TABLE 4. Character ROM Addressing

Address Range

R/W

Description

0x0000–0x2FFF

R

ROM Character Fonts, 190 two-color Character Fonts that are

read-only. The format of the address is as follows: A15-A14: Always

zero. A13-A16: Character value (0x00-0xBF are valid values)

A5-A1: Row of the character (0x00-0x011 are valid values) A0: Low

byte of line when a zero. High byte of line when a one. The low

byte will contain the first eight pixels of the line with data Bit 0

corresponding to the left most bit in the Character font line. The

high byte will contain the last four pixels and data. Bits 7-4 are

"don’t cares". Data Bit 3 of the high byte corresponds to the right

most pixel in the Character Font line.

0x3000–0x3FFF

R

ROM Character Fonts, 64 four-color Character Fonts that are

read-only. The format of the address is as follows: A15-A14: Always

zero. A13-A16: Character value (0xC0-0xFF are valid values)

A5-A1: Row of the character (0x00-0x011 are valid values) A0: Low

byte of line when a zero. High byte of line when a one. The low

byte will contain the first eight pixels of the line with data Bit 0

corresponding to the left most bit in the Character font line. The

high byte will contain the last four pixels and data. Bits 7-4 are

"don’t cares". Data Bit 3 of the high byte corresponds to the right

most pixel in the Character Font line.

0x4000–0x7FFF

-

Reserved

DISPLAY PAGE RAM

This address range (0x8000–0x81FF) contains the 512 characters, which comprise the displayable OSD screens. There must be

at least one End-Of-Screen code (0x00) in this range to prevent unpredictable behavior. NOTE: To avoid any unpredictable

behavior, this range should be cleared by writinga0tobit3ofthe FRMCTRL1 Register, 0x8400, immediately after power up.

There may also be one or more pairs of End-Of-Line and Skip Line codes. The codes and characters are written as 8-bit bytes,

but are stored with their attributes in groups of 13 bits. When writing, one byte describes a displayed character (CC), Attribute

Code (AC), End-Of-Screen (EOS), End-Of-Line (EOL) or Skip Line (SL) code.

When reading characters from RAM, bit 1 of the Character Font Access Register (0x8402) determines whether the lower 8 bits

or upper 4 bits of the Page RAM are returned. Table 5 gives the lower byte read, which is the first 8 character code bits when bit

1 of the Character Font Access Register is a 0. Table 6 gives the upper byte read, which is 4 attribute code bits when this bit is

set to a 1.

TABLE 5. Page RAM Lower Byte Read Data

Address Range

D7

D6

D5

D4

D3

D2

D1

D0

0x8000–0x81FF

CHAR_CODE[7:0]

TABLE 6. Page RAM Upper Byte Read Data

Address

D7

D6

D5

D4

D3

D2

D1

D0

0x8000–0x81FF

XXXX

ATTR_CODE[3:0]

RAM Data Format

Each of the 512 locations in the page RAM is comprised of a 12-bit code consisting of an 8-bit character or control code, and a

4-bit attribute code. Each of the characters is stored in sequence in the page RAM in bits 7:0. Special codes are used between

lines to show where one line ends and the next begins, and also to allow blank (or ‘skipped’) single scan lines to be added

between character lines. Table 7 shows the format of a character stored in RAM. Note that even though this is a 12- bit format,

reading and writing characters and codes is done in 8-bit bytes.

TABLE 7. Page RAM Format

ATTRIBUTE CODE

CHARACTER CODE

ATT[3:0]

CC[7:0]

Bits 11–4 (Character Code): These 8 bits define which of the 254 characters is to be called from the character ROM. Valid

character codes are 0x02-0xFF. Bits 3–0 (Attribute Code): these 4 bits address the attribute table used to specify which of the 16

locations in RAM specify the colors and enhanced features to be used for this particular character. Two seperate attribute tables

www.national.com

26