Saturday, 19 July 2014

Duet, 20x4 LCD screen and SD card.

Although the Duet communicates over Wifi and I can connect to it with a web browser on my phone, I still get many requests for a LCD based controller. I agree that being able to use a simple controller for quick, common functions is a good idea, things like homing the printer, preheating, starting a print etc. Think3dPrint3d have been providing controllers for older electronics that do this since we started, first with the Panelolu then with the Panelolu2 so it's time to port this to the Duet.

I decided to keep it simple to start with, so rather than get the Panelolu2 working, which would require I2C and headaches with 3.3V and 5V logic, I started with the PanelOne, a very basic LCD, Encoder, SD card combination which we are using on our Kossel Mini Kits.

Using a 5V LCD with 3.3V Microprocessor

This was the first hurdle to overcome. The HD44780 standard that these character LCD displays work on states 2.7 to 5V should work fine for the logic signals, however the LCD screens incorporating these chips are a lot more restrictive: 4.8-5.2V for the logic signals. Given the conflicting information I decided to modify a PanelOne prototype board by hand and give it a go. The key is to keep the logic signals separate from the backlight voltages. The schematic is below:

The major change is the addition of the "VCC" power into pin 2 of the LCD from pin 8 of header P1. Previously this was a straight 5V, now this should be able to be either 5V or 3.3V depending on the logic of the Microprocessor we are interfacing with. I connected this up and edited the "Hello World" sketch within Arduino 1.5.7 (1.5.6 r2 works as well) to use the following pin definitions:
                            Arduino pin number  Duet Pin Name  Expansion header pin
 * LCD RS pin to digital pin D67                 PB16          (32)
 * LCD Enable pin to digital D23                 PA14          (10)
 * LCD D4 pin to digital pin D19                 PA10 RXD0     (14)
 * LCD D5 pin to digital pin D18                 PA11 TXD0     (13)
 * LCD D6 pin to digital pin D17                 PA12 RXD1     (12)
 * LCD D7 pin to digital pin D16                 PA13 TXD1     (11)
LiquidCrystal lcd(67, 23, 19, 18, 17, 16);

Happily this worked!

I tested this with two other LCDs (old generic ones off eBay) and it worked with them as well. However as it's outside the guidelines of the LCD manufacturer's datasheets there is no guarantee that all types of HD44780 character LCDS will work in the same way.

Reading an SD card over SPI

The Duet's build in SD card supports SD2.0 and uses a 4 bit HSMCI interface. However it is often convenient to use an SD card reader mounted next to the screen, so I want to support SD over SPI which is how older 3D printer electronics and hardware interface with SD cards. The Arduino environment has a simple SPI SD library that worked straight away. The only customisation required is to make sure SD.Begin() uses the correct CS pin:


For this example I used the SPI0_NPCS0 pin which is arduino digital pin 77. I combined the listfiles example sketch with the LCD display to list the first 4 files on a SD card:

The code (an adaptation of the listfiles example sketch):

#include <SPI.h>
#include <SD.h>
#include <LiquidCrystal.h>

File root;
// initialize the library with the numbers of the interface pins

LiquidCrystal lcd(67, 23, 19, 18, 17, 16); //RS,E,D4,D5,D6,D7

void setup()
  lcd.begin(20, 4);
  lcd.print("Reading Files...");
  delay(2000); //delay or else the message will not be displayed

  if (!SD.begin(77)) {
   lcd.print("initialization done");
   delay(2000); //delay or else the message will not be displayed
   lcd.print("listing 4 files");
   delay(2000); //delay or else the message will not be displayed
   root ="/");
   printDirectory(root, 0);

void printDirectory(File dir, int numTabs) {
  int row=0;
   while(true) {
     if(row>3) return;
     File entry =  dir.openNextFile();
     if (! entry) {// no more files
     for (uint8_t i=0; i<numTabs; i++) {
       lcd.print(' ');
     if (entry.isDirectory()) {
       printDirectory(entry, numTabs+1);
     } else {
       // files have sizes, directories do not
       lcd.print("  ");
       lcd.print(entry.size(), DEC);


This sketch, and the adapted Kicad schematic are all on the Think3dPrint3d github.


I will test more LCD screens and hopefully will be able to get the next batch of PanelOnes compatible with both 3.3V and 5V.

The big job will be to port and adapt the Marlin menu system to RepRap Firmware. If anyone wants to assist with this project then drop me a line and I may be able to help with some hardware (duet board + LCD screen).