Hello all,

I want to add some on-chip debugging capability to a free 8051 FPGA core (http://opencores.org/project,light52). In its present state, this core has no debug features and you have to program the FPGA every time you want to update the MCU firmware; this is too cumbersome a procedure for any practical application.

So I want to add the usual basic OCD capabilities: C source level debugging, breakpoints and variable examination, plus some means to upload object code onto the FPGA without re-running the synthesis.

I think the best approach would be implementing an existing debugging interface, that is, making the chip behave like a commercial 8051 chip so that it can be used with an existing IDE; for example the free IDE from SiLabs.
This IDE can optionally use a RS-232 adapter for debugging, and in principle I might replicate the funcionality of that adapter as an FPGA module to be bolted on to the MCS51 core. Except that I haven't found the serial protocol documented anywhere.

My questions are:

1) Does anybody have the documentation for SiLabs debugging protocol over RS-232?
2) What other IDEs or debuggers may I consider as debugging hosts (free if at all possible)?
3) What would you do if you wanted OCD on an existing FPGA core?


I am aware of other possibilities that are well documented (e.g. ccdebug or IAR's c-spy), but those would force me to write a driver or some other form of on-host program. I would like to avoid that if possible. I may have to write some sort of conversion tool for the chosen IDE's object code format, but that would be a second stage of the project; the first stage is connecting to the CPU using standard tools.


For all I know this problem may have been solved already but I haven't been able to find what I'm looking for. Before I embark in a reverse engineering sub-project I wanted to check with you; after all, this forum is *the* place to learn about 8051 stuff...

If I can't easily add OCD to the core, at least I will add some form of ISP-over-serial using some standard, free tool. That would be much easier but I want to try the best solution first.

Please note that the core in its present state is not fit for actual, real-world use (it has not passed any serious verification test bench), and the debugging features are not going to change that. I strongly advise you NOT to use it.


All advice is welcome, thank you for your thoughts!