I bought some cheap HD44780-compatible LCD-displays on ebay to have
something to play with (never used them before). The seller supplied
some tutorials that I'm working with. I get the display to work on my
breadboard when using 8-bit mode (controlled by a PIC 16F628A), but I
cannot make it work using 4-bit mode. When I use 4-bit, D7-D4 is
connected to RB3-RB0. When 8-bit, D7-D0 is connected to RB7-RB0.

I've switched both the PIC and LCD to eliminate these sources of
errors, so I'm probably doing something wrong in my code. Nothing else
than the LCD-display and my Wisp-programmer is connected to the PIC.

Here's the code:

#include <htc.h>

__CONFIG(INTIO & WDTDIS & PWRTDIS & MCLREN & UNPROTECT & BORDIS & LVPDIS);

#define RS RA2
#define RW RA3
#define E  RA4
#define D4 RB0
#define D5 RB1
#define D6 RB2
#define D7 RB3

void lcd_wait(void) {
    unsigned char i;
    unsigned int j;

    for (i = 0; i < 5; i++) {}
    for (j = 0; j < 6000; j++) {}
}

void init_delay(void) {
    unsigned int i;

    for (i = 0; i < 23456; i++) {}
}

void init_MCU(void) {
    TRISA = TRISB = 0;                          // All outputs
    PORTA = PORTB = 0;                          // Clear garbage
    OSCF = 1;                                   // 4MHz INTOSC
    CMCON = 0b00000111;                         // Disable comparators
    VRCON = 0;                                  // Disable voltage
reference-module
    T1CON = 0;                                  // Disable Timer1
    CCP1CON = 0;                                // Disable CCP
}

void lcd_cmd(unsigned char cmd) {
    // According to the tutorial, only D7-D4 is used when 4-bit mode
    // is selected. RB3-RB0 goes to D7-D4 so we'll mask and shift
    // the commands.
    PORTB = (cmd & 0xF0) >> 4;
    E = 1;
    lcd_wait();
    E = 0;

    PORTB = cmd & 0x0F;
    E = 1;
    lcd_wait();
    E = 0;
}

void lcd_cmd_8bit(unsigned char cmd) {
    PORTB = cmd;
    E = 1;
    lcd_wait();
    E = 0;
}

void lcd_putc(char letter) {
    RW = 0;
    RS = 1;
    lcd_cmd(letter);
}

void lcd_print(char *str) {
    char *p = str;
    while (*p)
        lcd_putc(*p++);
}

void main(void) {
    char snase[]  = "ABC";
    init_MCU();
    init_delay();

    // RA1 is a LED (for debugging)
    RA1 = 1;

    // This works fine in 8-bit, but not in 4-bit
    lcd_cmd(0b00111000);          // Set 4-bit transfer, 2 lines
    lcd_cmd(0b00001111);          // Display on, cursor on, blink, underline
    // lcd_cmd_8bit(0b00111000);          // Set 4-bit transfer, 2 lines
    // lcd_cmd_8bit(0b00001111);          // Display on, cursor on,
blink, underline

    lcd_print(snase);

    RA1 = 0;
    while(1) {
        NOP();
    }
}

What happens in the 4-bit mode is that the display responds to my
commands, but instead of printing three letters (ABC) I get like
"OOOOOO\\\\\\NNNNNN". That is, each letter is repeated and deformed.

But - even weirder - is that this result is not 100% reproducible.
Sometimes the display turns on and cursor blinks *before* the command
is sent, and when this happens no garbled text is printed. This is the
reason I tried to change displays, but the behaviour is similiar on
everyone. 8-bit mode works fine for everyone.

Can anyone shed some light on this? Is my lcd_cmd() erronious?


-- 
- Rikard - http://bos.hack.org/cv/
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist