Electronic – AVR program behavior incorrect after disabling reset fuse

avrmicrocontroller

I am using an ATtiny13 to drive 15 LEDs from five I/O pins (Charlieplexed). I am using ADC0 (pin1) as input from a voltage divider to provide a speed control. In order use ADC0 properly, I need to disable the reset fuse when I write the program.

The problem is, the program behaves differently when the reset fuse is disabled. Instead of all 15 LED's lighting, only 9 are. The 9 LEDs that light involve all 5 I/O pins, in both polarities, so I don't believe I've accidentally changed any configuration to those pins.

Here is the command line I use to flash the AVR (using avrdude on wintel) and set the fuses:

avrdude -c usbtiny -p attiny13 -U flash:w:program.hex -U lfuse:w:0x6a:m -U hfuse:w:0xfe:m

And the schematic:

ATtiny13 Charlieplex Schematic

Before setting the fuses, the program works perfectly, lighting the 15 LEDs as expected. The input on ADC0 will reset the micro when the voltage drops low enough, as expected, but otherwise speed control works. Here is a chart to show how the LEDs are connected:

LED #    AVR PINs    I/O PINs
 1       5-6         1-2
 2       6-5         2-1
 3       5-7         1-3
 4       7-5         3-1
 5       5-2         1-4
 6       2-5         4-1
 7       5-3         1-5
 8       3-5         5-1
 9       6-7         2-3
10       7-6         3-2
11       6-2         2-4
12       2-6         4-2
13       6-3         2-5
14       3-6         5-2
15       7-2         3-4

After disabling the reset fuse, LEDs 1 through 9 work, 10 through 15 do not. There is a slight flicker detectable on LED 13. ADC0 speed control works through the full range, without resetting the micro.

I am thoroughly confused. Can anyone advise?

Edit:

In case it matters, this is using an SOIC-8 package. Previously I've used a DIP-8 and not had this problem.

Best Answer

I discovered the solution to this problem!

I wrote a batch file to compile the code, convert to hex, and write to the microcontroller in one go. To avoid accidentally disabling the reset fuse, I wrote a second batch file.

At some point, I was experimenting with different optimization levels with avr-gcc. My command line parameters for avr-gcc included -O3 in both files. I discovered that this level of optimization was unreliable and caused some issues. I changed it back to -Os but only in the first batch file, not the second.

So, the code behavior change was a result of an incorrect optimization, which was used only in the script I used to also disable the reset fuse. It was finally caught by an astute coworker who noticed the difference in the two scripts.

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