For my job, I was tasked with creating a board to interface with a 2.5V device we've made. So, to keep things simple, I decided to interface it with an ATmega328P-PU running the Arduino firmware at 2.5V. All the specifications say the chip has an operating voltage of 1.8V-5.5V, so I didn't bother testing any circuit at 2.5V until today. That was a dumb mistake.
I was testing the low voltage setup by running the simple blink sketch on a breadboard and found it would stop working just below 2.7V. The power supply would also show 0mA current draw as opposed to the 2mA it was showing before. I checked the pin with a DMM and the LED unplugged to confirm that no output was coming from the pin.
After some research, I found a site that seemed to say that, at lower voltages, it needs to run at a slower speed. So, I burned the bootloader to have the chip used the 8Mhz internal oscillator. However, it still turns off below 2.7V.
How can I get access to these lower voltages? I am guessing that this has to do with some internal fuse setting, but can't find anything about it. I'm well within the operating voltage for both the I/O pins and Supply rail. I know it can run at 2.5V as the Arduino BT can have an input voltage as low as 2.5V. Is there something I'm forgetting?