot rescue it, which is the non-obvious
part. The reply must be in hand by the LAST rising edge of the header
transfer, and the clock is paused after that edge, so no edge exists
during the gap for anything to update.
The device ID is a constant and belongs to the physical interface
rather than to the register file, so answering it here skips both
crossings. `byte 0 == 0x00` identifies the query exactly: bit 7 clear
selects command space, bit 6 clear selects read, and the register
number is 0.
"""
byte_idx = Signal(3) # bytes completed this transaction, saturating
id_mode = Signal()
with m.If(rx_bit == 7):
with m.If(byte_idx != 7):
@@ -140,56 +157,54 @@ class SPIMode3Slave(Elaboratable):
with m.Case(2): m.d.comb += id_byte.eq(0x02)
with m.Case(3): m.d.comb += id_byte.eq(0x02)
with m.Default(): m.d.comb += id_byte.eq(0x00)
id_sel = Signal()
m.d.comb += id_sel.eq(id_mode & (byte_idx >= 1) & (byte_idx <= 4))
return id_sel, id_byte
# ── TX: launch MISO a FULL CYCLE EARLY, on the RISING edge ────────
# The host samples MISO on the rising edge. The obvious thing is to
# drive it on the preceding FALLING edge, which gives the output path
# a HALF period -- 18.5 ns at 27 MHz. That is not enough on this
# part, and the shortfall is irreducible:
#
# MISO changes at (falling edge AT THE CONNECTOR)
# + input pad delay
# + GLOBAL CLOCK NETWORK INSERTION <-- the big one
# + IOB clock-to-Q
# + output pad delay
# ... and the host still needs its own setup time on top.
#
# Measured on a GameCube 2026-10-03 with PHASE_METER=1 (the engine
# drives a constant 0x80 per byte, so the host reads the engine's byte
# phase directly, with the register file and both byte FIFOs out of the
# path). Per-speed dump, first two slots per EXI speed index:
#
# 0:8080 1:8080 2:8080 3:8080 4:8080 5:C040
#
# Perfect to 16 MHz, one bit late at 27 MHz. An 8-slot dump at 27 MHz
# read C0 40 40 40 C0 40 40 40 -- a UNIFORM one-bit lag (ones at bit
# 1, 9, 17, 25, 33, ...), with extra highs at bits 0 and 32 where
# libogc splits the read into two 4-byte transfers and MISO tri-states
# to its pull-up in the gap.
#
# Crucially, ~9 ns was removed from the fabric output path first (the
# EXI_J4 pad merge, then the CS mux, then the whole fabric route by
# moving the output into an SB_IO register) and the result stayed BYTE
# IDENTICAL. Nine nanoseconds out of an 18.5 ns budget cannot change
# nothing if propagation delay were the whole story, so the remaining
# cost is the part no RTL change can touch: getting the clock in and
# the data back out of the chip.
#
# So stop shrinking the delay and DOUBLE THE BUDGET. Present the bit
# the host will sample at rising edge N on rising edge N-1, giving the
# whole path a FULL period (37 ns at 27 MHz) instead of half. The
# output then changes shortly AFTER the edge on which the host samples
# the previous bit, which is safe in both directions: the insertion
# delay that caused the problem now covers the host's hold time, and
# a full period covers its setup.
#
# `tx_out` is therefore clocked on the RISING edge and loads at
# rx_bit == 7 -- one bit-time earlier than a falling-edge shifter
# loading at rx_bit == 0. init=0xFF parks MISO high under CS reset.
# This also removes `tx_hold` and the whole exiclk_n domain: the
# engine now has ONE clock edge and ONE phase reference.
def _tx_bits(self, m, rx_bit, id_sel, id_byte):
"""Shift MISO out, launching each bit a FULL CYCLE EARLY.
The host samples MISO on the rising edge. The obvious thing is to
drive it on the preceding FALLING edge, which gives the output path a
half period -- 18.5 ns at 27 MHz. That is not enough on this part,
and the shortfall cannot be removed by any RTL change, because of what
the path is made of:
MISO changes at (falling edge AT THE CONNECTOR)
+ input pad delay
+ GLOBAL CLOCK NETWORK INSERTION <- the big one
+ IOB clock-to-Q
+ output pad delay
... and the host still needs its own setup time on top.
About 9 ns was removed from the fabric output path in three steps and
the console result stayed BYTE IDENTICAL each time. Nine nanoseconds
out of 18.5 cannot change nothing if propagation were the whole story,
so the rest is the irreducible part: getting the clock into the chip
and the data back out.
So the fix doubles the budget instead of shrinking the delay: present
the bit the host samples at rising edge N on rising edge N-1, giving
the whole path a FULL period (37 ns at 27 MHz). The output then
changes shortly AFTER the edge on which the host sampled the previous
bit, which is safe at both ends -- the insertion delay that caused the
problem now covers the host's hold time, and a full period covers its
setup.
If a LOW speed index ever regresses while 27 MHz still works, that is
the HOLD edge of this window (the launch being too early), not a setup
problem.
`tx_out` is therefore clocked on the rising edge and loads at
`rx_bit == 7`, one bit time earlier than a falling-edge shifter
loading at `rx_bit == 0`. `init=0xFF` parks MISO high under CS reset.
`tx_load` pops once per byte boundary unconditionally. That is
sufficient because the register file pushes its response during the
header's trailing pad byte, and necessary because the pops are also
what drains whatever a previous transaction left in the FIFO.
"""
tx_out = Signal(8, init=0xFF)
tx_src = Signal(8) # the byte loaded at the next byte boundary
m.d.comb += tx_src.eq(
@@ -202,13 +217,11 @@ class SPIMode3Slave(Elaboratable):
m.d.exiclk += tx_out.eq(Cat(0, tx_out[:-1]))
m.d.comb += self.spi_miso.eq(tx_out[7])
# What tx_out[7] WILL be after the next RISING edge -- this is what the
# What tx_out[7] WILL be after the next rising edge, which is what the
# SB_IO output register captures. Mirrors the load/shift choice above
# exactly; T6 proves the two agree bit for bit.
m.d.comb += self.miso_next.eq(Mux(rx_bit == 7, tx_src[7], tx_out[6]))
return m
# ── Testbench ─────────────────────────────────────────────────────────────
@@ -248,11 +261,16 @@ if __name__ == "__main__":
# A STRICT Mode 3 master. Unlike a convenience model that parks MOSI long
# before the edge, this one changes MOSI exactly ON the falling edge and
# samples MISO in the CLK-low phase. That is what makes the edge
# convention observable: a slave that sampled MOSI on the falling edge
# would capture the previous bit, and one that drove MISO on the rising
# edge would come out a bit late. Both bugs shipped to hardware against a
# lax master that could not see them.
# samples MISO in the CLK-LOW phase. Both details are what make the edge
# conventions observable at all:
#
# * A slave that sampled MOSI on the falling edge would capture the
# PREVIOUS bit. A master that parks MOSI early cannot see that, and
# a Mode 2/Mode 3 mix-up reached silicon against one that did.
# * MISO must be read in the clock-low phase, because the engine drives
# it ON the rising edge. Sampling just after raising the clock sees
# the NEXT bit in a zero-delay simulation, which would report
# everything one bit early while the hardware was correct.
async def spi_byte(ctx, mosi_val, probe_rx=False):
miso = 0
rx = None
@@ -284,9 +302,9 @@ if __name__ == "__main__":
ctx.set(dut.tx_byte, 0)
await ctx.delay(HP * 4)
# T1 — RX samples MOSI on the RISING edge, MSB first.
# rx_byte/rx_valid are combinational and peak during the final bit,
# which is the instant the parent's FIFO captures them.
# T1 — RX samples MOSI on the RISING edge, MSB first. rx_byte and
# rx_valid are combinational and peak during the final bit, which is
# the instant the parent's FIFO captures them.
await cs_low(ctx)
seen = []
for b in (0x80, 0x00, 0x31, 0x00):
@@ -297,7 +315,10 @@ if __name__ == "__main__":
check("T1 rx_valid", [v for v, _ in seen], [1, 1, 1, 1])
# T2 — TX: the byte latched at the end of slot N appears in slot N+1,
# MSB first. Slot 0 is the reset value, before any load.
# MSB first. Slot 0 is the header byte: the host ignores MISO there
# and nothing has been loaded yet, so it reads the CS-reset parking
# value (tx_out init=0xFF, which is what holds MISO high while
# deselected). The bits the host actually uses start at slot 1.
await cs_low(ctx)
ctx.set(dut.tx_byte, 0xA5)
s0 = await spi_byte(ctx, 0x00)
@@ -306,10 +327,6 @@ if __name__ == "__main__":
ctx.set(dut.tx_byte, 0x00)
s2 = await spi_byte(ctx, 0x00)
await cs_high(ctx)
# Slot 0 is the header byte: the host ignores MISO there and the
# engine has not loaded anything yet, so it reads the CS-reset parking
# value (tx_out init=0xFF, which is what keeps MISO high while
# deselected). The bits the host actually uses start at slot 1.
check("T2 miso slot0 (pre-load)", hex(s0), hex(0xFF))
check("T2 miso slot1", hex(s1), hex(0xA5))
check("T2 miso slot2", hex(s2), hex(0x3C))
@@ -333,16 +350,18 @@ if __name__ == "__main__":
# T5 — a transaction that BEGINS ON A RISING EDGE. The host asserts
# CS while CLK is already low, so the first edge the device sees is a
# rise and the transaction carries one fewer FALLING edge than rising.
# rise and the transaction carries one fewer falling edge than rising.
# A TX phase counted on falling edges slips one bit here and never
# recovers; a phase taken from rx_bit cannot.
#
# THIS IS THE HARDWARE BUG. On a GameCube the phase-meter build read
# 0x20 (= 0x80 >> 2) in every slot, identically at 1 MHz and 27 MHz.
# Nothing in the old testbench started a transaction this way -- every
# case asserted CS with CLK high -- so the falling and rising counts
# always matched and the slip was structurally invisible, exactly as
# the Mode 2/3 mix-up was invisible to a master that parked MOSI early.
# THIS IS A HARDWARE BUG THIS TEST EXISTS TO CATCH. With a
# free-running falling-edge counter the console's phase-meter build
# read 0x20 (= 0x80 >> 2) in every slot, identically at 1 MHz and
# 27 MHz -- speed independence being the tell that it was not a
# propagation problem. Every earlier test asserted CS with CLK high,
# so falls and rises always matched and the slip was structurally
# invisible. When adding SPI tests, vary the edge the transaction
# starts on.
ctx.set(dut.spi_clk, 0) # CLK already LOW ...
ctx.set(dut.spi_cs_n, 0) # ... when CS asserts
await ctx.delay(HP)
@@ -355,11 +374,11 @@ if __name__ == "__main__":
check("T5 rising-edge start, slot1", hex(s1), hex(0xA5))
check("T5 rising-edge start, slot2", hex(s2), hex(0x3C))
# T6 — miso_next must equal what spi_miso BECOMES on the next RISING
# T6 — miso_next must equal what spi_miso BECOMES on the next rising
# edge. The synthesised build drives the pad from an SB_IO register
# clocked on that edge (synth.py), so if this ever diverges the
# hardware and the simulation stop agreeing about MISO -- which is
# precisely the class of bug that has cost this design the most time.
# clocked on that edge (synth.py), so if these ever diverge then the
# hardware and the simulation stop agreeing about MISO -- precisely
# the class of bug that has cost this design the most time.
ctx.set(dut.spi_clk, 1)
ctx.set(dut.spi_cs_n, 0)
await ctx.delay(HP)
@@ -369,7 +388,7 @@ if __name__ == "__main__":
for _bit in range(8):
ctx.set(dut.spi_clk, 0)
await ctx.delay(HP)
predicted = ctx.get(dut.miso_next) # before the RISING edge
predicted = ctx.get(dut.miso_next) # before the rising edge
ctx.set(dut.spi_clk, 1)
await ctx.delay(HP)
if ctx.get(dut.spi_miso) != predicted:
@@ -379,9 +398,10 @@ if __name__ == "__main__":
# T7 — the device-ID fast path. A command-space query (byte 0 = 0x00,
# byte 1 = 0x00) must be answered 04 02 02 00 straight from this
# domain, with the tx FIFO empty: the register file cannot make the
# round trip inside a 2-byte header's one byte time, which on a
# console returned the ID one byte late (00 04 02 02).
# domain, with the tx FIFO deliberately feeding 0x00 so that only the
# fast path can be the source. The register file cannot make the
# round trip inside a 2-byte header's one byte time; on a console it
# returned the ID one byte late.
ctx.set(dut.id_arm, 1)
ctx.set(dut.tx_byte, 0x00) # FIFO deliberately contributes nothing
await cs_low(ctx)
@@ -392,9 +412,10 @@ if __name__ == "__main__":
check("T7 device ID", [hex(b) for b in got],
[hex(x) for x in (0x04, 0x02, 0x02, 0x00)])
# T8 — and it must be GATED. Once the GC's first NCRA reset clears
# id_phase, address 0 reads back NCRA, so the fast path must stop
# hijacking the reply and let the FIFO through.
# T8 — and the fast path must be GATED by id_arm, so that a build
# which disarms it gets the FIFO byte through untouched. That is what
# the PHASE_METER bitstream relies on: its constant 0x80 pattern would
# otherwise be replaced by ID bytes in the slots it measures.
ctx.set(dut.id_arm, 0)
ctx.set(dut.tx_byte, 0x5A)
await cs_low(ctx)