This is an excerpt from A. Dyess, W. King, and T. Greer, " HILFD-1400," ECE 4542: Design II, Department of Electrical and Computer Engineering, Mississippi State University, Fall 2003:

The device's control system will have to spend time processing commands and refreshing the LED arrays and will incorporate a microcontroller in connection with digital logic to divide the work. The microcontroller will be the element of the system that is responsible for conforming to the HD44780 specification, which is the basis for several design constraints. The communication standard dictates the response time of the system, the initialization time of the system, the number of wires used for interfacing, and the minimum number of characters to be mapped by the system. These standards place requirements on our microcontroller such as the speed, the amount of RAM available, and the number of I/O pins.