ST6385 datasheet(55/84 Pages) STMICROELECTRONICS

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ST6365, ST6375, ST6385 ST6367, ST6377, ST6387

ON-SCREEN DISPLAY (Cont’d)

When modifying the picture display (i.e.: a bar

graph for an analog control), it is important that the

switching on of the GE bit is done before the end

of the flyback time (D in Figure 31). If the GE bit is

set after the end of the flyback time then the OSD

will not start until the beginning of the next frame.

This results in one frame being lost and will result

in a Flicker on the screen. One method to be sure

to avoid the flicker is to wait for the VSYNC inter-

rupt at the start of the flyback; once the VSYNC in-

terrupt is detected, then the GE bit can be set to

zero, the characters changed, and the GE set to

one. All this should occur before the end of the fly-

back time in order not to lose a frame. The correct

edge of the interrupt must be chosen. The VSYNC

pin may alternatively be sampled by software in or-

der to know the status; this can be done by reading

bit 4 of register E4h; this bit is inverted with respect

to the VSYNC pin.

6 - An OSD end of line Bar is present in the

ST638x ROM, EPROM and OTP devices when

using the box background mode.

The bar appears at the end of the line in the back-

ground mode when the last character is a space

character, the first format character is defined with

S=0 (size 0)and the box background is not dis-

played during the space. The bar is the colour of

the background defined by the space character.

To eliminate the bar:

a. If two backgrounds are used then the bar

should be moved off the screen by using large

word spaces instead of character spaces. If

there are not enough spaces before the end of

the line, then the location of the valid characters

should be moved so they appear at the end of

the line (and hence no bar); positioning can be

compensated using the horizontal start register.

b. If only one background is used, then the other

background should be disabled in order to elim-

inate the bar.

7 - The OSD oscillator external network should

consist of a capacitor on each of the OSD oscilla-

tor pins to ground together with an inductance be-

tween pins. The user should select the two capac-

itors to be the same value (15pF to 25pF each is

recommended). The inductance is chosen to give

the desired OSD oscillator frequency for the appli-

cation (typically 56µH).

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