4x20 LCD display code

16 Mar 2010 . Edited: 17 Mar 2010

I just switched from a 2x20 LCD to a 4x20.  I loaded the example code and reset the row offset values based on my controller from Crystal Fontz.

I can not get the example code for the 4x20 LCD to compile.  I have imported directly from this link.


The standard 2x16 LCD code works fine.  I am not sure what the issue is.  Any help is appreciated.


It flags on line 40 with this error.

"No instance of constructor "mbed::DigitalOut::DigitalOut" matches the argument list (E289)" in file "/lcdtest/TextLCD.cpp"

17 Mar 2010 . Edited: 17 Mar 2010

Hi James,

The problem is related to the way pins are specified (the code looks a little dated). If you import it as files in to your project, then change the constructor to use PinName rather than int for the pins:

TextLCD(int rs, int rw, ...


TextLCD(PinName rs, PinName rw, ...

all should spring in to life. You'll have to change it in a couple of places, but hopefully that is about it.

The error is basically saying the compiler couldn't match the constructor you were (indirectly) requesting i.e. DigitalOut(int); with any it could find i.e. DigitalOut(PinName);

We're aiming to relaunch the cookbook soon, so we'll aim also to clean everything up at the same time as there is a fair amount of code from our beta up there which isn't quite compatible and needs some love.


17 Mar 2010

Simon,  Thank you for the reply.  I see exactly what you describe in the example.  I also updated the line to include the LCD layout.


TextLCD::TextLCD(PinName rs, PinName rw, PinName e, PinName d0, PinName d1,
PinName d2, PinName d3, int columns, int rows) : _rw(rw), _rs(rs),
_e(e), _d(d0, d1, d2, d3), _columns(columns), _rows(rows) {


However, I get a new error related to this.

"No instance of overloaded function "mbed::TextLCD::TextLCD" matches the specified type (E493)" in file "/lcdtest/TextLCD.cpp"

It also calls for a bracket "{" before the colon ":" at  : _rw(rw),"


I know I am close but do not have the code background to fix this.  If I copy the code from the 2x16 example it still flags.

17 Mar 2010

Hi James,

Can you publish the example if possible to give the full problem context? The "It also calls for a bracket "{" before the colon ":" at  : _rw(rw)," sounds fishy and I suspect it is therefore a simple typo/mistake elsewhere (only simple when once you've been burnt by obscure C error messages enough times of course!).


17 Mar 2010

Simon Here is the example code.

"main.cpp" This is a test code.


#include "mbed.h"
#include "TextLCD.h"

Serial usb(USBTX, USBRX); //define USB connection pins

Serial megasquirt(p28, p27); //define serial commection pins

/*LCD 2x20 parallel code pin assignment*/
TextLCD lcd(p21, p22, p23, p24, p25, p26, p30, 20, 4); // (rs, rw, e, d0, d1, d2, d3, n_column, n_rows)

DigitalOut myled(LED1);

int main() {
                usb.baud(115200); //define USB connection speed

/*    lcd.locate(0,1);
    lcd.printf("test2"); //
    lcd.printf("test3"); //
    lcd.printf("test4"); */





my "TextLCD.cpp"


/* draft mbed TextLCD 
 * (c) 2007/8, sford
#include "TextLCD.h"

#include "mbed.h"

using namespace mbed;

 * useful info found at http://www.a-netz.de/lcd.en.php
 * Initialisation
 * ==============
 * After attaching the supply voltage/after a reset, the display needs to be brought in to a defined state
 * - wait approximately 15 ms so the display is ready to execute commands
 * - Execute the command 0x30 ("Display Settings") three times (wait 1,64ms after each command, the busy flag cannot be queried now). 
 * - The display is in 8 bit mode, so if you have only connected 4 data pins you should only transmit the higher nibble of each command.
 * - If you want to use the 4 bit mode, now you can execute the command to switch over to this mode now.
 * - Execute the "clear display" command
 * Timing
 * ======
 * Nearly all commands transmitted to the display need 40us for execution. 
 * Exceptions are the commands "Clear Display and Reset" and "Set Cursor to Start Position" 
 * These commands need 1.64ms for execution. These timings are valid for all displays working with an 
 * internal clock of 250kHz. But I do not know any displays that use other frequencies. Any time you 
 * can use the busy flag to test if the display is ready to accept the next command.
 * _e is kept high apart from calling clock
 * _rw is kept 0 (write) apart from actions that uyse it differently
 * _rs is set by the data/command writes

TextLCD::TextLCD(PinName rs, PinName rw, PinName e, PinName d0, PinName d1, 
    PinName d2, PinName d3, int columns, int rows) : _rw(rw), _rs(rs), 
    _e(e), _d(d0, d1, d2, d3), _columns(columns), _rows(rows) {

//    _rows = 2;
//    _columns = 16;

    _rw = 0;
    _e  = 1;
    _rs = 0; // command mode

    // Should theoretically wait 15ms, but most things will be powered up pre-reset
    // so i'll disable that for the minute. If implemented, could wait 15ms post reset
    // instead
    // wait(0.015); 
    // send "Display Settings" 3 times (Only top nibble of 0x30 as we've got 4-bit bus)
    for(int i=0; i<3; i++) {
        wait(0.00164);      // this command takes 1.64ms, so wait for it
    writeNibble(0x2); // 4-bit mode
    writeCommand(0x28);    // Function set 001 BW N F - -  
    writeCommand(0x6);  //  Cursor Direction and Display Shift : 0000 01 CD S (CD 0-left, 1-right S(hift) 0-no, 1-yes

int TextLCD::_putc(int value) {
    if(value == '\n') {
    } else {
    return value;

int TextLCD::_getc() {
    return 0;

void TextLCD::newline() {
    _column = 0;
    if(_row >= _rows) {
        _row = 0;
    locate(_column, _row); 

void TextLCD::locate(int column, int row) {
    if(column < 0 || column >= _columns || row < 0 || row >= _rows) {
        error("locate(%d,%d) out of range on %dx%d display", column, row, _columns, _rows);
    _row = row;
      _column = column;
      int address=0;
      if (_rows > 2) {
        // row 0 : 0x0->0x13
        // row 1 : 0x40->0x53
        // row 2 : 0x14->0x27
        // row 3 : 0x54->0x67
        switch (_row) {
            case (0) : address = 0x00 + _column;
            case (1) : address = 0x40 + _column;
            case (2) : address = 0x14 + _column;
            case (3) : address = 0x54 + _column;
      else {
         // memory starts at 0x80, and is 40 chars long per row
          address = 0x80 + (_row * 40)  + _column; 

void TextLCD::rows(int rows) {
    _rows = rows;

void TextLCD::columns(int columns) {
    _columns = columns;

void TextLCD::cls() {
    writeCommand(0x01); // Clear Display
    wait(0.00164f);        // This command takes 1.64 ms
      locate(0, 0);

void TextLCD::reset() {

void TextLCD::clock() {
    _e = 0;
    wait(0.000040f);  // most instructions take 40us
    _e = 1;    

void TextLCD::writeNibble(int value) {
    _d = value;

void TextLCD::writeByte(int value) {
    writeNibble(value >> 4);
    writeNibble(value >> 0);

void TextLCD::writeCommand(int command) {
    _rs = 0;

void TextLCD::writeData(int data) {
    _rs = 1;
    if(_column >= _columns) {



/* draft mbed TextLCD 
 * (c) 2007/8, sford

#include "Stream.h"
#include "DigitalOut.h"
#include "BusOut.h"
#include "mbed.h"

namespace mbed {

class TextLCD : public Stream {


    TextLCD(int rs, int rw, int e, int d0, int d1, int d2, int d3);
    void rows(int rows);
    void columns(int columns);
    virtual void locate(int row, int column);
    virtual void cls();    
    virtual void reset();

    void clock();
    void writeData(int data);
    void writeCommand(int command);
    void writeByte(int value);
    void writeNibble(int value);
    virtual int _putc(int c);        
    virtual int _getc();
    virtual void newline();                
    int _rows;
    int _columns;
    int _row;
    int _column;    
    DigitalOut _rw, _rs, _e;
    BusOut _d;






17 Mar 2010

Hey James,

I have written a quick program for a 20X4 LCD screen which compiles fine and is using the latest LCD library. Try and see if it works for you.


17 Mar 2010

I think the library files are incorrect with respect to an offset.  Locating at (0,0) to print "TEST" I get te following [location - printed character]:

(3,19) - "E" and (0,0) - "DU5".

I wonder if the memory location for the character tables, offset values for screen location and possibly timing are incorrect for this particular display.  These were some of the things I was trying to correct when I ran into the compile issue mentioned above.


17 Mar 2010

Btw Vlad, your code (and the other variants) work great on the 2x20 LCD dislplay on my desk.  It is an Optrex DMC20261 compared to the 4x20, Crystal Fontz CFAH2004k.


I really appreciate the effort here.  I have some code from an Atmel Atmega32 that I will try to port over to the mbed.  I know it works well with the 4x20 display.

17 Mar 2010

My mistake, Have you tried Simons LCD test code, it will show you your RAM offsets

Its about half way down the thread at: http://mbed.org/forum/mbed/topic/195/

17 Mar 2010

Hi James,

A problem in your code is that the header file (TextLCD.h) still uses int rather than PinName - hence the constructors not matching. If you change that it'll compile (with a warning, that won't be a problem).


17 Mar 2010

I thought I fixed those.  I found the problem.  The program now compiles yet still yielded the same results.

I am in the process of reconfiguring the initialization sequence.  My display calls for a slightly different process.  I will let you know if I get it to work.

I think I may have a timing issue.  It seems that every few resets I can get the display to recognize and print the correct characters.  the problem is that it is intermittant and that makes trouble shooting difficult.


24 Mar 2010

Still no go on the 4 line display.  2 line works perfectly.

I can get the first two lines of the 4 line display to work with the 2 line code but I get no output with the number rows set to 4.  I am not sure what the issue is.


14 May 2010


Have you succeeded on the 4 line display?

If not, please refer to my project on the notebook.



17 May 2010

I did succeed.  I went through the manual and changed the initialization delays.  My device was a bit slower and would get confused.


Thank you.