AVR Programmer, AVRISP mkII: About a bug

I had a good debugging session over the last few nights and want to write down my notes here.

The AVR Programmer initially worked with a target ‘328p, but it hung when programming a second build of the board, and then again on a particular ATtiny13A that I was using for the Hypna Go Go. Oddly, it worked for another ‘13A that I was using on the same project. The board was flaky, and as a result, I couldn’t keep my promise to myself to never again ISP-program another AVR without a programmer I’d built myself.

A few nights ago, while getting ready for the latest and greatest version of the board to arrive from the factory, I decided to take a look at what was going on. I got out my Saleae Logic and watched the SPI traffic. Perfect… except that the target ‘13A never responded on MISO. I swapped in a known-good programmer, confirmed it worked, but then saw that the logic signal patterns were identical. The target wasn’t responding, but the known-good programmer was programming it!

I suspected bad logic probe connections… nope. They were fine.

Getting a bit frantic, I zoomed out in the Logic app, and immediately figured out why the target wasn’t responding. The first cycle through the initialization sequence (the only cycle I’d been examining), it always failed. But the second cycle succeeded, and I never saw this until I zoomed out.

Next question: why was the known-good good? I re-read the datasheet for the ‘13A and saw that if the programmer couldn’t guarantee SCK was low on target powerup (which indeed applied in both my and the known-good programmer cases), an alternate way to get into programming mode was to pulse /RESET high for a bit. I could see the known-good doing this between the first bad and second good cycles, and my firmware was doing the same thing, so what was the difference?

Back to my programmer under the logic analyzer. Zoom out… and see that in fact /RESET was staying low consistently. Aha! That’s the problem. But why was it happening?

It turns out that the LUFA code doesn’t pulse /RESET by sending it high. Instead it tristates the pin. This makes sense because you want your programmer to disconnect from the target when you’re done so that /RESET can go back high on its own. This is fine, except that my programmer has a 74AHC125 buffer between the ‘32u2 and the ISP outputs, and a tristate output entering the buffer leaves as a low logic signal. This is why my programmer wasn’t pulsing /RESET, and it’s why programming sometimes worked if the powerup sequence happened to be right, or if the chip in question (such as most 328p’s or some 13A’s) happened to be a more tolerant sample.

Once I understood the problem, the fix was easy. Change LUFA to pulse /RESET with explicit logic levels, and insert buffer enable/disable code at the appropriate places. So far, it’s been running flawlessly.