[pdf] AVR340: Direct Driving of LCD Using General Purpose IO

Atmel / Microchip created this Application note on how to drive the low-power and low-cost LCD segment displays.

However, LCDs should be energized with AC signals, due to their internal capacitor-like structure. No worries: microcontrollers can send AC signals thanks to PWM-like on-off sequences. But thinking about this AC signal and how it energizes LCD segments is rather complex if you've never done it before.

In fact, the electrical hookup here can be more complex than you expect. Its still a cheap hookup only requiring resistors directly connected to the pins, but... you need to think about this.

Note: If you're only doing a simple "Static" LCD, a simpler application note (https://ww1.microchip.com/downloads/en/Appnotes/doc2569.pdf) will be sufficient.

But some LCD screens, especially the ones with more segments, will have multiplexing. And multiplexing requires a more complex signal and electrical hookup to accomplish successfully.