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Bricked RT-AC68U when downgrading CFE

Discussion in 'Asuswrt-Merlin' started by fatice, Aug 25, 2019.

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  1. fatice

    fatice Occasional Visitor

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    Hi,

    I was trying to downgrade the CFE for overclocking, because CFE 1.1.1.2 does not allow overclocking. The original firmware were:
    CFE 1.1.1.2
    Firmware: RT-AC68U_384.13_0
    Hardware: REV E1 board rev 1.90

    After backing up the original CFE and obtained the modified CFE, I flashed the 1.0.2.0 US AiMesh.

    After the reboot command, the router never booted up. I have tried press and hold WPS, press and hold the RESET for recovery mode, none of them put the router in recovery mode. Power LED never turned on (After I opened up the router, I can see the power LED is dimmed on.) and USB LED is fully solid.

    I then open up the router and connected a USB to serial adapter to the serial pins on the router. No serial output at all (tried 115200bps, 9600bps, swapping tx / rx ...). I have tried two USB to serial adapters (I know they work), nothing showed up.

    Is there a jump or wire that disables the serial console in this revision of the hardware?

    Is there any way I can flash back my original CFE (I do have a backup)?
    From what I gathered, there is no known way to JTAG it, is de-solder, program CFE and re-solder is the only way?

    Anyone can do it for a price?

    Thanks.
     
    Last edited by a moderator: Aug 26, 2019
  2. RMerlin

    RMerlin Super Moderator

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    You flashed an invalid CFE. Your router is now permanently bricked, since the serial interface is handled by the CFE.
     
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  3. fatice

    fatice Occasional Visitor

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    Thanks for the reply. May I know how to determine if the CFE is valid? Isn't all CFE is interchangeable for all RT-AC68U?
     
  4. RMerlin

    RMerlin Super Moderator

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    No. There are multiple hardware revisions, using different cpus, nand chips and power amplifiers, so you need the cfe specific to your hardware.

    Sent from my SM-T720 using Tapatalk
     
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  5. fatice

    fatice Occasional Visitor

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    Thanks. Lesson learnt.
     
  6. AndreiV

    AndreiV Very Senior Member

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    пішли на риболовлю
    First lines on the web page in question :

    WARNING
    DO NOT flash if any of the following applies:

    1. Your router is not one of the supported models below
    2. You are not 100% sure what you are doing
     
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  7. fatice

    fatice Occasional Visitor

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    I have flashed my AC68U to advanceTomato, back to Merlin. Decided to stick to Merlin. I did not know different revision of hardware are not just minor hardware changes. That resulted in total bricking of my router. I don't know what I don't know.
     
    Last edited: Aug 26, 2019
  8. fatice

    fatice Occasional Visitor

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    Now I bricked a perfectly good router, I might upgrade to a newer one, but prefer to stay in Merlin. Is the AC86U a good alternative since it has a dual core 1.8GHz instead of 1.4Ghz for the AC88U?
     
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  9. RAH-66

    RAH-66 Occasional Visitor

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    AX88U with quad core
     
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  10. fatice

    fatice Occasional Visitor

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    AC88U is just 30% more, AX88U is double in price.
     
  11. CaptainSTX

    CaptainSTX Very Senior Member

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    A good AC86 is great but this router model seems to have more problems than other ASUS routers. Read the one and two star reviews on Amazon for some of the common issues. While some reviewers on any site are idiots many of the reviews on Amazon were written by knowledgeable reviewers who were long term ASUS users until purchasing an AC86.

    If you decide to go ahead with the purchase, buy from someplace with a good return policy and whatever you do don't risk purchasing a refurbished unit.

    I personally have had to RMA four AC86s. The one I received last week seems to be better as I have only had to factory reset it once.
     
  12. CrashXRu

    CrashXRu Occasional Visitor

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    AC86U - it's a good choice

    only keep in mind that all new routers are very hot
     
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  13. RMerlin

    RMerlin Super Moderator

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    Biggest upgrade isn't in the core clock, but in the fact it's a completely different CPU (Cortex A53 instead of Cortex A7). That CPU has hardware-based AES support, which will greatly enhance performance of OpenVPN, among other things.
     
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  14. fatice

    fatice Occasional Visitor

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    Thanks for the heads up. I will do more research on that.

    Back on the AC68U, I am wondering other SoC like the TI ones can boot from USB, is it possible for the BCM4708A0 boot from USB? That would be great for de-bricking.
     
  15. RMerlin

    RMerlin Super Moderator

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    Without a bootloader, there's nothing that can be done. The CFE is what initializes the hardware. You basically just wiped the BIOS of a motherboard.
     
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  16. fatice

    fatice Occasional Visitor

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    Broadcom has very limited documentation on its chips. Unlike TI, which very open about its chips. There is a bank of resistors for the TI chip I worked with to select the chip to boot from TFTP, EMMC or USB.
     
  17. fatice

    fatice Occasional Visitor

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    Incircut flash programming clip does not require to desolder the NAND chip.


    Its a good debricking tool, but the cost is the same as an RT-AC68U.
    This might be similar to the commissioning tool used by Asus.
     
    Last edited: Aug 28, 2019
  18. fatice

    fatice Occasional Visitor

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    Another thought, my mistake was putting a CFE modified from cfeditor.feng.si. According to the website's comments, the resulting CFE works with TM-AC1900. I could do the following to debrick my original RT-AC68U using a TM-AC1900.

    1. Buy a TM-AC1900.
    2. Desolder the flash chips from both router.
    3. Put the flash chip from RT-AC68U to TM-AC1900.
    4. If the CFE works, I can put the TM-AC1900 into recovery mode and restore the router to advancedTomato.
    5. Once the TM-AC1900 is restored, swap back the flash chips.
    6. Then bootup the RT-AC68U and put back the backup CFE.
    7. If the original CFE works, RT-AC68U should be able to go into recovery mode.

    The downside is that I have to desolder the flash chips 3 times and resolder them another 3 times. Also, a spare TM-AC1900 that does not support Merlin (Maybe I can use it as an accesspoint).
     
  19. Val D.

    Val D. Senior Member

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    I don't know where do you live, but RT-AC1900P router (fastest revision of RT-AC68U with 1.4GHz CPU) is currently on sale in BestBuy Canada for about USD100. I'm sure it comes down to this price from time to time in other countries as well. All this repair with chips re-soldering and buying a donor router is not worth it.
     
  20. fatice

    fatice Occasional Visitor

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    I am in Australia, everything is more expansive here. We never have RT-AC1900P here. The lowest normal 1Ghz RT-AC68U I can find is AUD$170.

    BTW, Just found out that ever since Amazon.com.au was setup, we can no longer buying from Amazon.com. For this reason, the TM AC1900 option might be off the table.
    If I am buying a new one, I might get the AC86U, not using its wifi as I have a dedicated AP.
     
    Last edited: Sep 3, 2019