Accessing JTAG pins using Virtual JTAG Megafunction

Yes, your intention has been clear so far. I was mainly wondering what you exacty did when you stated:

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I can't get TDO to work as an output

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As far as I'm aware of, the generic VJTAG block is working fine. But it has to be operated with the respective JTAG codes to get a response.

I agree, that it would be a straightforward way for many production and maintenance purposes to operate the Altera supplied SFL or SFL enhanced MegaFunction by your PC application. At present, there are two parts missing:

- a documentation of the low level SFL protocol

- an interface to the Altera JTAG stack in another way than tcl scripting

I already mentioned, that the first point can be basically solved by analyzing the alt_sfl.vhd code. Personally, I didn't need it yet.

Because the VJTAG protocol has been disclosed with the V2.0 user manual (and previously in an Altera patent, by the way), you are at least able to operate it over a third party JTAG interface.

For the Altera JTAG adapters and the respective software interface, tcl scripting is the only supported access method up to now. For the JTAG UART, there's also a dll interface. sld_hapi.dll is an existing unsupported interface to the JTAG stack. If you are able to guess the purpose of it's exported functions, you can control it.