I was supposed to build stairs for my deck today but it's raining so I decided to make a circuit for proper 3.3V to 1.8V level conversion for the JTAG wiring.
There has been speculation about the voltage drop across the diodes being too high when the SMC drives low which could cause boot failures. I tried measuring it in an experiment and found it to be 0.62V with my 1N4148 diodes. Damn, we want close to zero volts!
I've attached a link to a new wiring scheme that uses 2 transistors, and 2 resistors instead of 2 diodes. The circuit provides electrical isolation when SMC is driving 3.3V (the transistor is off, disconnecting the JTAG which then pulls high), and when the SMC is driving low, the transistor turns on, providing a low voltage of about 100 mV to the JTAG. This means it is just as safe as diodes, but provides more reliable zero-level voltages.
Here is the wiring diagram. You don't have to use 2N3904, I also tired 2N3714 which I had kicking around and it worked fine. PLEASE DOUBLE CHECK THE PINOUT OF YOUR TRANSISTOR BEFORE YOU USE IT!!! Some BJTs are C-B-E and some are B-C-E.
** NEW CIRCUIT HERE ***
*** THE DIODE METHOD IS 100% SAFE, BUT NOT NECESSARILY 100% EFFECTIVE, THIS CIRCUIT PROVIDES BOTH ***
I tried rebooting my Falcon about 30 times to make sure it worked okay. Previously, I've had ROL issues where occasionally the lights don't behave correctly. I had hoped to resolve that issue, unfortunately voltage levels are not the problem.
Another suspicion floating around is the J2D2 pin 7 to 4 connection. This wire is supposed to be connected to 1.8V supply. People have suggested it is no longer connected via zero-ohm resistor. I have instructions below on how to confirm that your J2D2 Pin 7 is shorted to 1.8V supply.
This means my ROL issue has to do with the precise timing of the exploit (probably because we are borrowing a ROL I/O on the SMC).
So, if you are having ROL issues, this will not help you. If you are having actual boot issues, this circuit might help (hopefully some stubborn Zephyrs that can successfully boot occasionally can try this out.)
Since this forum is for technical info and research, for those interested I've included in separate posts below how to confirm your J2D2 pin 7 is wired to 1.8V (you don't need a resistor here, really) and also the theory of operation behind the voltage conversion circuit.