RT-AC68U will only boot with jtag cable attached.

[boke]

This is a CellSpot router that has been converted. I have verified and re-flashed the cfe and tried Asus stock firmware, Asuswrt-Merlin, Tomato, and DD-WRT.

Without a jtag cable attached it appears to boot normally. All LEDs are lit and the switch works for lan comunication between wired PCs. But I have no internet access, no wifi, and no web-admin access.

With a JTAG cable attached the router boots and runs normally every time.

I thought I had a bad flash but that's not the problem. I wish I would have not voided the warranty by opening it up

Anyone have any ideas?

 

[hggomes]

You have a serial cable connected, not JTAG, that's quite odd, the boot process shouldn't be different with or without serial cable connected, clear your NVRAM via serial console and reboot or HW reset your router.

 

[boke]

Tried that many times. Same results. Its definitely very strange. I also tried another power supply.

 

[hggomes]

Dump the boot process with serial cable connected, then do it again but this time without serial cable attached and dump the dmesg message (you will need to connect serial cable this moment after boot fails) and check what is wrong there.

 

[boke]

Tried that too. Got no serial output at all when it wouldn't boot but I did not try sending a command (don't know what I was thinking). I will try a dmesg dump shortly.

 

[hggomes]

 

[boke]

No serial communication at all when it freezes.

But I think I know what the problem is. I think the serial port is picking up stray inputs.

I set a timer on my phone and started connecting the serial cable at various intervals to see where the problem started. I started out a t 10 seconds and increased by 1s each time I booted.
I narrowed it down to between 29 and 30 seconds into the boot process.
The serial output at this point looked to be "Hit ENTER for console..."
If I connected the serial cable just before that message would be displayed the modem will boot normally. If I wait until just after it freezes.

So I connected a 1K pull down resistor between the TX and GND pins(probably should use a 10k as soon as I find one).

Boots up every time now

Do you think it would be ok to leave it that way permanently?

 

[hggomes]

I was gonna to suggest you the exact process you did and solution, so it seems you are a mind reader.

No problem at all, leave it that way, 10K shouldn´t be needed.

 

[boke]

Great its running fine now.
One change though in case anyone else ever tries this.
I had to move the pull down resistor to TX and GND or else the router would boot but had no internet access.

Thanks for your help!!

 

[L&LD]

Can anyone explain in layman's terms why grounding out TX would allow this to boot up normally and stall without it?

 

[boke]

The serial port allows you to send interactive keyboard commands just like you can with SSH or Telnet.
There is a point in the boot process which briefly waits for the enter key to be pressed. If it is, the boot process stops and waits for more input. That's where mine was stopping. If you press Ctrl-C you can also stop the boot.

Mine must have been receiving a lot of "noise" on the serial input. Which were interpreted as random key presses (ie: Cat walking on the keyboard).

With the right combination anything could happen. I was also experiencing some random reboots when I could get it booted. It remains to be seen, but I'll bet this was the cause.

By grounding the TX pin I effectively turned off serial port.