Recently, I discovered that with the Samsung S7, you can’t downgrade certain firmware versions.
These are firmware releases for Sprint’s branded Samsung S7, the SM-G930P (P means Sprint). Note the number in the yellow column:
In late November, I was on the QJ3 update (last three digits of firmware). Its number is 5.
I tried to switch to the SM-G930U firmware. That would make my S7 an unlocked phone, which should free it of Sprint-specific customizations.
At the time, the latest Sprint-compatible* SM-G930U firmware was G930UUEU4BQJ5. Note the 4! When I tried to install that, I got an error saying that the version I am trying to install is lower than the fused version. This was the error:
SW REV CHECK FAIL : [ABOOT]FUSED 5 > BINARY 4
Note that “ABOOT” means the Android bootloader. This is a small, critical piece of software that kicks off everything else when you start the phone. The yellow-highlighted number in the above picture is in fact the bootloader version!
On the S7, Samsung has eFuses that indicate the bootloader version. When enough eFuses are tripped for bootloader version 5, I can’t install bootloader version 4. When an eFuse trips, the circuit permanently changes. I can’t un-set these eFuses.
Samsung’s eFuses became notorious in the custom ROM community a few years ago. Samsung’s Knox security would trip a warranty fuse if you installed unauthorized firmware. You can’t reset the warranty fuse. Warranty fuse-tripped phones work fine, but there are anecdotal reports of Samsung refusing warranty service due to tripped warranty fuses.
This firmware change didn’t trip the Knox warranty fuse. It’s because all US-market Samsung S7 phones are hardware-identical: the only difference between Sprint, AT&T, unlocked, etc. S7s are the software! All Samsung- or carrier-issued firmware types, intended for the US market, are Samsung-authorized on any Samsung S7.
Back to the story: When you install firmware on Android phones, you are installing four pieces of software: the bootloader, modem (handles the cell network communications), Android core software, and carrier- and region-specific Android customizations. It turns out this version lockout doesn’t stop you from installing older modem software. Due to my determination, I managed to install older SM-G930U (unlocked) modem software. I was still on current-version SM-G930P (Sprint-branded) for everything else. Because of that version mismatch, I got security warnings:
The phone still worked fine.
Why didn’t I just get the newer firmware and be done with it? No download site had QJ3 despite it being out for a few weeks!
About a week later, I could download the correct firmware. I installed it, and everything went back to a happy state. I’ve since installed two more over-the-air updates from Sprint with no problems.
*I don’t understand why an “unlocked” phone has carrier-specific variants! Isn’t unlocked supposed to mean “not carrier-specific”? Here are SM-G930U variants (source):
- Bluegrass Cellular (LRA)
- Nextech / C-Spire (ACG)
- Sprint (cdma) (SPR)
- Tracfone (TFN)
- USA (TMK)
- USA (USC)
- USA (AT&T) (ATT)
- USA (T-Mobile) (TMB)
- USA (Verizon) (VZW)
- Unknown (AIO)
- Unknown (BST)
- Unknown (XAA)
- Unknown (XAS)
- Virgin Mobile USA (VMU)