Arduino Pro 8mhz bootloader problem (ATmega328p)

Hey,

I'm having an issue with an ATmega328p. I was planning on using it in a project that runs on 3.3v instead of 5v, so I got an 8 MHZ crystal (with capacitors) and an ATmega328p. I was planning on using the Pro 8mhz bootloader on it. The bootloading process seems to go over without a hitch, at which point I do the "upload using programmer" option to upload my "blink" sketch to the chip. (I changed the blinking LED to pin 9 so that it would not be using the SCK pin)

The blink sketch works just fine running off of the 5v from my Arduino Uno. However, when I disconnect the Uno and attempt to power the chip with a 3.3v regulator, it is totally inert. I've checked the voltages everywhere when the 3.3v regulator is on, and it's not a power problem. It's not the LED/resistor combination either because if I connect them across 3.3v, the LED lights up.

I thought maybe it was a bug in the bootloader program, like it didn't set the "brownout detect" fuse to the correct value or something like that. I can tell that my external crystal is working and the chip is running off of it, because it only works (on 5v) when said crystal is connected. I've already checked and rechecked the "board" and "programmer" options.

Never mind, it turned out to be a PEBKAC error. I didn't hook up AVCC properly (the wire was loose so there was no actual voltage on that pin) and for whatever reason, this caused the chip to work on 5v but NOT on 3.3v. I didn't think to check the power and ground connections simply because it worked fine on 5v :grin:

Not sure why AVCC (for the analog to digital converter) would be required ONLY for 3.3v operation. I guess only the AVR engineers would know- or maybe not :grin:

Somewhat embarrased, but I figured that I can't be the only one who had this problem and I was hoping to "save" someone from this very situation :grin: