2 min read

When your SFP module keeps rebooting: the problem is often the power supply

When your SFP module keeps rebooting: the problem is often the power supply

You plug your SFP/ONU module into your USB-to-TTL adapter, open your serial terminal, and suddenly you see this repeating over and over again:

U-Boot 2011.12-lantiq-gpon-1.2.24 (Nov 03 2014 - 22:46:28), Build: falcon_sfp_linux
Board: SFP
DRAM: 64 MiB
Now running in RAM - U-Boot a
ROM: V1.1.4 ROM: CFG 0x00000006
ROM: SFLASH-4
hw fuse format 1

[...]

Before you even get the chance to hit Ctrl+C, it restarts.
No prompt, no shell, just an endless loop.

Good news: your module isn’t dead — it’s just starving.

The real cause: not enough power

Most USB-to-TTL adapters include a 3.3 V pin that you can use to power your board or module.
But here’s the catch: that pin is usually very weak — it can only supply around 150–200 mA at best.

A GPON SFP module like the Huawei MA5671A typically draws:

  • 300–500mA continuously when running (but does not matter here)
  • up to 300mA in short bursts during initialization (before console is available)
  • around 100mA once boot is aborted using console (not shown here)
90 first seconds after boot

When your adapter can’t deliver that much current:

  • the voltage drops,
  • the module “brown-outs” (loses power momentarily),
  • U-Boot restarts,
  • and the loop continues forever.

That’s why you can’t interrupt U-Boot — it never stays on long enough to accept input.

The Fix: give it a proper power source

The solution is simple: power the module from a stable 3.3 V supply that can provide at least 500 mA (1 A recommended to be safe).

Wiring Diagram

USB-TTL TX     →  SFP-TTL RX
USB-TTL RX     →  SFP-TTL TX
USB-TTL GND    →  SFP-TTL GND 
External GND   →  SFP-TTL GND (yep, same as above)
External 3.3V  →  SFP-TTL VCC (in place of USB-TTL 3.3V)

The important part: all grounds must be connected together.
Your external power supply and USB-to-serial adapter need a common ground for UART communication to work correctly.

Once Properly Powered

Once you feed the module with a reliable 3.3 V source, it boots normally.
No more reboots, no more frustration — you can finally interrupt U-Boot, access the console, and flash your firmware.

Even a small SFP module can be picky about power.
Before blaming the firmware, always double-check your power supply — it’s usually the culprit.

Thomas

Thomas