The dragon is a lot smaller in person than it looks in the pictures. It can program AVR's using ISP or JTAG (like the STK600) but can also debug via JTAG. It also can debug using other methods.
I spent a great deal of time this morning making sure I was plugging it in the right way. I've heard rumors that since the Dragon is cheap, it also is easy to break, so I definitely didn't want to plug in any cables backward.
I hooked my ATMega324P up to my STK600 (using my new routing card!) and then just to be safe, I removed the target voltage jumper from the STK600. I then plugged in the JTAG interface from the STK600 to the Dragon and fired up AVR Studio. The Dragon gave an error message about the target device not being powered. At that point I decided it was safe to put the target voltage jumper back into the STK600 and give it some power.
Things started working at that point. I was able to put AVR Studio in JTAG mode, program the AVR, and then start hardware debugging. Exciting!
I was seeing the serial RX interrupt handler trigger over and over again which was odd so I looked at the pins on the STK600 again to see if they were correct. Since I had previously had it wired up for the ATMega2560, there was a chance I hadn't rewired it correctly. And sure enough, I had the vsync signals plugged into the AVR's RX/TX pins hehehe. So the vsync pulse was triggering the RX interrupt.
After I corrected the pin assignment, trouble started. AVR Studio would try to program the AVR and get corrupted results. So I decided to try using the STK600 to program the AVR instead. I got the same problem. Hmmmm.. well I was relieved that I hadn't broken the Dragon already hehehe. So I started thinking that maybe something I had plugged in was interfering with the JTAG pins. I pulled up one of my schematics for the dexter PCB and saw that most (all?) of the 324P's JTAG pins were on pin group C. So I just unplugged my header from pin group C and that fixed the problem for now. I was able to start programming again and debugging.
I am using pins C0 and C1 for the status and command strobes so I will need to look at the 324P's datasheet to see if I need to reassign these temporarily in order to make JTAG work. As JTAG is not needed for the final product this would only be a temporary change but it would be more convenient to keep the JTAG pins exclusively for debugging than to have to juggle them back and forth. That's one reason I chose the 324P is it has more pins to play with.
So once I sort out the JTAG pin assignments then I can power up the Dragon's Lair machine and set some breakpoints in the debugger to see what the ploblem is with my code.