Dot matrix LCD : the CGRAM

Dot-matrix LCD.

First, let us review the internal structure of the controller that manages the LCD module:

 

 

 

From the block diagram we see that the integrated circuits of the genus HD44780 have three different memory areas:

• CGROM, characters generator in ROM (read only memory, non-erasable)
• CGRAM, characters generator in RAM, readable and writable memory
• DDRAM, memory  that matches the characters on the display

Some compatible controllers implement an additional memory area:

• SEGRAM, programmable area for segment icons

 

All these controllers, however, manage characters in the format 5x8 or 5x11 points (5 columns x 8 lines or 11 lines), whose matrices are available in the character generator in ROM (CGROM). 

 

The procedure for presenting a character on the display consists of a few simple actions:

1. The controller, via the data bus (DB0-DB7) receives from 'host a hexadecimal code that is placed in an input buffer. The state of the line RS determines whether it is a command or a data to display.
    
2. If it is a datum, the code is saved in the DDRAM, the typical capacity of 80 cells of 8 bits each. All or part of these cells, depending on the size of the LCD panel, correspond to the characters displayed.
   
3. For the actual display, the controller sends the data in the DDRAM as addresses of a character generator (CG), the output of which is composed of 40 bits.
   These bits are processed by a system of parallel-serial shift register, that, through the appropriate timings, the controller uses for the ignition of points on the LCD panel. The 40-bit output from the GC are organized as an array of 8 lines and 5 columns. The 1 bits correspond to a point displayed, the zero bits are blanks.
   

Below, the character generator uses the ASCII codes, for which bytes 41h input as data, corresponds to the display of a capital letter.

Thus, the task of writing a character is simply in send data corresponding to the character you want in the DDRAM and the controller takes care of the complex self-management tasks LCD panel. If you want to write to the display, the letter 'A', just the sequence of statements:

movlw  0x41
call   senddatatolcd

senddatatolcd  is a subroutine that transfers the data from the accumulator to the W DDRAM controller.
During this operation, the lines RS and RW will assume the right level, by communicating to the controller that it is a data write:

 

Command

Control

Data

Function

Execution time

Write data to DDRAM

RS

RW

D7

D6

D5

D4

D3

D2

D1

D0

Send a data to DDRAM for display

37-100us

1

0

data to DDRAM

As mentioned, the DDRAM has a width of 80 bytes, which corresponds to a character in 'visible area on the LCD panel according to the number of lines and characters per line available.
For example, for a display with 4 lines of 20 characters each, all 80 bytes in memory DDRAM will be visible. In a display of one row to 8 characters, only 8 locations DDRAM will be displayed.

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