Some Notes for HD44780 Based LCD Character Displays

LCD Module Interface

These character LCD modules usually have a standard 14 pin interface, commonly using pins with 0.1 inch between centres. If the module has an LED backlight, there are usually extra pins 15 and 16 to power the LEDs. The pinout is as follows:

 1.  Ground
 2.  Vcc  +3.3V to +5V
 3.  Contrast Adjustment
 4.  Register Select (RS) 0 for a command, 1 for data
 5.  Read/Write (R/W)  0 for Write, 1 for Read
 6.  Clock (Enable) Falling edge initiates transfer of data
 7.  Data Bit 0 (not used in 4-bit mode)
 8.  Data Bit 1 (not used in 4-bit mode)
 9.  Data Bit 2 (not used in 4-bit mode)
10.  Data Bit 3 (not used in 4-bit mode)
11.  Data Bit 4
12.  Data Bit 5
13.  Data Bit 6
14.  Data Bit 7
15.  Backlight Anode (+)
16.  Backlight Cathode (-)

Some older modules have a double row of 7 pins at one end of the module. You can use a 14 way ribbon cable and 14 pin IDC connector to connect up the module. More usually, there is a 14 or 16 pin single pin row, either above or below (sometimes both!) the display.

Contrast Adjustment, Pin 3, needs a voltage setting of between Vcc and Ground. Some models need a slight negative voltage here to get good contrast, but a lot of modules have adequate contrast when this pin is at Ground. For variable contrast it is often enough to wire up a protentiometer between Vcc and ground and connect pin 3 to the slider, and then adjust the pot for the required contrast.

Read/Write, Pin 5, can be tied to ground, as long as all there is no requirement to read from the module. All data and commands must be written slow enough to ensure that the module has completed any previous command or data writes. See the timings from the datasheet, and below.

Control & Timing

The 44780 controller takes time to execute various instructions, or to action interface changes. While it is busy it keeps the BUSY flag high, so polling this will tell you when the command has finished or when the next character can be sent. Alternatively, simply wait at least the time the datasheet quotes for various functions:

  Enable cycle time           500ns min    
  Enable pulse width, high    230ns min   
  Enable rise and decay time   20ns max
  Address setup time           40ns min   RS and R/W lines must be valid 40ns before enable pulse arrives 
  Address hold time            10ns min   RS and R/W must be valid at least 10 ns after enable goes low
  Data delay time             160ns max   On read data valid within 320ns of enable going high
  Data hold time, read          5ns min   On read data valid at least 20ns after enable goes low
  Data setup time             195ns min   On write data lines must be valid at least 195 ns before enable goes low
  Data hold time, write        10ns min   On write data lines must be valid for at least 10ns after enable goes low

8 Bit mode and 4 Bit mode

In 8 bit mode each byte on data or command is transferred in one cycle of the enable pin.

In 4 bit mode each byte is sent as 2 by 4 bit nibbles on data pins 4 - 7. The first 4 bit nibble is the high order 4 bits of the byte, and the second 4 bit nibble is the low order 4 bits. While this is obviously slower, it needs less controller pins - useful when the controlling device pin usage is getting tight.

Some Useful References

Jim Jackson <jj@franjam.org.uk> Wed Dec 30 20:22 GMT 2015

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