Building Display Pages

THE OSD WINDOW

The Display Page RAM contains all of the 8-bit display

character codes and their associated 4-bit attribute codes,

and the special 12-bit page control codes — the End-Of-Line,

Skip-Line parameters and End-Of-Screen characters. The

LM1236 has a distinct advantage over many OSD Genera-

tors in that it allows variable size and format windows. The

window size is not dictated by a fixed geometric area of

RAM. Instead, 512 locations of 12-bit words are allocated in

RAM for the definition of the windows, with special control

codes to define the window size and shape.

Window width can be any length supported by the number of

pixels per line that is selected divided by the number of

pixels in a character line. It must be remembered that OSD

characters displayed during the monitor blanking time will

not be displayed on the screen, so the practical limit to the

number of horizontal characters on a line is reduced by the

number of characters within the horizontal blanking period.

The EOS code tells the OSD generator that the character

codes following belong to another displayed window at the

next window location. An EOS code may follow normal char-

acters or an SL code, but never an EOL control code,

because EOL is always followed by an AC plus an SL code.

WRITING TO THE PAGE RAM

The Display Page RAM can contain up to 512 of the above

listed characters and control codes. Each character, or con-

trol code will consume one of the possible 512 locations. For

convenience, a single write instruction to bit 3 of the Frame

Control Register (0x8400) can reset the page RAM value to

all zero. This should be done at power up to avoid unpre-

dictable behaviour.

Display Window 1 will also start at the first location (corre-

sponding to the I

always contain the Skip-Line (SL) code associated with the

first line of Display Window 1. The attribute for this SL code

must be written before the SL code itself, and will be stored

in the lower four bits of this memory location. Subsequent

locations should contain the characters to be displayed on

line 1 of Display Window 1, until the EOL code or EOS code

is written into the Display Page-RAM.

The Skip-Line parameters associated with the next line must

always be written to the location immediately after the pre-

ceding line’s End-Of-Line character. The only exception to

this rule is when an End-Of-Screen character (value 0x0000)

is encountered. It is important to note that an End-Of-Line

character should not precede an End-Of-Screen character

(otherwise the End-Of-Screen character will be interpreted

as the next line’s Skip-Line code). Instead, the End-Of-

Screen code will end the line and also end the window,

making it unnecessary to precede it with a EOL. The I

Format for writing a sequence of display characters is mini-

mized by allowing sequential characters with the same at-

tribute code to send in a string as follows:

Byte #1: I

Byte #2: LSB Register Address

Byte #3: MSB Register Address

Byte #4: Attribute Table Entry to use for the following

Skip-Line code or characters

Byte #5: First display character, SL parameter, EOL or

EOS control code

Byte #6: Second display character, SL parameter, EOL or

EOS control code

Byte #7: Third display character, SL parameter, EOL or

EOS control code

Byte #n: Last display character in this color sequence, SL

parameter, EOL or EOS control code to use the

associated Attribute Table Entry.

TABLE 15. Sequence of Transmitted Bytes

This communication protocol is known as the Auto Attribute

Mode, which is also used by the LM1237 and LM1247. The

Attribute Entry (Byte #4, of the above) is automatically asso-

ciated with each subsequent display character or SL code

written. Please see examples of usafe for this mode in the

LM1237/LM1247 datasheets.

ENHANCED PAGE RAM ADDRESS MODES

Since the Page RAM in the LM1236 is 12 bits wide, usually two bytes of Page RAM information has to be sent to every location.

To avoid this, the LM1236 addressing control system has 3 additional addressing modes offering increased flexibility that may be

helpful in sending data to the Page RAM. Some of the left over bits in the Attribute byte are employed as data control bits to select

the desired addressing mode as shown in Table 16. This is identified as the first byte sent in a write operation or the Page RAM’s

upper byte read in Table 6.

TABLE 16. Attribute Byte

ATTRIBUTE Byte

X

DC[1]

DC[0]

X

ATT[3:0]

AUTO ATTRIBUTE MODE

The Auto Attribute mode is the standard LM1237 mode that is described above in the WRITING TO THE PAGE RAM section. The

attribute byte is shown in Table 17.

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