ST63156 datasheet(57/86 Pages) STMICROELECTRONICS

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To avoid a bad visual impression, it is important

that the GE bit is set before the end of the flyback

time when changing characters. This can be done

inside the VSYNC interrupt routine. The following

diagram can explain better:

Notes: A - Picture time: 20 mS in PAL/SECAM.

B - VSYNC interrupt, if enabled.

C - Starting of OSD oscillator, if GE = 1.

D - Flyback time.

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 the

end of the flyback time (D in Figure 68). If the GE

bit is set after the end of the flyback time then the

OSD will not start until the begining 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 in-

terrupt at the start of the flyback; once the VSYNC

interrupt 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 order to know the status; this can be

done by reading bit 4 of register E4h; this bit is in-

verted with respect to the VSYNC pin.

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

ST63P1xx piggyback and ST631xx ROM, EPROM

and OTP devices when using the background

mode. If this bar is present with software running in

the piggybacks then it is also present on the ROM

mask version. If the end of line bar is seen to be

eliminated by software in the piggyback, then it is

also be eliminated in the ROM mask version.

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 backround is not displayed

during the space. The bar is the colour of the back-

ground defined by the space character. To elimi-

nate 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 charac-

ters should be moved so they appear at the

end of the line (and hence no bar); positioning

can be compensated using the horizontal start

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

background should be transparent in order to

eliminate the bar.

7 - The OSD oscillator external network should

consist of a capacitor on each of the OSD oscillator

pins to ground together with an inductance be-

tween pins. The user should select the two capaci-

tors 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 56

µH).

Figure 68. OSD Oscillator ON/OFF Timing

ON-SCREEN DISPLAY (Continued)

ST63140,142,126,156

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