Embedded Lab

HD44780 based LCD displays are very popular for embedded projects because they are cheap, easy to interface, can display characters, consume power lot less than seven-segment displays, and most of the present day compilers have in-built library routines for them. However, the only disadvantage is that they require at least 6 I/O pins of microcontroller. Well, you may ask, isn’t that less than what seven-segment displays require? Yes, that’s true but there are circumstances where you don’t have left enough pins for LCD display. For example, if you are going to design a temperature sensor based on a PIC12F683 microcontroller, which has just 6 I/O pins, you won’t have pins for interfacing a LCD. One solution for cases like that is to use serial LCD’s. Serial LCD’s available in the market cost more than double of HD44780 based standard one. The serial LCD’s still have the standard LCD module but it has an extra built-in driver module that receives data/command from a host microcontroller in serial format and convert it to appropriate parallel format suitable for HD44780 driver input.

The goal of this project is to show how to make a 3-wire serial LCD driver for low pin-count microcontrollers like PIC12F series using a serial-in parallel-out shift register (74HC595 in this case). PIC12F683 has been chosen here to test the end product. Since 74HC595 is just a shift register with no intelligence of its own, the burden goes to the host microcontroller (PIC12F683) to appropriately transfer the LCD character/command data and control signals in serial format. This part of the intelligence is embedded in the firmware inside PIC12F683. So, a portion of program flash memory inside PIC12F683 is sacrificed for this purpose. The required three pins of PIC12F683 performs following functions.

1. Serial data transfer to 74HC595
2. Clock signal for the Shift/Storage register
3. Enable signal to HD44780 LCD Module

74HC595 Shift Register

74HC595 is a high-speed 8-bit serial in, serial/parallel-out shift register. It also has a storage register and 3-state outputs. The two registers (shift and stoarge) have separate clocks, SH_CP and ST_CP respectively. Data in the shift register is shifted on the positive-going transitions of SH_CP, and the content of shift register will be transferred to the storage register on a positive-going transition of the ST_CP. If both the clocks are together, the shift register will be one clock ahead of the storage register. The output enable (OE) pin, when pulled Low, enables the 8-bit data of the storage register to show up at the parallel output (Q0-Q7).

HD44780-based character LCD

HD44780 based character LCD displays use 14 wire connections: 8 data lines (D0-D7), 3 control lines (RS, E, R/W), and three power lines (Vdd, Vss, Vee). Some LCDs come with backlight features that help reading display data in low illumination conditions. They have two additional connections  (LED+ and LED-).

Pin Description

Control pins
The control pin R/W determines if the data transfer between the LCD module and an external microcontroller are actual character data or command/status. When the microcontroller needs to send commands to LCD or to read LCD status, it must be pulled low. Similarly, this must be pulled high if character data is to be sent to and from the LCD module.

The direction of data transfer is controlled by the R/W pin. If it is pulled Low, the commands or character data is written to the LCD module. And, when it is pulled high, the character data or status information from the LCD registers is read. In this project, we will use one way data transfer, i.e., from microcontroller to LCD module, so the R/W pin will be grounded permanently.

The enable pin (E) initiates the actual data transfer. When writing to the LCD display, the data is transferred only on the high to low transition of the E pin.

Power supply pins
LCD module data sheets recommend to use +5V d.c. supply for operation, but some LCDs may work well for a wider range of volatge (3.0 to 5.5 V). The Vdd pin (2) should be connected to the positive power supply and Vss (1) to ground. Pin 3 (Vee) is for display contrast adjusment, which is usually connected to a voltage between 0 and 2V by using a preset potentiometer.

Data pins
Pins 7 to 14 are data lines (D0-D7). Data transfer to and from the display can be achieved either in 8-bit or 4-bit mode. The 8-bit mode uses all eight data lines to transfer a byte, whereas, in 4-bit mode, a byte is transferred as two 4-bit nibbles. In the later case, only the upper 4 data lines (D4-D7) are used. This technique is beneficial as this saves some input/output pins of microcontroller.

For further details on LCDs, it is recommended to read these two articles from Everyday Practical Electronics magazine : How to use intelligent LCDs

3-wire LCD using shift register

Clock, Data, and Enable lines are served through GP1, GP5, and GP2 ports.

 /* 3-wire Serial LCD using 74HC595
    Rajendra Bhatt, Sep 6, 2010 */

sbit Data_Pin at GP5_bit;
sbit Clk_Pin at GP1_bit;
sbit Enable_Pin at GP2_bit;

// Always mention this definition statement
unsigned short Low_Nibble, High_Nibble, p, q,  Mask, N,t, RS, Flag, temp;

void Delay_50ms(){
 Delay_ms(50);
}

void Write_LCD_Nibble(unsigned short N){
 Enable_Pin = 0;
 // ****** Write RS *********
 Clk_Pin = 0;
 Data_Pin = RS;
 Clk_Pin = 1;
 Clk_Pin = 0;
 // ****** End RS Write

 // Shift in 4 bits
 Mask = 8;
  for (t=0; t<4; t++){
   Flag = N & Mask;
   if(Flag==0) Data_Pin = 0;
   else Data_Pin = 1;
   Clk_Pin = 1;
   Clk_Pin = 0;
   Mask = Mask >> 1;
  }
  // One more clock because SC and ST clks are tied
  Clk_Pin = 1;
  Clk_Pin = 0;
  Data_Pin = 0;
  Enable_Pin = 1;
  Enable_Pin = 0;
}
// ******* Write Nibble Ends

 void Write_LCD_Data(unsigned short D){
 RS = 1; // It is Data, not command
 Low_Nibble = D & 15;
 High_Nibble = D/16;
 Write_LCD_Nibble(High_Nibble);
 Write_LCD_Nibble(Low_Nibble);
 }

void Write_LCD_Cmd(unsigned short C){
 RS = 0; // It is command, not data
 Low_Nibble = C & 15;
 High_Nibble = C/16;
 Write_LCD_Nibble(High_Nibble);
 Write_LCD_Nibble(Low_Nibble);
}

void Initialize_LCD(){
 Delay_50ms();
 Write_LCD_Cmd(0x20); // Wake-Up Sequence
 Delay_50ms();
 Write_LCD_Cmd(0x20);
 Delay_50ms();
 Write_LCD_Cmd(0x20);
 Delay_50ms();
 Write_LCD_Cmd(0x28); // 4-bits, 2 lines, 5x7 font
 Delay_50ms();
 Write_LCD_Cmd(0x0C); // Display ON, No cursors
 Delay_50ms();
 Write_LCD_Cmd(0x06); // Entry mode- Auto-increment, No Display shifting
 Delay_50ms();
 Write_LCD_Cmd(0x01);
 Delay_50ms();
}

void Position_LCD(unsigned short x, unsigned short y){
 temp = 127 + y;
 if (x == 2) temp = temp + 64;
 Write_LCD_Cmd(temp);
}

void Write_LCD_Text(char *StrData){
 q = strlen(StrData);
 for (p = 0; p<q; p++){
  temp = StrData[p];
  Write_LCD_Data(temp);
 }

}

char Message1[] = "3-Wire LCD";
char Message2[] = "using 74HC595";

void main() {
CMCON0 = 7;  // Disable Comparators
TRISIO = 0b00001000;  // All Outputs except GP3
ANSEL = 0x00; // No analog i/p

Initialize_LCD();

do {
 Position_LCD(1,4);
 Write_LCD_Text(Message1);
 Position_LCD(2,2);
 Write_LCD_Text(Message2);
 Delay_ms(1500);
 Write_LCD_Cmd(0x01);  // Clear LCD
 delay_ms(1000);
} while(1);

}

 
Download source and HEX files

Output

Reference:

• How to use Intelligent Liquid Crystal Displays, by Julyan Ilett, (February, March 1997 EPE)
• Two-wire LCD:ATM18 display for the Elektor AVR Project, by Jurss and Rudolph, (elektor -5/2008)