I thought I’d use my trusty CP2102 USB-UART adapter. But as it turns out, it only works up to 1 Mbps, while the Orange Pi 4 is running at 1500000 bauds. Oops.
My other attempt was to try the Raspberry Pi‘s serial port, but that also didn’t work. The data coming in was also just a random garbage.
I tried using a random Arduino‘s on board serial module CH340 (while having Reset hooked up to GND so that it bypasses the Atmel chip) but that for some reason didn’t produce any data on the high baud rate. Even though based on the specification of the chip the baud rate should be supported.
My next attempt was with an ESP32 dev board, hooking it up similarly to an Arduino. No luck, turns out there is again the CP2102 chip. There was some output from the Orange Pi 4, but again it was garbage since it can’t keep up at this speed.
Finally I found a lonely DAPlink module that is specifically designed for ARM programming. Among other things it has a UART with 3.3V TTL levels. And bingo, this one was fast enough to finally read the Orange Pi 4’s debug UART port.