When I started reverse engineering the Edgecore AS5610-52X, I expected the port LEDs to be a simple software toggle. I was wrong. It’s a masterclass in distributed computing involving a PowerPC CPU, a PCIe-to-AXI bridge, and two dedicated “LED processors” running their own bytecode.

The hidden micro-engines

The CPU — a Freescale P2020 — doesn’t actually touch the LEDs. Instead, the Broadcom BCM56846 ASIC contains two tiny, independent stack-machine microcontrollers (LEDUP0 and LEDUP1).

  • The program: these controllers run a 256-byte program at 30 Hz.
  • The task: they scan port status bits (link, activity, speed) and “pack” them into a serial chain.
  • The chain: 52 bicolor LEDs = 104 signals. The hardware clocks out two bits per LED — amber first, then green.

The PCI Express “Heisenbug”

Getting the CPU to talk to these controllers was the real challenge. Accessing the LED registers requires navigating the iProc PCIe-AXI bridge (PAXB).

I hit a hardware quirk that took some effort to pin down: to remap the memory window (BAR0) to see the LED registers, you cannot use standard memory-mapped I/O. On this iProc revision, MMIO writes to the IMAP0_7 register don’t persist. You have to drop down to PCI config-space writes to make the mapping stick.

”Leddance” — taking total control

To make these LEDs do what I wanted, I bypassed the factory logic entirely:

  • The passthrough: I replaced the stock 256-byte program with a custom 51-byte passthrough script.
  • The framebuffer: that turned the LED DATA RAM into a raw framebuffer.
  • The result: by sending patterns from userspace through a custom kernel module, the entire front panel refreshes in under a millisecond — comfortably within the 30 Hz hardware tick.

The takeaway

In high-performance networking, even a blinking light is a high-optimization target. By offloading the blink logic to a dedicated micro-engine, the main CPU stays focused on moving terabits of data.

It’s a reminder that in systems programming, “simple” features often sit on top of a mountain of intentional, complex engineering.