# ATtiny85 absolute ratings for outputs?

The ATtiny85 datasheet specifies an absolute maximum negative voltage for any pin of -0.5 volts relative to ground.

Consider this partial schematic:

MOSI and MISO are the respective ATtiny85 pins and the CLOCK terminal is connected to the coil of a lavet stepper motor - which means it's just an electromagnet coil.

It works, but when I had the outputs on a scope to examine some of the timing, I happened to notice a negative going spike as the output is turned off, despite the presence of the flyback diode. I quickly realized that the magnitude of the spike is the forward voltage of the diode - in this case, 1 volt.

The question I have now is, does that absolute maximum (actually, absolute minimum in this case) rating apply to pins configured as outputs set to LOW?

I can replace the 1N4148s with Schottky diodes with a forward voltage of only 0.5 volts... but is it worth bothering with?

• Shottkey diodes may very well improve performance. To be safe you should consider driving your stepper from an external transistor or an H-bridge. – jippie Aug 17 '14 at 17:02
• I don't think that's necessary. This is a stepper motor in name only. It's actually a clock movement. There's virtually no torque being imposed and the stepper pulses are 30 mS wide at 1 Hz. Since the current is going through two 100 ohm resistors even if the coil was a dead short there would be only 16.5 mA flowing (Vcc is 3.3 volts). – nsayer Aug 17 '14 at 18:59

• Not so fast! It's not clear that the same current limit applies to the protection diodes inside the chip as applies to its transistors. – Chris Stratton Aug 18 '14 at 20:47