LCD Interface

• Based on HD44780 LCD controller (Hitachi).
• Block Diagram
  • E – enable
  • RS – register select
  • R/W – read/write (1 – read, 0 – write)
  • Data – 8 data bits
    • The data bits are bidirectional, but rarely is there a need to read from it.
• There are two types of information that can be written to the data pins:
  • Data (RS should be set (high)).
  • Command (RS should be cleared (low)).
  • A few useful commands:
    • clear – clears the display.
    • home – move cursor to home (first) position.
    • blink – cursor blinks.
    • auto-increment cursor position.

• Data bits are hardwired to PORTA.
• E hardwired to PORTC pin 7.
• RS hardwired to PORTC pin 6.
• WR hardwired to PORTD pin 6 (active low).
• RD hardwired to PORTD pin 7 (active low).

• Clear RS.
• Clear R/W.
• Delay 2ms.
• Set E.
• Send command to PORTA.
• Clear E.
• Delay 2ms.
• Set RS.
• Set R/W.
• Delay 2ms.

• Set RS.
• Clear R/W.
• Set E.
• Output data to PORTA.
• Clear E.
• Set R/W.
• Delay 2ms.

Send out sequence of commands:

• 0×38 – data length = 8, font is 5×7.
• 0x0f – display on, cursor on, cursor blink.
• 0×01 – clear display.
• 0×06 – auto-increment cursor.

Must repeat the command write process for each of these commands.

Example function to write a command to the LCD:

; Writes value in r20 to the LCD as a command
; Assumes ports configured for LCD and RD is set (PORTD, 7 is high)
; Dependencies: delay2ms
lcd_cmd_write:
        ; Set up LCD for accepting a command
        cbi    PORTC, 6     ; RS low --> put in command mode
        cbi    PORTD, 6     ; WR low --> enable write
        sbi    PORTC, 7     ; E high --> enable LCD interface
        out    PORTA, r20   ; write command to data pins
        ; Place LCD back in default configuration
        cbi    PORTC, 7     ; E low --> disable LCD interface
        sbi    PORTC, 6     ; WR high --> disable write
        sbi    PORTD, 6     ; RS high --> put in data mode
        rcall  delay_2ms    ; delay  TODO: must write this function
        ret

You should be able to write functions for:

• delay_2ms – delay for two milliseconds.
• lcd_data_write – write value in R20 to the LCD as data.
• lcd_init – sends a sequence of initialization commands to LCD.
• lcd_port_init – configures ports used by LCD.
• lcd_string_write – write the null terminated string stored in program memory (indicated by the Z pointer) to the LCD.
• lcd_clear – clears the LCD screen.