AVR Freaks

EDIT: This turned out to be user error. Please see my post further down in the thread.

Regarding the average (and oversampling) feature, after a few experiments, I now have a clear picture of what is happening. And I am afraid to say that the averaging feature has one heck of an error in its implementation, rendering it more or less unusable.

We know from the XMega E manual chapter 24.8.3, that it can average over up to 1024 samples (I am not taking oversampling into account yet). Well, it does make 1024 samples and add them all together. Unfortunately, it adds them in a 16 bit register
This means that we can't use the full input range of the ADC, or the 16-bit sum register will overflow. In order for the 1024 sample average to work properly, the sum should be kept in a 22 bit register, or the input restricted to the equivalent of a 6 bit ADC.

The average feature can be used as long as the sum doesn't overflow the 16 bit register. This is only guaranteed if the average is made up of no more than 16 samples. This also means that the oversampling described in chapter 24.8.4 only works properly when trying to oversample to 13 or 14 bits resolution.

I made a small test program on the E5 Xplained board, that outputs all values from 0 to 4095 on DAC 0, and then reading it with the ADC. The ADC is set up for oversampling to 15-bit resolution, as per table 24-4. I have attached the result. The X axis is DAC output, and Y is what the ADC was reading back.

I haven't really thought that far.
I was planning on reducing my testprogram to the simplest possible and adding some useful comments, and then present it here so others could either verify my findings, or tell me what I am doing wrong :twisted:

If this really is a bug in the E5, then the next step would probably be reporting it to Atmel.

My mistake all the way.
It was RESOLUTION in CTRLB that must be set to MT12BIT when using averaging, regardless of oversampling or not. Now everything works perfectly.

Of course. Attached.

Look at the end of adc_init, and select one of the AVGCTRL settings, or make your own. Run the code on a XMEGA-E5 Xplained board. Once started, the code will cycle through 0-4095 on the DAC, and read the ADC. Results will come out on the USB-serial port at 115200 baud.

Capture the output into a text file, and open it with a spreadsheet program to plot a graph. Or just look at it directly in text.

I know that both DAC and ADC are missing their calibration initialization, but they are not available in the production signature row on my two boards (just 0x00).