Datasheet

PmodCLP™ Reference Manual

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Page 4 of 6

2.3 CGRAM and DDRAM

The LCD controller contains a character-generator ROM (CGROM) with 192 preset 5×8 character patterns (see

table 4 below), a character-generator RAM (CGRAM) that can hold 8 user-defined 5×8 characters, and a display

data RAM (DDRAM) that can hold 80 character codes. Character codes written into the DDRAM serve as indexes

into the CGROM (or CGRAM). Writing a character code into a particular DDRAM location will cause the associated

character to appear at the corresponding display location. Display positions can be shifted left or right by setting a

bit in the Instruction Register (IR). The write-only IR directs display operations (such as clear display, shift left or

right, set DDRAM address, etc). A busy flag shows whether the display has competed the last requested operation;

prior to initiating a new operation, the flag can be checked to see if the previous operation has been completed.

The display has more DDRAM locations than can be displayed at any given time. DDRAM locations 00H to 27H map

to the first display row, and locations 40H to 67H map to the second row. Normally, DDRAM location 00H maps to

the upper left display corner (the “home” location, and 40H to the lower left. Shifting the display left or right can

change this mapping. The display uses a temporary data register (DR) to hold data during DDRAM /CGRAM reads

or writes, and an internal address register to select the RAM location. Address register contents, set via the

Instruction Register, are automatically incremented after each read or write operation. The LCD display

uses ASCII character codes. Codes up through 7F are standard ASCII (which includes all “normal” alphanumeric

characters). Codes above 7F produce various international characters.

The predefined characters available to display (from the KS0066 datasheet) are shown in the table below: