LCD Controller

This page describes a VHDL module for driving alphanumeric LCDs. It drives the very common HD44780 chip, a driver IC found on many LCD modules.

Overview
LCD VHDL Driver Module
Xilinx PicoBlaze Example
Interface PCB
Further LCD Information

Overview

This LCD Driver module was designed with two aims in mind:

Simple: Make writing to the display as simple and flexible as possible.

From a hardware point-of-view, it would be simplest to connect the LCD pins directly to the microcontroller's general purpose IO (GPIO) pins. The disadvantage is you need to support the LCD protocol in software. Instead, it should be possible to write character data directly to memory locations that map to character positions.
Fast: Make updating the screen as fast as possible.

The LCD is a fairly slow device to operate. Communicating directly with the LCD over GPIO (without using interrupts) would require some long delays, during which the microcontroller is doing nothing. Instead, the microcontroller should be able to write character data without the need to pause between or after writes.

These two aims are achieved using the system below. The finite state machine (FSM) simplifies the operation by taking care of the LCD interface protocol. The off-screen buffer makes it fast, as it can accept new character data as fast as the microcontroller can write it.

LCD Driver Overview

LCD VHDL Driver Module

The LCD driver modules makes use of the LCD's 4bit data mode. This saves IO, simplifies PCB routing and is still fast enough for quick full screen refreshes. The port definition for the module is shown below. To meet the LCD's timing requirements the module must be supplied with the frequency of the clk input. This is set with the CLK_FREQ generic.

The off screen buffer is loaded using dIn, charNum and wEn. This is a very simple interface. The screen location specified by charNum is written with the value specified by dIn on a rising clock edge when the wEn input is high. The module is written for a 2x20 character LCD, so the charNum input ranges from 0 to 39. The module can be very easily adjusted for different display sizes.

The actual LCD is connected to the driver module via lcdData, lcdRS and lcdE. These connect directly to the LCD connections with the same name. As the module only writes character data to the LCD, there is no need for a lcdRW signal.

File Excerpt: LCDDriver4Bit.psm

entity LCDDriver4Bit is
   generic (CLK_FREQ    : positive);       -- Frequency of clk input in Hz
   port (  clk          : in    std_logic;
            reset       : in    std_logic;

            -- Screen Buffer Interface
            dIn         : in    std_logic_vector(7 downto 0);
            charNum     : in    integer range 0 to 39;
            wEn         : in    std_logic;

            -- LCD Interface
            lcdData     : out   std_logic_vector(3 downto 0);
            lcdRS       : out   std_logic;
            lcdE        : out   std_logic);
end LCDDriver4Bit;

Download the module using the link below.

LCDDriver4Bit.vhdl - LCD Driver Module

Note: This design continually updates the LCD module, even if the off screen buffer (within the FPGA) does not change. This is not particularly efficient. However, if you're worried about power consumption or excessive IO traffic, it would be a simple modification to the VHDL in order to only update the LCD module when the off screen buffer has been changed.

Xilinx PicoBlaze Example

The LCD driver module is very easy to interface to the Xilinx PicoBlaze microcontroller core. The example system shown below is based on the Digilent Nexys 2 Development Board, but is easily used with other setups.

LCD Driver connected to Xilinx PicoBlaze

LCD connected to Digilent Nexys

The PicoBlaze code shown below reads in the state of the switches and uses this value to set the value in the LCD character screen buffer. The LCD Module then takes care of writing this to the actual display.

File: lcd.psm

; Simple LCD Test code

CONSTANT LCD_CHARNUM, 00
CONSTANT LCD_DATA,    02
CONSTANT SWITHCES,    01

init:
   load    s0, 00                   ; Initialise address reg to zero

loop:
   output  s0, LCD_CHARNUM          ; Write LCD Char Address

   input   s1, SWITHCES             ; Read Switches
   output  s1, LCD_DATA             ; Write LCD Data

   add     s0, 01                   ; Increment address
   compare s0, 28                   ; Check for address 40 (0x28)
   jump    NZ,   loop

   load    s0, 00                   ; Reset address to zero
   jump    loop

Here are the files:

System.vhdl - Top level module

ProgramROM.vhdl - Program ROM that connects to PicoBlaze

System.ucf - Constraints file (for Digilent Nexys 2 FPGA Board)

lcd.psm - PicoBlaze Assembler program

Interface PCB

Below is a simple PCB for interfacing the LCD to a Digilent Nexys2 FPGA development board. The pot is used to set the LCD's contrast.

Below is the connections for the Nexys2 JA1 IO port (the connector nearest the power socket). Note: the connector pin numbers differ between the development board header and the interface PCB header. Next to it is a pin diagram for the LM032XMBL LCD module.

Note: The LCD uses 5V IO whereas the FPGA uses 3.3V IO. As the LCD is only being written to, level translation is not required, as the 3.3V output signals can drive the 5V inputs, with a slight loss in noise immunity.

The Lab Book Pages

LCD Controller

Overview

LCD VHDL Driver Module

Xilinx PicoBlaze Example

Interface PCB

Further LCD Information