Rikard,

When I run an LCD in 4 bit mode, I always ground the 4 bits on the LCD that 
I'm not using, (ie.lcd bits 0-3).
The reasoning here is that if these pins float, you're likely to get 
something other than what you want during initialization because you have to 
put the unit in 4 bit mode during initialization.   Also, make sure you're 
actually putting it in 4 bit mode.  On your code below, the 4 bit mode and 
the 8 bit mode commands are the same.  Although the 8 bit commands are 
commented out, make sure it's correct for 4 bit mode.     I would check, but 
I don't have the command sheet at hand.
Let us know what you find out what the problem was.


Regards,   Jim




----- Original Message ----- 
From: "Rikard Bosnjakovic" <bos@grastorpsik.se>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, March 30, 2009 9:37 AM
Subject: [EE] Problems with LCD-display in 4-bit mode


>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
> 

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