docs: frame sync runs as PicoRV32 firmware, zero extra RTL

Reuses existing syndrome_wt register and converged flag.
No additional hardware needed on the ASIC.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/project_report.md

@@ -277,13 +277,33 @@ A valid codeword satisfies all parity checks (syndrome weight = 0). A random 256
 
 If the offset drifts (e.g., clock slip), the receiver first searches locally within +/-16 positions of the last known offset. Only if that fails does it fall back to full acquisition. Local re-sync is nearly instant since it screens ~33 candidates instead of 256.
 
-### 7.4 RTL Implications
+### 7.4 Implementation: PicoRV32 Firmware (Zero Extra RTL)
 
-The frame sync logic is simple hardware:
+Frame sync runs entirely in firmware on the Caravel PicoRV32. No additional hardware is needed -- the decoder already exposes everything the algorithm requires:
-- **Syndrome screening:** reuses the same syndrome checker already in the decoder. Just feed it hard decisions from different offsets.
+
-- **State machine:** ACQUIRE -> LOCKED -> RESYNC (on decode failure) -> local search -> fallback to ACQUIRE
+- **SYNDROME_WT register (0x5C):** read back syndrome weight after a 1-iteration "quick check"
-- **No extra memory:** operates on the incoming LLR stream, one window at a time
+- **STATUS.converged flag:** tells firmware whether a full decode succeeded
-- **Firmware option:** could also run entirely in PicoRV32 firmware if area is tight, since it's not time-critical (only runs once at startup or after link loss)
+- **LLR_IN port:** firmware writes LLRs at each candidate offset alignment
+
+The firmware loop:
+
+```c
+for (offset = 0; offset < 256; offset++) {
+    write_llrs(stream + offset);     // load 256 LLRs at this alignment
+    start_decode(max_iter=1);        // 1 iteration = quick syndrome screen
+    if (read_syndrome_wt() < 50) {   // promising candidate
+        start_decode(max_iter=30);   // full decode
+        if (read_converged())        // confirm with next 2 frames
+            if (confirm_next_frames()) return offset;
+    }
+}
+```
+
+**Why firmware, not a hardware FSM:**
+- Frame sync only runs at link startup or after losing lock -- not on the data path
+- PicoRV32 at 150 MHz screens all 256 offsets in well under 1 ms
+- Thresholds and confirmation count are tunable without a respin
+- Saves ~0.05 mm^2 that would otherwise go to a rarely-used state machine
 
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