Maybe using some proprietary serial communication scheme isn't the smartest way to do this. I'd say this should be obvious to everyone. Consequently, I conclude that there's a really compelling reason to do that.

What is it?

Nearly every ASIC supplier with whom I've had contact includes JTAG as a means for testing as well as a debugging tool for their products. If you choose not to use that, it's your choice, but it's to your advantage to do so, however, inconvenient and awkward it may prove to be. You can certainly debug and test with JTAG, so it's a vehicle you shouldn't ignore.

Why don't you want to use the "default" internal UART? You can include as many of them as you like, so an external one or an internal one will behave more or less the same, depending on how you build your ASIC.

If you'd spent 5 minutes searching the web for 8051 monitor, you'd have a list of at least a dozen of them. I did that and found plenty. You should do this yourself, though, so you get an idea of how foolish not searhcing first makes you appear to be.

You need, perhaps, to be more specific about just what you mean by ASIC. To many of us, that means a custom-built hard-logic circuit made to order for us in a silicon foundry. I've not been involved in such projects since the '80's, aside from FPGA-to-ASIC migrations. If you're "up" to such a task, you must have at least $2 million U.S. for startup, licensing, and foundry charges, as well as software tools, and you need a team of long-experienced engineers to accomplish the task, which will take, typically, about a year.

It's not something you're going to pull off in your back bedroom next weekend.

RE