Hitachi HD44780 LCD controller

The Hitachi HD44780 LCD controller is an alphanumeric dot matrix liquid crystal display (LCD) controller developed by Hitachi in the 1980s. The character set of the controller includes ASCII characters, Japanese Kana characters, and some symbols in two 28 character lines. Using an extension driver, the device can display up to 80 characters. The HD44780 is one of the most popular character LCDs ever made, with numerous third-party displays utilizing its 16-pin interface and instruction set for compatibility.
Architecture.
The Hitachi HD44780 LCD controller is limited to monochrome text displays and is often used in copiers, fax machines, laser printers, industrial test equipment, and networking equipment, such as routers and storage devices.
Compatible LCD screens are manufactured in several standard configurations. Common sizes are one row of eight characters (8×1), and 16×2, 20×2 and 20×4 formats. Larger custom sizes are made with 32, 40 and 80 characters and with 1, 2, 4 or 8 lines. The most commonly manufactured larger configuration is 40×4 characters, which requires two individually addressable HD44780 controllers with expansion chips as a single HD44780 chip can only address up to 80 characters.
Character LCDs may have a backlight, which may be LED, fluorescent, or electroluminescent.
Character LCDs use a 16 contact interface, commonly using pins or card edge connections on 0.1 inch (2.54 mm) centers. Those without backlights may have only 14 pins, omitting the two pins powering the light. This interface was designed to be easily hooked up to the Intel MCS-51 XRAM interface, using only two address pins, which allowed displaying text on LCD using simple MOVX commands, offering cost effective option for adding text display to devices. The pinout is as follows:
The nominal operating voltage for LED backlights is 5V at full brightness, with dimming at lower voltages dependent on the details such as LED color. Non-LED backlights often require higher voltages (I.E. electroluminescent).
Mode selection.
In 8-bit mode all transfers happen in one cycle of the enable pin with all 8 bits on the data bus and the RS and RW pins stable. In 4-bit mode, data are transferred as pairs of 4-bit "nibbles" on the upper data pins, D7-D4 with two enable pulses and the RS and RW pins stable. The four most significant bits (7-4) must be written first, followed by the four least significant bits (3-0). The high/low sequence must be completed each time or the controller will not properly receive further commands. 
Selecting 4-bit or 8-bit mode requires careful selection of commands. There are
two primary considerations. First, with D3-D0 unconnected, these lines will
always appear low (0b0000) to the HD44780. Second, the
LCD may initially be in one of three states:
State3 may occur, for example, if a prior control was aborted after sending
only the first 4 bits of a command while the LCD was in 4-bit mode.
The following algorithm ensures that the LCD is in the desired mode:
The same command is sent three times, Function Set with 8 bit interface D7-D4 = 0b0011, the lower four bits are don't care, using single Enable pulses. If the controller is in 4 bit mode the lower four bits are ignored so they can't be sent until the interface is in a known size configuration. 
Starting in state 1 - 8 bit configuration
 Send Function Set command. Command will be executed, set 8 bit mode. 
 Send Function Set command. Command will be executed, set 8 bit mode. 
 Send Function Set command. Command will be executed, set 8 bit mode. 
Starting in state 2 - 4 bit configuration, waiting for first 4 bit transfer
 Send Function Set command. First four bits received. 
 Send Function Set command. Last four bits, command accepted, set 8 bit mode. 
 Send Function Set command. Command will be executed, set 8 bit mode. 
Starting in state 3 - 4 bit configuration, waiting for last 4 bit transfer
 Send Function Set command. Last four bits, unknown command executed. 
 Send Function Set command. In 8 bit mode command will be executed, else first 4 bits. 
 Send Function Set command. 8 bit command will be executed or last 4 bits of previous command, set 8 bit mode. 
In all three starting cases the bus interface is now in 8 bit mode, 1 line, 5×8 characters. If a different configuration 8 bit mode is desired an 8 bit bus Function Set command should be sent to set the full parameters. If 4 bit mode is desired 0b0010 should be sent on D7-D4 with a single enable pulse. Now the controller will be in 4 bit mode and a full 4 bit bus Function Set command sequence (two enables with command bits 7-4 and 3-0 on subsequent cycles) will complete the configuration of the Function Set register.
Instruction set.
The HD44780 instruction set is shown below:
DDRAM is Display Data RAM and CGRAM is Character Generator RAM. The DDRAM is 80 bytes (40 per row) addressed with a gap between the two rows. The first row is addresses 0 to 39 decimal or 0 to 27 hex. The second row is addresses 64 decimal to 103 decimal or 40 to 67 hex. 
The CGRAM is read/write memory used to encode up to 8 characters in the character generator. It consists of 64 fields at addresses 0 to 3F. Each field is 5 bits mapping to a row of pixels of each character. Each 8 fields in the CGRAM are used for each character. The lower 3 bits of the character codes from 0-7 and 8-15 select the groups of 8 fields in the CGRAM memory.
Reading and writing to the DDRAM is done by setting the RS input high during bus transfers. The DDRAM must also be selected by using the Set DDRAM address command which selects the DDRAM for access and also sets the starting address for DDRAM access. 
Likewise reading and writing to the CGRAM is done by setting the RS input high during bus transfers. The CGRAM must also be selected by the Set CGRAM address command which selects the CGRAM for access and also sets the starting address for CGRAM access. 
The execution times listed in this table are based on an oscillator frequency of 270 kHz. The data sheet indicates that for a resistor of 91 kΩ at 5V Vcc the oscillator can vary between 190 kHz and 350 kHz resulting in wait times of 52.6 us and 28.6 us instead of 37 us. If a display with the recommended 91 kΩ resistor is powered from 3.3 volts the oscillator will run much slower. If the busy bit is not used and instructions are timed by the external circuitry, this should be taken into account.
Font.
The original HD44780 character generator ROM contains 208 characters in a 5×8 dot matrix, and 32 characters in a 5×10 dot matrix. More recent compatible chips are available with higher resolution, matched to displays with more pixels.
Two versions of the ROM have been developed:
The 7-bit ASCII subset for the Japanese version is non-standard: it supplies a Yen symbol where the backslash character is normally found, and left and right arrow symbols in place of tilde and the rubout character.
A limited number of custom characters can be programmed into the device in the form of a bitmap using special commands. These characters have to be written to the device each time it is switched on, as they are stored in volatile memory.