2-wire LCD interface using PIC16CF84

Alphanumeric LCD displays have become very popular for microcontroller applications because they can add a lot to a project in a variety of different ways. A text message giving the user instructions as well as feedback can make the application seem much more “professional” and easy to use. I like to use LCD’s to help debug applications, with breakpoints set to display variable and I/O conditions and they are a lot cheaper than using a microcontroller emulator. To top it off, surplus LCD’s can be found for a dollar or less.

The most popular LCD interface is the Hitachi 44780 based LCD controller chip which provides a fairly easy to work with interface and low power consumption. The major drawback of the interface is the perceived complexity of working with the interface. This perception has been promoted by the lack of good (i.e. well translated) and accurate datasheets and web site information.

2-wire LCD interface using PIC16CF84Often the biggest stumbling block to using alphanumeric LCD displays is the number of pins required to control them. For the Hitachi 44780, twelve pins are required from the microcontroller to interface to the display for it to work in eight bit mode. For many smaller microcontrollers, twelve pins are not available or will be better served in the application. To be fair, this can be reduced to six by using the 44780’s “Four Bit” mode, but this can still be more than acceptable for most applications.

A popular solution that only requires one pin from the microcontroller to the LCD is the use of “Serial LCD Interfaces”  to the data and signals necessary for the Hitachi 44780 controllers.

Many of these products (such as the SLI-OEM) are excellent and can provide useful product interface and debugging information. The only drawback to them is the need for properly timed NRZ serial data which may be difficult or even impossible to guarantee in some applications.

In this case, different approaches have to be made. The most popular one is to use synchronous serial data (requiring a “clock” and “data”) pin to load a serial-in/parallel-out shift register with the data bits and “R/S” pin information. The “E” Strobe Pin is driven directly by the microcontroller to latch in the data from the LCD.

2-wire LCD interface using PIC16CF84 SchematicThe project presented in this article is an enhancement of this circuit. By combining the shift register’s “Data Line” with the most significant bit of the shift register, the “E” Strobe can be implemented without resorting to a separate line for the function. The 1 K resistor and diode act as an “AND” gate.

The operation of the resistor/diode “AND” gate may not be immediately obvious. When the shift register bit is low, the diode pulls the connection to the “E” pin low. When the shift register bit is high, the diode will not cause any current flow from the connection at the “E” pin to the shift register. The resistor from “Data” to the “E” pin is a current limiting resistor. When the shift register bit is low and the data bit is high, then the current through the resistor will be limited to 5 mA (for a 5 Volt logic application). At the “Data” side of the resistor, the voltage will still be high, even though the diode is pulling the “E” pin low.


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