Two wire Interface for LCD-with shift register

Using an LCD for a microcontroller such as the Arduino takes up quite a number of pins and actually doesn’t leave that much pins for other tasks. There are various solutions, like using an HC595 shiftregister (requiring three wires), a PCF 8574 or MCP23008 to create an I2C option or using a shift register such as the 74LS164. The added circuit does just that. It works on an Attiny85 as well.

A Library (by Malpartida) supporting this circuit can be found here: https://bitbucket.org/fmalpartida/new-liquidcrystal/downloads

A 100nf decoupling capacitor is necessary and should be placed near the  IC. Shiftregisters as the 74LS164 cause sudden changes to the current drawn from it’s power, and hence introduce noise. To counter this, a “decoupling capacitor” should be used as close to the shiftregister power pins as possible (Vcc and GND). Otherwise, the shiftregister may get faulty data and then the LCD won’t work as intended. This is also dependent on other factors, like wire lengths etc.

The resistor and the diode form an AND gate. They could be replaced by a proper AND gate, but that would increase the component count. The circuit can be easily built on a piece of 16×16 stripboard, even a 11×12 piece if you connect the LCD with wires rather than a connector.

As the top circuit doesn’t really show the logical functions of the 74LS164 but merely the lay out of the chip, I added the circuit below to show its logical function:
Program:

#include <LiquidCrystal_SR.h>
LiquidCrystal_SR lcd(8,7,TWO_WIRE);
//                   | |
//                   | \-- Clock Pin
//                   \---- Data/Enable Pin

void setup(){
lcd.begin(16,2);               // initialize the lcd
lcd.home ();                   // go home
//
}
void loop(){
lcd.home ();
lcd.print("LiquidCrystal_SR");
lcd.setCursor (0, 1 );
lcd.print("2 wire");
}

Proof of concept

It is possible to let the backlight be taken care of by the software as well. Connect the circuit below to pin 4 of the SN164 and to the backlight pins of the LCD:

Backlight

The stripboard should be adapted like this:

With backlight regulation

The full circuit like this:

As I still had an old Seiko M1632 display with no back-light and a DIL 2×7 connector rather than a SIL 1×16 connector, I also quickly made a design on perfboard for that:

for Seiko M1632

It is a 9×14 piece. If you are cramped for space you can do it on an 8×11 piece, smaller even if one uses a small standing  variable resistor. For the practical circuit I chose a slightly bigger 9×20 piece (no use trimming an already small piece), and no.. you don’t want to see the bottom side 🙂 :

As pin 4 of the 164 is used for the LCD backlight, but the Seiko doesn’t have a backlight, I decided  later to add an LED, connected through a 1 k resistor to pin 4 and be able to switch the LED on and off with the backlight command.

With regard to the potmeter for the contrast… if using a potmeter is not desirable for any reason, it is possible to set the Vlc to a voltage that is most optimal for most situations. The circuit shows how. Over the diode a voltage of about 0.65 Volt can be expected which is ideal for most situations:

8 bit Two wires

Above circuits are all for LCD in 4 bit mode as that is the most often used mode. However, Mike MacLaren pointed me to a circuit that uses a shiftregister (that link is dead, try this one) (either a 164 or a 595) that uses two wires and does full 8 bit addressing of the LCD. ( His program is for  a PIC, Check here as well)

Two Wire 8bit LCD with 164 Shiftregister

The above circuit uses an HC164 shiftregister, but Mike’s site also has a circuit for a 595 and a driver program for a PIC as well as an Arduino microprocessor. This circuit does not cater for software backlight control, but it has full 8 bit control, should you need that