USAGE OF CG RAM

Character Generator (CG) RAM is a useful accessory. It does not have to be used or attended to during any normal display operation. CG RAM allows the creation of up to 8 special character or symbols. Once programmed, the newly formed characters may be accessed as if they were in the "normal" CG ROM. This ROM contains 192 unchangeable characters. Thus the CG RAM expands the character representation available to the user.

NOTE: This is a RAM, and must be reprogrammed if display power is interrupted. If used regularly, programming can be made part of the initialization routine.

There are two distinct areas of RAM within the display module. The main area, 80 bytes wide, is dedicated to the display and is called Display Data (DD) RAM. CG RAM consists of 64 bytes which range from 40 to 7F (hex), or 4 5x10 (or 5x11) symbols. 40-47 locate the first, custom 5x7 character. 40 is the top row of this character, 47 is the 8th row. Similarly, 48-4F locate the second CG character, and 78-7F locate the 8th custom character. The locations 40-7F are the CG "Programming" locations only. Once programmed, these special characters are displayed by writing to character font locations 00-07 (hex). 00 will return that character residing in locations 40-47, 01 returns 48-4F. etc. (See Font Chart).

While the CG RAM byte is 8 bits wide, only the 5 least significant bits appear on the LCD. Thus D4 represents the left-most dot and D0 the right-most dot. To illustrate, loading a CG RAM byte with 1F turns all dots in that row on; loading a byte with 00 turns all dots off. All 7 or 8 rows must be programmed at each desired CG location.

Programming procedure is:

a) with RS=0 enter the address of the top row of the character to be programmed (i.e. 40,48,50, etc.)

b) with RS=1 enter pattern data for row 1 (top row)

c) continue to enter pattern data for rows 2-8; it is not necessary to enter additional addresses if the module has been initialized with command 06 (auto increment of cursor).

This procedure may be continued until all CG bytes have been loaded.

The CG RAM can create an attractive, "reverse-video" 3 x 5 pattern. Numerals look especially good in this format. Most letters can be executed. The limitation of 8 characters can be circumvented by creating a "library" of custom symbols, each totaling 8, resident in the host system. Eight custom symbols can be displayed at any ONE time. The CG RAM can be periodically reloaded as display requirements change. If you reload a CG location which is currently on the display, the change will be immediately apparent. Displays employing multiple controllers (ie. 4 x 40, 2 x 80. 4 x 80) may create 8 symbols per controller. The CG RAM adds interest and flexibility to the LCD module.