Capturing HDMI video on a Raspberry Pi, for a PiKVM-style remote console, a stream box or a test rig, has long meant a TC358743 bridge board limited to 1080p at 30 to 60 frames per second. Engineer adouglass52 has released three open-hardware HDMI to MIPI CSI-2 boards built around the Rockchip RK628H that reach 4K30 and 1080p120 on a single 4-lane link, with drivers that run on standard Raspberry Pi OS.
What the boards do
The boards came out of building WarpKVM, the maker's own KVM. All three use the RK628H, which turns an HDMI input into a MIPI CSI-2 camera stream, the same interface a Pi camera uses. The 22-pin board measures 45 x 30 mm, offers 4K30 and 1080p120 and passes HDMI audio out on an I2S header. The 30-pin board adds an on-board DAC and a 3.5 mm jack and was tested on a Firefly ROC-RK3576-PC. A dual 22-pin board uses two 4-lane links for 4K60 capture; that circuit was proven on the maker's own shield, but the board itself has not been fabricated yet.
On a Raspberry Pi CM4 with four camera lanes, the 22-pin board measured 4K30 and 1080p120, against the maker ratings of 1080p50 for a 2-lane TC358743 board and 1080p60 for a 4-lane one. A Pi 4B only exposes two camera lanes, so it tops out at 1080p60 and needs a 22-pin to 15-pin cable. The Pi 5 has not been tested yet. Single-link boards also work with the cheaper RK628D.
Driver fixes
The maker says documentation for these chips is poor, and the stock Rockchip driver had real bugs. It ignored EDID changes until the cable was replugged, never advertised audio, and could lock up the bridge after a hot HDMI replug until a power cycle. The repo fixes all of these. For the Pi, the maker wrote a standalone driver that talks to the RK628H over I2C and uses standard V4L2, so it runs without Rockchip's kernel. One script builds and installs it, and a check script finds the bridge and captures test frames.
Our take
This is a useful release for anyone building KVMs, capture devices or HDMI test equipment on a CM4. Schematics, Fusion and EAGLE files, gerbers and JLCPCB BOMs are published under CERN-OHL-S, with the software under GPL. The same maker shared a CM4-format RV1126B vision module earlier this week. If you are new to the CSI interface, start with our guide to using a Pi camera module.
Image credit: WarpKVM / adouglass52, cropped and resized.
About the author: Aswinth Raj is an electronics engineer and the editor of CircuitDigest, where he has published more than 150 hands-on projects and tutorials across Arduino, PIC, ESP32, Raspberry Pi and EV battery systems. He selects the stories on this page for what engineers and makers can actually learn or build from. Connect on LinkedIn
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