I think you'll find that even that is much quicker than twiddling lines via boundary scan. At best, JTAG boundary scan is basically one giant shift register ge

I think you'll find that even that is much quicker than twiddling lines via boundary scan. At best, JTAG boundary scan is basically one giant shift register generally hundreds to thousands of bits long. Every cycle you'd need to shift out the whole thing even if you change the state of one signal.