Replaces 54 MHz oversampling with a slave clocked by EXI CLK itself:
exiclk posedge sample MOSI (Mode 3 samples on edge 2)
exiclk_n negedge drive MISO (must settle between the master's samples)
CS_N async reset for both
EXI CLK (pin 44) is at die (6,0) = global buffer net 6 on the 5k-sg48, so it
can drive a real clock network with no board change.
This removes the capture domain entirely. Bit-level timing no longer
crosses a clock boundary -- MISO is clock-to-Q instead of 3 ticks of a foreign
clock -- and the only crossings left are the byte FIFOs, which have a whole
byte-time (~296 ns) of slack rather than a fraction of a bit.
AsyncFIFO needs a posedge clock for its internal reset CDC, so the FIFO sides
use reset-less POSEDGE domains on the same pin; the engine pops on the posedge
and hands the byte to the negedge output stage through a register. The FIFOs
must not see CS: resetting one side of an AsyncFIFO desynchronises its gray
pointers.
Two bugs found and fixed along the way:
* one pop per byte boundary is all that is needed -- pops during the header
drain whatever the previous transaction left (replacing the CS-fall flush)
and the turnaround-byte push lands in data slot 0. No slot-threshold or
header-length logic at all.
* rx_byte/rx_valid had to become COMBINATIONAL. Registered, they need one
more clock edge, and after a transaction's final bit the clock stops and
CS rises -- so the last byte of every transaction was lost (a 4-byte write
landed as DE AD BE 00, a 34-byte TX as 15 bytes).
At the HARDWARE rate (EXI_HALF=1, 27 MHz) the device ID, NWAYS, PAR
read/write and the W5100->SPRAM RX path now all pass -- none of which has ever
worked at 27 MHz before. Testbenches now drive the SPI clock with real
delays, since there is no capture domain to count ticks against.
STILL BROKEN: the SPRAM/DMA stream is offset by one byte (its round trip
exceeds the single turnaround byte), and TX lands 15 of 34 bytes. Do not
flash this yet.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Replaces the invented 2-byte header (addr split across both bytes, 2-bit
length field) with the protocol libogc and Swiss actually speak:
command (bit7=0, 6-bit reg) [reg, 00, 00, data] one 4-byte transfer
register (bit7=1, 16-bit reg) [0x80|w<<6, ah, al, 00] + data until CS
This was not an optimisation: the old decoder read Swiss's NWAYS read
(80 00 31 00) as 'write 0x31 to NCRA', and bit 0 of 0x31 is RESET, so every
register read would have reset the device.
Changes:
* 16-bit address; no length field -- register reads and writes now stream
until CS, like the SPRAM path already did
* RHDR_LO starts the SPRAM fetch as soon as the address completes, using
the trailing header byte as the prefetch window the protocol provides
* command space serves EXI regs 0x02/0x03 (interrupt mask/status), the only
two Swiss uses
* EXI_GetID gets its own path: it offers just ONE header byte of warning, so
the first ID byte is pushed the instant byte 0 identifies the query
Two timing subtleties, both found by simulation and commented in place:
* MISO is driven LIVE from the tx FIFO head, so a byte pushed while the pad
byte is on the wire appears in the pad slot. A filler byte is parked
there (MISO is don't-care through the header) so the first real response
lands in data slot 0.
* the prefetch must be one-shot; re-firing while waiting for the pad byte
consumed the register stream early and shifted reads by a byte.
All testbenches updated to the real format and passing: bba_register_file
(7), bba_top (8 incl. the full W5100->SPRAM->GC loop and TX), exi_j4,
exi_capture, spi_mode3_slave, tx_frame_drain, spram_arbiter,
rx_frame_assembler, w5100_parallel_master. gc_bba_test rebuilt against the
new protocol.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Factors the EXI_J4 pad merge out of synth.py so the synthesis path and the
test exercise the same code. Verifies the device ID (04 02 02 00) enumerates
over SP1 with J4 idle, over J4 with SP1 idle, and over SP1 again afterwards,
plus a register read via J4 and the full cs_n truth table.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Roflin