You can try looking for kernel options with "MUIC", "USB" or "UART" in the name in the downstream kernel's defconfig - if you find a seemingly SOC-specific option, grep through the kernel source to find the driver (hint: don't include the CONFIG_ part). Once you've found the model of your MUIC, look for similar chips in the [[Micro-USB Interface Controller|MUIC]] page to find the resistance values you should use for your cable. If you're unsure, look up your MUIC chip's datasheet; if you can't find one, or the datasheet does not have the resistance value table, use the default values.
===Finding the UART ports on your device's main board===
Sometimes there is no other solution but to poke at random motherboard pins to find the serial port. This is mostly a lucky process,
but you can: * send either garbage, or a repeating pattern to the serial port at <code>/dev/ttysomething</code> (if you have a kernel where it's working) , and then try to find the tx pin with a scope.* * If you can't, you can also reboot your phone and hope it writes something during boot (but that's really trying luck) * tx is usually good enough for debugging, but rx should be quite close, with a high impedance, or more likely a pulling resistor. ** a pull-up or pull-down resistor will make the pin look like gnd or vcc
==Building the cable==