Fix extended native CRT protocol compatibility

[wizzomafizzo]

Summary

• Accept both legacy native-video magic 0x5A50 and frontend extended 0x5A51, decoding their different mode/vertical-offset layouts explicitly.
• Widen signed vertical path and implement the common safe -31..+9 pixel / -14..+2 line trim window while preserving legacy clamps and line/field totals.
• Cover exact PAL control word, wider trims, legacy compatibility, unknown magic, and progressive/interlaced timing in RTL regressions.

Verification

All six Icarus suites passed. Quartus compile and setup/hold/recovery/removal/pulse-width timing gate passed; timing-gate Python tests passed. Hardware PAL output visible with unchanged extended word 0x5A5100F6 after explicitly loading new RBF; updated HDMI scanout/OSD also checked.

Analog CRT quality remains unqualified. Kernel/module ABI unchanged. Load this Menu before enabling Slint native CRT; legacy writers remain supported. Replacing RBF and restarting Main alone does not program FPGA—use normal load_core path. Rollback RBF preserved.

Related frontend: ZaparooProject/zaparoo-frontend#457
Related Main: ZaparooProject/Main_MiSTer#26

Summary by CodeRabbit

• New Features
  • Added support for extended video control blocks with expanded signed horizontal and vertical offset ranges.
  • Added support for PAL and 480i timing adjustments using the extended control format.
  • Preserved compatibility with legacy control blocks, including their existing offset limits.
• Bug Fixes
  • Invalid control modes now safely default to mode 0.
  • Improved handling of extreme timing offsets while preserving correct video field timing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 965ea0d5-d544-4f6a-bd7e-daa66bc2d1b0

📥 Commits

Reviewing files that changed from the base of the PR and between e8c8680 and 5c2cb49.

📒 Files selected for processing (5)

• rtl/native_video_reader.sv
• rtl/native_video_timing.sv
• rtl/native_video_top.sv
• tb/native_video_reader_tb.sv
• tb/native_video_timing_tb.sv

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The video path now supports legacy 0x5A50 and extended 0x5A51 control blocks. It carries signed 6-bit vertical offsets, applies expanded horizontal and vertical timing ranges, preserves legacy clamping, and adds protocol and timing coverage.

Changes

Extended video offsets

Layer / File(s)	Summary
Reader protocol handling rtl/native_video_reader.sv, tb/native_video_reader_tb.sv	The reader recognizes 0x5A50 and 0x5A51, parses extended mode and signed offsets, applies legacy clamping, widens v_offset_out, and rejects unknown magic values. The reader testbench covers extended, legacy, and invalid blocks.
Timing offset ranges rtl/native_video_timing.sv, rtl/native_video_top.sv, tb/native_video_timing_tb.sv	The timing path carries signed 6-bit vertical offsets and clamps horizontal offsets to -31..+9 and vertical offsets to -14..+2. The synchronizer and timing tests use the widened signal and verify NTSC, PAL, and 480i timing.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant DDR
  participant native_video_reader
  participant native_video_top
  participant native_video_timing
  DDR->>native_video_reader: Provide 0x5A50 or 0x5A51 control block
  native_video_reader->>native_video_top: Output mode and signed offsets
  native_video_top->>native_video_timing: Synchronize and forward offsets
  native_video_timing->>native_video_timing: Clamp offsets and calculate sync starts

Loading

Merge Risk: ⚪ Minimal · up to 5c2cb

The extended protocol and signed offset paths are consistent across the reader, top-level, timing module, and covered testbenches, with no unresolved merge risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: support for extended native CRT protocol compatibility. It is concise and directly related to the protocol, RTL, and testbench updates.

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