BT261 datasheet(12/56 Pages) ETC

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Bt261

30 MHz Pixel Clock Monolithic CMOS HSYNC Line Lock Controller

Brooktree®

CIRCUIT DESCRIPTION

VSYNC* Output

L261_H

VSYNC* Output

The VSYNC register deﬁnes the clock count from the falling edge of nongated

CSYNC* at which point CSYNC* is sampled to determine if the video signal is in

the vertical interval. Since nongated CSYNC is used to start the VSYNC counter,

two samples per horizontal line are taken during the vertical interval because of

equalization and serration pulses.

For each scan line that the sample is a logical zero, the VSYNC* output is a log-

ical zero. Thus, the VSYNC sample register should be programmed so that the

sample occurs well after the end of CSYNC* and before subsequent equaliza-

tion/serration pulses. VSYNC is output on the rising edge of PCLK.

FIELD Output

The FIELD output is derived from the vertical sync information. By positioning

the FIELD gate start/stop values a half-line interval apart, the half-line delay in

vertical sync during the second ﬁeld's vertical interval can provide a signal to dis-

tinguish the ﬁelds.

The FIELD output is clocked by VSYNC*. Therefore, FIELD start and stop

values should not coincide with VSYNC or VSYNC + HCOUNT/2. The FIELD

GATE start and stop values deﬁne the clock count, from the beginning of the hor-

izontal counter, at which to set the input to a register high (start value) or low (stop

value). This register is clocked by VSYNC, which occurs half a line later in the

transition from ﬁeld 1 to ﬁeld 2. The FIELD start and stop values must be separated

by, approximately, HCOUNT/2, so that opposite states are reported on subsequent

ﬁelds. The polarity is such that, with the FIELD GATE start value programmed

less than the stop value, FIELD output is toggled high to indicate the beginning of

ﬁeld 1 and toggled low to indicate the beginning of ﬁeld 2. The polarity of the

FIELD pin may be inverted by swapping the start and stop register values.

Figure 1 illustrates the operation of the FIELD gate and FIELD output.

BT261 Datasheet(PDF) 12 Page - List of Unclassifed Manufacturers