Hi All,

I have a weird issue with a debricked TP-Link WR841ND V8.0. It now only boot when a USB to UART (cp2102) Controller is connected to the serial port (based on my tests only GND and TX are needed). So it looks like it
somehow need the 3,3 V from the USB to UART Controller.

When starting with connected Controller it boot up and runs fine but as soon as I power off and power on again
(without the USB to UART Controller connected) only the power LED is on and I do not see anything on putty.

Probably I have broken the board during my first tests with a USB to Serial Controller (should have read more
before starting with debricking the device).

Anyone know if and how this can be fixed or should I just throw it away?

Thanks!

I think you somehow connected the TX to another trace, which now injects noise. So the bootloader will be interrupted as it detects an 'any key'.

Apparently the connection to your serial device is enough to keep the line low. (or high, doesn't matter).

Remedy: find that short and remove it, or connect the TX to the ground, preferably with a resistor. The value doesn't really matter, something between 100 Ohm and 4k will do, but plain short is a bit short.

(Last edited by Mijzelf on 10 Sep 2016, 08:43)

Thanks for the Info's Mijzelf!

As I did not have a resistor to test what you have suggested I tried some other things
and found out that if I shorten TX and VCC (of the serial ports) the device is also starting.

Can that break the board if I leave it this way or is this generally not recommended as
now TX permanently has 3,3 Volt from VCC?

For TX that is not a problem. The port has a high impedance, and so you can safely connect it to ground or VCC. But I don't know about that unknown noise generator. You have connected that to 3.3V also.

Don't know to be honest what you mean with "noise generator". Before TX and GND was connected to the USB to UART Controler. Now I just shorten TX and VCC. So it seem that TX need 3,3 V (whyever).

Currently the device is up and running based on my first checks (WLAN, LAN, DHCP, Routing, etc..) but the System LED is permanently going ON and OFF which due to documentation mean that there is a system error but I don't know what the error is.. probably because I have shorten TX and VCC..

If you think it is not dangerous to have TX and VCC shorten I will leave it now how it is and see how long it will work.

Thanks again for your help!

My theory is that something is injecting noise in TX, causing the bootloader to stop on an 'any key'. This injection can be caused by a short wire from some dataline to TX.
That is that 'noise generator'. By shorting TX and VCC, you also pull that data line (or whatever) to VCC. Without knowing what it is, it's impossible to say if it will hurt.

Impossible. Autoboot will only stop if you type tpl which of course can't be "noise injected".