Moving-message displays are ideal to get your message (advertisements, greetings, etc) across in an eye-catching way. You can make an LCD show a brief moving message by interfacing it to a microcontroller.

Here’s an AVR-based moving-message display that uses a 16×2 LCD display incorporating HD44780. The 16×2 LCD can display 16 characters per line and there are two such lines.

Circuit description
Fig. 1 shows the circuit for AVR ATmega16-based moving-message display on an LCD. It consists of an ATmega16 microcontroller, a 16×2 LCD, an SPI6 connector and a power supply section.

 Fig. 1: Circuit for AVR microcontroller-based moving-message display on the LCD
Fig. 1: Circuit for AVR microcontroller-based moving-message display on the LCD
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To derive the power supply for the circuit, 230V AC mains is stepped down by a 9V, 250mA secondary transformer, rectified by bridge rectifier module BR1A and filtered by capacitor C1. The voltage is regulated by a 7805 regulator. LED1 glows to indicate the presence of power in the circuit. The regulated 5V DC powers the entire circuit including SPI6 connector.

Port-C pins PC4 through PC7 of the microcontroller (IC2) are connected to data lines D4 through D7 of the LCD. The LCD control lines—read/write (R/W), register-select (RS) and enable (E)—are connected to PD6, PC2 and PC3 of IC2, respectively.

Why AVR microcontroller? AVR is faster and more powerful than 8051 microcontroller, yet reasonably cheaper and in-circuit programmable. Most AVR development software are free as these are Open Source. Moreover, discussions and tutorials on the AVR family of processors are available on the Internet.

ATmega16 is a high-performance, low-power 8-bit AVR microcontroller. It has 16 kB of in-system self-programmable flash, 1 kB of internal SRAM, 512 bytes of EEPROM, 32×8 general-purpose working registers and JTAG Interface (which supports programming of flash, EEPROM, fuse and lock bits).

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Some of the on-chip peripheral features are:
1. Two 8-bit timers/counters with separate pre-scaler and compare modes
2. One 16-bit timer/counter with separate pre-scaler, comparator and capture modes
3. Four pulse-width-modulation channels
4. 8-channel, 10-bit analogue-to-digital converter
5. Byte-oriented two-wire serial interface
6. Programmable serial USART
7. Master/slave serial peripheral interface
8. Programmable watchdog timer with separate on-chip oscillator

LCD display module
The project uses a Hitachi HD44780-controlled LCD module. The HD44780 controller requires three control lines and four or eight input/output (I/O) lines for the data bus. The user may choose to operate the LCD with a 4-bit or 8-bit data bus. If a 4-bit data bus is used, the LCD will require a total of seven data lines—three lines for sending control signals to the LCD and four lines for the data bus. If an 8-bit data bus is used, the LCD will require a total of eleven data lines—three control lines and eight lines for the data bus.

The enable control line is used to tell the LCD that the microconroller is sending the data to it. To send data to the LCD, first make sure that the enable line is low (0). When other control lines are completely ready, make enable pin high and wait for the LCD to be ready. This time is mentioned in the datasheet and varies from LCD to LCD. To stop sending the data, bring the enable control low (0) again. Fig. 2 shows the timing diagram of LCD control lines for 4-bit data during write operation.

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When the register-select line is low (0), the data is treated as a command or special instruction (such as clear screen and position cursor). When register-select is high (1), the text data being sent is displayed on the screen. For example, to display letter ‘L’ on the screen, register-select is set to high.

When the read/write control line is low, the information on the data bus is written to the LCD. When read/write is high, the program effectively queries (or reads) the LCD. This control command can be implemented using ‘C’ programming language.

For 4-bit interface data, only four bus lines (D4 through D7) are used for data transfer. Bus lines D0 through D3 are disabled. The data transfer between HD44780 and the microcontroller completes after the 4-bit data is transferred twice.

Fig. 2: Timing diagram of LCD control lines for 4-bit data during write operation
Fig. 2: Timing diagram of LCD control lines for 4-bit data during write operation

Controlling a standard numeric LCD is not that difficult. To display text on the LCD, correct library files for the LCD are needed. Many LCD libraries are available on the Internet, which are used in various applications. You may get confused which library is suitable for your application.

Libraries for LCDs found in AVRLIB library occupy unnecessary program memory space. To solve the problem, you can write your own library for LCD control.

Software program
This project demonstrates sending the text to the LCD controller and scrolling it across the LCD. For the project, AVR Studio 4 and WINAVR software need to be installed in your PC. Three program codes are used here—movm.c, lcd2.c and lcd2.h. The movm.c contains the text message to be scrolled on the LCD. lcd2.c and lcd2.h are the library files. The programming technique given here may not be the best as it uses a simple logic, but it works pretty fine.

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