ST92196A datasheet(124/268 Pages) STMICROELECTRONICS

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- ON SCREEN DISPLAY CONTROLLER (OSD)

OSD CONTROLLER (Cont’d)

8.5.5 Programming the Display

The row-wise RAM buffer contains the description

of the characters to display:

– Row and character attributes (color, shape etc.)

– Horizontal shift code

– Character codes (addressing the Font ROM)

While one row buffer is displayed on the screen,

the CPU has time to prepare the content of the

next character row by filling up the second row

buffer. At the time the next row must be displayed,

the Display controller will point to the second row

buffer, allowing the CPU to start loading data into

the first row buffer for the following row. An inter-

rupt request is generated each time the buffer

pointer toggles.

Note: The display and the mouse share the same

interrupt line. In configurations with mouse, the

DINT and MOINT bits in the OSDFBR register can

be used to determine the interrupt cause.

The Vertical location of the next character row on

the screen is programmed by software through the

Event Line value (Refer to Figure 65). The vertical

position of the beam is memorized by the Line

counter which counts the TV horizontal synchroni-

zation pulses (called here "Scan Line"). When the

Scan Line counter matches the Event Line value

the buffer toggle mechanism is activated.

8.5.5.1 OSDRAM Mapping

The OSDRAM is mapped in segment 22h of the

memory space.

In addition to row buffers, it is used to store color

palettes and information concerning mouse point-

er.

Note: The reset value of the OSDRAM contents is

undefined.

General Overview

The Figure 59 gives a general overview of the OS-

DRAM mapping.

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