LCD, 4 bit data mode

Fabien Le Mentec May 21, 2013 Coded in C for the ATMEGA328P

LCD driving using 4 bit data mode to reduce the GPIO count

/* note: lcd model MC21605A6W */
/* note: DB0:3 and RW must be grounded */
/* note: see https://github.com/texane/lcmeter for usage */

#include <stdint.h>
#include <avr/io.h>

#define LCD_POS_DB 0x02
#define LCD_PORT_DB PORTD
#define LCD_DIR_DB DDRD
#define LCD_MASK_DB (0x0f << LCD_POS_DB)

#define LCD_POS_EN 0x06
#define LCD_PORT_EN PORTD
#define LCD_DIR_EN DDRD
#define LCD_MASK_EN (0x01 << LCD_POS_EN)

#define LCD_POS_RS 0x07
#define LCD_PORT_RS PORTD
#define LCD_DIR_RS DDRD
#define LCD_MASK_RS (0x01 << LCD_POS_RS)

static inline void wait_50_ns(void)
{
  __asm__ __volatile__ ("nop\n\t");
}

static inline void wait_500_ns(void)
{
  /* 8 cycles at 16mhz */
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
  __asm__ __volatile__ ("nop\n\t");
}

static inline void wait_50_us(void)
{
  /* 800 cycles at 16mhz */
  uint8_t x;
  for (x = 0; x < 100; ++x) wait_500_ns();
}

static inline void wait_2_ms(void)
{
  wait_50_us();
  wait_50_us();
  wait_50_us();
  wait_50_us();
}

static inline void wait_50_ms(void)
{
  /* FIXME: was _delay_ms(50), but not working */
  uint8_t x;
  for (x = 0; x < 25; ++x) wait_2_ms();
}

static inline void lcd_pulse_en(void)
{
  /* assume EN low */
  LCD_PORT_EN |= LCD_MASK_EN;
  wait_50_us();
  LCD_PORT_EN &= ~LCD_MASK_EN;
  wait_2_ms();
}

static void lcd_write_db4(uint8_t x)
{
  /* configured in 4 bits mode */

  LCD_PORT_DB &= ~LCD_MASK_DB;
  LCD_PORT_DB |= (x >> 4) << LCD_POS_DB;
  lcd_pulse_en();

  LCD_PORT_DB &= ~LCD_MASK_DB;
  LCD_PORT_DB |= (x & 0xf) << LCD_POS_DB;
  lcd_pulse_en();
}

static void lcd_write_db8(uint8_t x)
{
  /* configured in 8 bits mode */

  /* only hi nibble transmitted, (0:3) grounded */
  LCD_PORT_DB &= ~LCD_MASK_DB;
  LCD_PORT_DB |= (x >> 4) << LCD_POS_DB;
  lcd_pulse_en();
}

/* exported interface */

void lcd_setup(void)
{
  LCD_DIR_DB |= LCD_MASK_DB;
  LCD_DIR_RS |= LCD_MASK_RS;
  LCD_DIR_EN |= LCD_MASK_EN;

  LCD_PORT_DB &= ~LCD_MASK_DB;
  LCD_PORT_RS &= ~LCD_MASK_RS;
  LCD_PORT_EN &= ~LCD_MASK_EN;

  /* small delay for the lcd to boot */
  wait_50_ms();

  /* datasheet init sequence */

#define LCD_MODE_BLINK (1 << 0)
#define LCD_MODE_CURSOR (1 << 1)
#define LCD_MODE_DISPLAY (1 << 2)

  lcd_write_db8(0x30);
  wait_2_ms();
  wait_2_ms();
  wait_500_ns();

  lcd_write_db8(0x30);
  wait_2_ms();

  lcd_write_db4(0x32);
  wait_2_ms();

  lcd_write_db4(0x28);
  wait_2_ms();

  lcd_write_db4((1 << 3) | LCD_MODE_DISPLAY);
  wait_2_ms();

  lcd_write_db4(0x01);
  wait_2_ms();

  lcd_write_db4(0x0f);
  wait_2_ms();
}

void lcd_clear(void)
{
  /* clear lcd */
  lcd_write_db4(0x01);
  wait_2_ms();
}

void lcd_home(void)
{
  /* set cursor to home */
  lcd_write_db4(0x02);
  wait_2_ms();
}

void lcd_set_ddram(uint8_t addr)
{
  lcd_write_db4((1 << 7) | addr);
  wait_2_ms();
}

void lcd_goto_xy(uint8_t x, uint8_t y)
{
  /* assume 0 <= x < 8 */
  /* assume 0 <= y < 2 */

  /* from datasheet: */
  /* first line is 0x00 to 0x27 */
  /* second line is 0x40 to 0x67 */
  static const uint8_t row[] = { 0x00, 0x40 };
  lcd_set_ddram(row[y] | x);
}

void lcd_write(const uint8_t* s, unsigned int n)
{
  wait_50_ns();

  LCD_PORT_RS |= LCD_MASK_RS;
  for (; n; --n, ++s)
  {
    lcd_write_db4(*s);
    wait_2_ms();
  }
  LCD_PORT_RS &= ~LCD_MASK_RS;
}

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