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[avr-chat] Noisy supply line with the xmega

From: Graham Davies
Subject: [avr-chat] Noisy supply line with the xmega
Date: Sat, 09 Apr 2011 10:34:58 -0400

As previously posted, I am bringing up my first xmega project using an ATxmega32A4. This is mainly an informative post, but I would welcome any discussion.

One function of the project is analog waveform generation. My first pass at this avoided the use of the DAC and DMA because I felt that figuring out how this all works would take too long (as you will see in a previous post, I was right). Instead, I set up a Timer / Counter in Dual-Slope PWM mode with the four Capture / Compare Channels producing a series of nested pulses that I added together with a resistor network. This worked beautifully and, of course, has zero impact on run-time firmware.

A potential problem, which I was aware of when I took this direction, is that the high values of the pulses don't come from some stable reference voltage, but come from the microcontroller digital supply. However, there is heavy filtering in my hardware to smooth out the waveform, which is of a fixed and known frequency, so expecting only random switching noise on the supply I was not too worried.

It turns out that the xmega generates strong, fixed frequency noise on the supply, which if course amplitude modulates my signal and totally messes me up. As far as I can tell, this is coming from the clock generation system. I have a 32.768 kHz crystal connected and I am using the built-in DFLL to fix-up the internal 32 MHz oscillator (RC or Ring, depending on which Atmel document you're reading). This appears to work well enough, but at power on, I can see this sawtooth waveform with a 12 us period kind-of swell up over a second or two and then and stabilize at about 120 mV peak-to-peak.

I have the usual 0.1 uF capacitors on all VCC pins, 0402 size, as close as you could wish with the other end through a via to the ground plane. The 3.3 V regulator sports a 2.2 uF output capacitor.

Also, athough as far as I know, there is nothing wrong with the quality of my 32.768 kHz crystal, setting low power mode for the oscillator did not work for the second board I made up, although it worked for the first. I have a Citizen 6 by 2 mm cylinder crystal with 35 kohm maximum series resistance and 6 pF load capacitance. I am not using external load capacitors. This should be just fine. I suspect the xmega oscillator.



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