So my tachometers suddenly started acting weird. When on the edge between black and white, the tachometers counted up like crazy. I figured the Schmitt triggers on the input pens on the Atmega must be a little too sensitive. They have a hysteresis of only 0.25 volts @ a supply voltage of 5V, thats a bit in the low end. So I added a 4093 quad nand schmitt trigger IC (hysteresis 0.9V), and after a little confusion (forgot one single connection on the circuit board - that gave really odd results), the tachometers now run flawlessly.
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