LM1237 datasheet(40/42 Pages) NSC

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Attribute Table and Enhanced Features (Continued)

TABLE 14. Attribute Tables and Corresponding Addresses (Continued)

Attribute Value (n)

Two-Color Attribute Table Address

Four-Color Attribute Table Address

1010b

0x8468–0x846B

0x8550–0x8557

1011b

0x846C–0x846F

0x8558–0x855F

1100b

0x8470–0x8473

0x8560–0x8567

1101b

0x8474–0x8477

0x8568–0x856F

1110b

0x8478–0x847B

0x8570–0x8577

1111b

0x847C–0x847F

0x8578–0x857F

BUTTON BOX FORMATION

The value of the most significant Enhanced Feature Bit

(EFB3) determines when to draw the left, right, bottom and

top sides of a Box. EFB1 denotes whether a box is raised or

depressed, and EFB2 denotes whether the box is normal or

‘heavy’. For normal boxes, the lowlight color is determined

by the color code stored in the register EF2. For the heavy

box feature, the lowlight is determined by the color code

stored in register EF3.

Boxes are created by a ‘pixel override’ system that over-

writes character cell pixel information with either the highlight

color (EF1) or low light shadow (EF2 or EF3) of the box. Only

the top pixel line of the character and the right edge of the

character can be overwritten by the pixel override system.

To form a complete box, the left hand edge of a box is

created by overwriting the pixels in the right most column of

the preceding character to one being enclosed by the box.

The bottom edge of a box is created by either —

•

overwriting the pixels in the top line of the character

below the character being enclosed by the box, or

•

overwriting the pixels in the top line of the skipped lines

below, in the case where skip lines are present below a

boxed character.

Characters should be designed so that button boxes will not

interfere with the character.

Some minor limitations result from the above box formation

methodology:

•

No box may use the left most display character in the

Display Window, or it will have no left side of the Box. To

create a box around the left most displayed character, a

transparent ‘blank’ character must be used in the first

character position. This character will not be visible on

the screen, but allows the formation of the box.

•

At least one skip line must be used beneath characters

on the bottom row, if a box is required around any char-

acters on this row in order to accommodate the bottom

edge of the box.

•

Skipped lines cannot be used within a box covering sev-

eral rows.

•

Irregular shaped boxes, (i.e., other than rectangular),

may have some missing edges.

Operation of the Shadow Feature

The shadow feature is created as follows: As each 12-bit line

in the character is called from ROM, the line immediately

preceding it is also called and used to create a ‘pixel over-

ride’ mask. Bits 11 through 1 of the preceding line are

compared to bits 10 through 0 of the current character line.

Each bit X in the current line is compared to bit X+1 in the

preceding line (i.e., the pixel above and to the left of the

current pixel). Note that bit 11 of the current line cannot be

shadowed. A pixel override output mask is then created.

When a pixel override output is 1 for a given pixel position,

the color of that pixel must be substituted with the color code

stored in the register EF3. Please see Figure 24 for an

example.

20023444

FIGURE 24. Operation of the Shadow Feature

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