An HD44780-type display can perform a number of functions, but we will just focus on the two most important functions we need, which is (a) init the display, and (b) display a line of text on the display. The HD44780 datasheet gives the required initialization sequence for both 4-bit and 8-bit mode (remember, we are using 4-bit), as well as the commands to write a character to a specific X-Y location on the screen.
The HD44780 datasheet gives a sequence of instructions called "Initialization by Instruction". This sequence is a universal initialization sequence, which will work no matter what state the display is in. It consists of 8 commands sent to the display, with specified delays between commands. There is a futher complication that some of the commands are sent as 8-bit commands, and some as 4-bit commands. I won't list the commands here since you can see them in the software listing to follow, in the function lcd_init().
There is one serious omission in all the datasheets I've seen. The Clear display function (0x01) does not indicate how long it takes to execute. Don't assume (as I did) that it is one of those 37us commands. It actually seems to be one of the 1.52ms commands. Some displays work without the long delay, but others don't. I speak from recent debugging session experience.
This function sets the display address according to the desired X-Y coordinates of the string, and then writes the string characters into the display memory.
The last important function is one to clear the entire display. This function sends out a single "clear screen" command.
Here is all the code to get the display running in 4-bit mode. The two delay functions are included in this listing even thought they might be in another file in a typical project.
// lcd.c// AVR version// 4-bit interface #define LCD_USE_BF // define to use busy flag #define MS_COUNT (F_CPU/14003) // depends on clock speed, toolchain and settings void nano_delay(void){} void tiny_delay(volatile u8 d){ while (--d != 0) ;} void ms_delay(u16 d){ while (d-- != 0) { volatile u16 i = MS_COUNT; while (i-- != 0) ; }} // LCD is Port A#define LCD_PORT PORTA#define LCD_DD_PORT DDRA#define LCD_IN_PORT PINA #define LCD_D4 _BV(PA4)#define LCD_D5 _BV(PA5)#define LCD_D6 _BV(PA6)#define LCD_D7 _BV(PA7)#define LCD_RS _BV(PA1) // 0=CMD, 1=DATA#define LCD_RW _BV(PA2) // 0=WR, 1=RD#define LCD_E _BV(PA3)#define LCD_BL _BV(PA0) #define LCD_CTRL (LCD_RS | LCD_RW | LCD_E)#define LCD_DATA (LCD_D4 | LCD_D5 | LCD_D6 | LCD_D7)#define LCD_BLITE (LCD_BL)#define LCD_BF (LCD_D7) #define DISP_INIT 0x30#define DISP_4BITS 0x20#define DISP_ON 0x0c#define DISP_OFF 0x08#define DISP_CLR 0x01#define CUR_HOME 0x02#define DISP_EMS 0x06#define DISP_CONFIG 0x28 #define DD_RAM_ADDR 0x00#define DD_RAM_ADDR2 0x40#define DD_RAM_ADDR3 (DD_RAM_ADDR+0x14)#define DD_RAM_ADDR4 (DD_RAM_ADDR2+0x14)#define CG_RAM_ADDR 0x40 #define LCD_LINES 4#define LCD_WIDTH 20 void lcd_strobe(void){ LCD_PORT |= LCD_E; // start E pulse nano_delay(); //tiny_delay(LCD_STROBE); LCD_PORT &= ~LCD_E; // end E pulse (must be >= 230ns) nano_delay(); //tiny_delay(LCD_STROBE);} void LCD_PORT_data(u8 d){ // write upper 4 bits of data to LCD data lines LCD_PORT = (LCD_PORT & ~LCD_DATA) | (d & 0xf0);} #ifdef LCD_USE_BFvoid lcd_wait(void){ u8 data; LCD_DD_PORT &= ~LCD_DATA; // all data lines to input LCD_PORT &= ~LCD_RS; // cmd LCD_PORT |= LCD_RW; // set to read do { LCD_PORT |= LCD_E; // 1st strobe, read BF nano_delay(); //tiny_delay(LCD_STROBE); data = LCD_IN_PORT; LCD_PORT &= ~LCD_E; nano_delay(); //tiny_delay(LCD_STROBE); LCD_PORT |= LCD_E; // 2nd strobe, don't read data nano_delay(); //tiny_delay(LCD_STROBE); LCD_PORT &= ~LCD_E; nano_delay(); //tiny_delay(LCD_STROBE); } while (data & LCD_BF); // loop while BF is set LCD_PORT &= ~LCD_RW; // set to write LCD_DD_PORT |= LCD_DATA; // all data lines to ouput}#endif void lcd_send_cmd(u8 cmd){#ifdef LCD_USE_BF lcd_wait();#endif LCD_PORT &= ~LCD_RS; LCD_PORT_data(cmd & 0xf0); // send hi 4 bits of cmd lcd_strobe(); LCD_PORT_data((cmd & 0xf) << 4); // send lo 4 bits of cmd lcd_strobe();#ifndef LCD_USE_BF tiny_delay(90);#endif} void lcd_putc(u8 c){#ifdef LCD_USE_BF lcd_wait();#endif LCD_PORT |= LCD_RS; LCD_PORT_data(c & 0xf0); // send hi 4 bits of data lcd_strobe(); LCD_PORT_data((c << 4) & 0xf0); // send lo 4 bits of data lcd_strobe();#ifndef LCD_USE_BF tiny_delay(90);#endif} void lcd_init(void){ LCD_DD_PORT = LCD_CTRL | LCD_DATA | LCD_BLITE; LCD_PORT &= ~LCD_RW; // set to write, permanently LCD_PORT &= ~LCD_RS; LCD_PORT &= ~LCD_E; ms_delay(15); // must be >= 15 LCD_PORT_data(DISP_INIT); lcd_strobe(); // pseudo 8-bit command ms_delay(5); // must be >= 4.1 LCD_PORT_data(DISP_INIT); lcd_strobe(); // pseudo 8-bit command ms_delay(1); // must be >= 100us LCD_PORT_data(DISP_INIT); lcd_strobe(); // pseudo 8-bit command ms_delay(1); LCD_PORT_data(DISP_4BITS); lcd_strobe(); // pseudo 8-bit command ms_delay(1); lcd_send_cmd(DISP_CONFIG); lcd_send_cmd(DISP_OFF); lcd_send_cmd(DISP_CLR); ms_delay(2); // undocumented but required delay for Clear display command lcd_send_cmd(DISP_EMS); lcd_send_cmd(DISP_ON); lcd_set_backlight(1);} void lcd_clear(void){ lcd_send_cmd(DISP_CLR); ms_delay(2); // undocumented but required delay for Clear display command} void lcd_display(int x, int y, const char *str){ int n = LCD_WIDTH - x; u8 addr; if ((y < 0) || (y >= LCD_LINES)) return; switch (y) { default: case 0: addr = DD_RAM_ADDR; break; case 1: addr = DD_RAM_ADDR2; break; case 2: addr = DD_RAM_ADDR3; break; case 3: addr = DD_RAM_ADDR4; break; } lcd_send_cmd(addr + x + 0x80); while (*str && n--) lcd_putc(*str++);} void lcd_set_backlight(u8 on){ if (on) LCD_PORT |= LCD_BL; // turn on backlight else LCD_PORT &= ~LCD_BL; // turn off backlight}