CS4954 datasheet(24/56 Pages) CIRRUS

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CS4954 CS4955

DS278PP4

therefore allow the user to have full control over to

when sending teletext data to this device.

The time tFD is the time needed to interpolate tele-

text input data and inserting it into the CVBS and

Y output signals, such that it appears between

tTTX = 9.8 µs and tTTX =12 µs after the leading

edge of the horizontal synchronization pulse. tFD

changes with the TV standard and the selected

teletext standard. Please refer to ITU-R BT.653-2

for more detailed information.

The time tPD is the pipeline delay time introduced

by the source that is gated by TTXRQ in order to

deliver teletext data. This delay is programmable

through the register TTXHD. For every active

HIGH transition at output pin TTXRQ, a new tele-

text bit must be provided by the source. The time

between the beginning of the first TTXRQ pulse

and the leading edge of H-sync is programmable

through the TTXHS register.

Since the beginning of the pulses representing the

TTXRQ signal and the delay between the rising

edge of TTXRQ and valid teletext input data are

fully programmable, the TTXDAT data is always

inserted at the correct position after the leading

edge of the outgoing horizontal synchronization

pulse.

The time tTTXWin is the internally used insertion

window for TTX data; it has a constant length

depending on the selected teletext standard which

allows insertion of 360 TTX bits (6.9375

Mbit/sec) (European teletext) or 296 TTX bits

(5.6427875 Mbit/sec) (WST PAL) or 288 TTX bits

(5.727272 Mbit/sec) (NABTS) or 296 TTX bits

(5.727272 Mbit/sec) (WST NTSC) respectively.

Using the appropriate programming, all suitable

lines of the odd field (TTXOVS through TTX-

OVE) plus all suitable lines of the even field

(TTXEVS through TTXEVE) can be used for tele-

text insertion. In addition it is possible to selec-

tively disable the teletext insertion on single lines.

This

can

programmed

setting

the

TTX_LINE_DIS1,

TTX_LINE_DIS2

and

TTX_LINE_DIS3 registers appropriately.

Note that the TTXDAT signal must be synchro-

nized with the 27 Mhz clock. The pulse width of

the TTXRQ signal varies between three and four

27 Mhz clock cycles. The variation is necessary in

order to maintain the strict timing requirements of

the teletext standard.

Table 5 shows how to program the TTXHS register

for teletext instantiation into the analog signals for

the various supported TV formats. TTXHS is the

time between the leading edge of the HSYNC sig-

nal and the rising edge of the first TTXRQ signal

and consists of multiples of 27 Mhz clock cycles

Note that with increasing values of TTXHS the

time tTTX increases as well. The time tFD accounts

for the internal pipeline delay due to processing,

synchronization and instantiation of the teletext

data. The time tPD is dependant on the TTXHD

Figure 12. Teletext Timing (Pulsation Mode)

Figure 13. Teletext Timing (Window Mode)

CVBS/Y

TTXRQ

TTXDAT

textbit #: 1

2345

tTTX

tTTXWin

tPD

tFD

CVBS/Y

TTXRQ

TTXDAT

textbit #: 1

2345

tTTX

tTTXWin

tPD

tFD

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