I'm using an ATmega1284p to control an LPD8806 LED strip. I am powering the light strip and the MCU separately with a common ground between them and the LPD8806 data & clock pins are connected to pin 6 (MOSI) and pin 8 (SCK) respectively. I'm using a 16MHz crystal oscillator with 22pF caps to ground, both pins 10 & 30 are going to 5V, and both pins 11 & 31 to ground.
Everything is working correctly except that I am seeing 16Hz spike noise on other MCU pins as soon as I connect the data and clock pins of the light strip. For example, here's a screen grab from the scope monitoring ADC0 (pin 40):
And, zoomed in:
As you can see, the spikes on ADC0 are around 500mV, so fairly significant. I've tried adding decoupling caps across Vcc to GND on both sides of the chip (10 to 11, and 30 to 31), but it didn't make any difference. I'm assuming the source of the signal is the SPI communication but I don't understand how it is affecting the other pins. I measure the spikes at 61ms, so it's right at 16.4Hz and very consistent. Any help is much appreciated, and please let me know if you would like any additional information.