Electronic – 74HC595 Shift register – output pin polarity


Most shift register tutorials I've seen have the output pins connected to ground (for instance: http://www.arduino.cc/en/Tutorial/ShiftOut), however I've seen at least one circuit that connects them to VCC instead (https://learn.adafruit.com/assets/7766).

These circuits both use the same chip (74hc595), and aside from reversing the LEDs look otherwise identical. is there something I'm missing?

Best Answer

Not really. High pins source current and low pins sink current, which means that outputs that lead to ground through LEDs are active high and outputs that lead to VCC are active low.

It may be an issue with logic families whose drive strength is highly asymmetrical, e.g. they can source much more current than they can sink or vice versa, but HCMOS has mostly symmetrical drive strength (within 20% or so).