How to tftboot with Xiaomi AX3600?

I'm new to OpenWrt and I'm trying to test a custom firmware build. AFAIK I'm supposed to use tftboot. I soldered in a TTL serial to USB cable. If connect I the USB cable and then turn on the router, it acts dead and won't start. If I turn the router on and wait 2 seconds before hooking in the USB cable, I get serial output that looks like this. I set the bootdelay to 10 but nothing happens when I press and buttons. If I hold reset while booting I get output like this. Again, keys do nothing when pressed. How do I get to an interactive boot menu?

I have already done all of that. I have the INT firmware and persistent ssh. I'm trying to test a custom local firmware build compiled from a dev git codebase.

in that case, you have already enabled serial console RW, as described in the thread above ?

I set uart_en=1 and boot_wait=on in nvram, if that's what you mean. I'm not sure if the cable is right. I'm using this cable.

Do you have a UART connection to the router working?

There is a recovery method that might work to send a custom firmware to the router.

Thanks but I'm a little apprehensive about hard flashing firmware that may be junk. My understanding is that tftpboot would load it through the network. I think it's supposed to respond to a button press during the boot wait countdown and it isn't, so I think maybe I should recheck the solder connection.

Disconnect your serial TTL from the router and short the Rx and Tx pins.
When done, you should be able to see what you type in your terminal app.

It's a simple test to check if the TTL works.

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On the device page, it says

no ttl pin voltage 1.8V

What does this mean exactly?

It means that the UART is running directly at SOC voltage, at 1.8 volts - not the more customary 3.3 volts. In case you want to connect to the serial console, you must use an adapter (or level shifter) operating at 1.8 volts.

E.g. several (higher priced-) FTDI FT232RL (not all!) USB2serial adapters can be switched between 1.8 volts, 3.3 volts or 5 volts (with a jumper), cp2102/ cp2104, ch340 or pl2303 based ones typically only support 3.3- and 5 volts.

The TP-Link Archer c2600 is in a similar boat, here you can find the alternative solution of using a Bi-Directional Logic Level Shifter Converter mediating between 1.8 volts on the UART and 3.3 volts on the USB2serial adapter.

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Thanks. It makes sense now. I think I might go with DSD TECH SH-U09C2. It has jumper adjustable voltage.

That's coincidentally exactly one of the devices I was thinking about (and the adapter I bought as well), but there are some slightly cheaper options using this chipset (and support 1.8V) as well.

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