P83C266BDR datasheet(62/92 Pages) PHILIPS

62 / 92 page

1999 Mar 10

Philips Semiconductors

Product speciﬁcation

Microcontrollers for PAL/SECAM TV

with OSD and VST

P8xCx66 family

17.8

Character ROM

128 character fonts may be stored in ROM: 126 customer

selected character fonts plus two reserved codes (the

Carriage Return code and the Space code).

Each character font is stored in a 12

× 19 dot matrix in

character ROM. However, only elements in rows 1 to 18

can be selected as visible dots on the TV screen. Row 0 is

used only in the North-West shadowing sub-mode and the

Border shadowing sub-mode when two character cells are

to be combined in a vertical direction to formulate a new

pattern. If combination of character fonts is not required

then row 0 should be filled with zeros. An example of a bit

pattern stored in ROM is shown in Fig.40.

A software package that helps the customer design the

character fonts on the screen and that also generates the

bit pattern HEX files automatically, is available on request,

from local Philips sales organisations. The package is run

under the MS-DOS environment for IBM compatible PCs.

17.9

Character ROM organization

ROM is divided into two parts: OSDL and OSDH.

The address of OSDL is from 0000H to 0FFFH and the

address of OSDH is from 1000H to 1FFFH.

The organisation of the bit patterns stored in ROM and the

file format to submit to Philips for customized character

sets is shown in Fig.40. Regarding Fig.40 the following

points should be noted.

1. Each character is structured using 38 bytes.

2. Row 0 of each font is reserved for vertical combination

of two fonts. In vertical mergence row 0 holds the

same code as row 18 of the previous font.

3. Binary 1 denotes visual dots.

4. ROM data files are in Intel HEX format on a byte basis.

Each byte is structured high nibble followed by low

nibble.

5. The unused bytes in ROM must be filled with FFH.

17.10 Combination of two or more font cells

Two (or more) character font cells may be combined in a

vertical or horizontal direction to create a new higher

resolution pattern.

The combination of two cells in a horizontal direction is

straight forward and presents no problems to the user.

All 4 background/shadowing sub-modes can be used.

However, the combination of two character font cells in a

vertical direction is more difficult and care must be taken

otherwise the new pattern may be created with gaps in its

shadowing.

• Row 0 in the character ROM is for use in the North-West

shadowing and Border shadowing sub-modes. If either

of these sub-modes is selected when combining two

character cells in a vertical direction then row 0 must

contain the bit pattern of row 18 of the font above it

otherwise a broken dot in the shadow may occur.

17.11 More about North-West shadowing and Border

shadowing sub-modes

Some special care must be taken when designing the

character bit pattern if the character is intended to be used

in the North-West shadowing or Border shadowing

sub-mode.

• North-West shadowing and Border shadowing

sub-modes are limited in the 18 display scan lines box

only in the vertical direction if the character in the next

row is not in North-West shadowing sub-mode

(otherwise as described in Section 17.10) if the shadow

of the last foreground bit pattern line is intended to be

seen as well. To get correct shadowing following the

character font then in:

– North-West shadowing: row 18 must be blank

– Border shadowing: row 1 and 18 must be blank.

• North-West shadowing and Border shadowing

sub-modes are not limited to the 12 horizontal OSD

dots. Shadows of the previous font cell may cross over

the 12 dot boundary and appear in the next font cell.

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