CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
1.3.7.5 Slave Interface
Without a Crystal
For price-sensitive applications, it is possible to remove the crystal found between
the XTALIN and XTALOUT ports and strictly utilize the incoming CLKI signal
as both the data transfer mechanism and internal main clock source for the
encoder. To complete this architecture, the data master must also program the
CX25870’s EN_XCLK bit to 1. This will trigger CLKI to be used for all
operations requiring a clock source and force the encoder to ignore any
oscillations received via its XTALIN and XTALOUT pins. The flicker filter and
timing blocks will utilize this asynchronous clock on the input side for data
processing, and the encoder will combine its internal PLL and CLKI in
conjunction with the DACs to transmit video from the device.
Since CLKI will be the only incoming frequency reference, the encoder uses
this signal to run its internal PLL for derivation of the video color subcarrier
(Fsc). Since PAL and SECAM televisions are not lenient in accepting color
subcarrier frequencies with more than 25 ppm error (i.e., Fsc 330 Hz), it is
critical the data master maintain a very high level of accuracy for the incoming
clock. In numerical terms, this means that the incoming clock should always
remain within a window of {ideal CLKI} 25 ppm. As an example, for
autoconfiguration mode #1, CLKI would have to reside in the range
[29.499270 MHz < ideal CLKI = 29.500008 MHz < 29.500746 MHz.]
Tight control of the incoming digital clock ensures that the CX25870
generates an analog Fsc of 4.433618 MHz 338 Hz for PAL-BGHI or 4.250000 /
4.406250 MHz 338 Hz for SECAM. Actual testing has found that excursions
outside this range result in loss of color for PAL and SECAM televisions and
sometimes affect NTSC sets in the same manner.
When the CX25870 is receiving an external clock, its serial bus is also
dependent on this incoming signal. As a result, the data master should never
disable the input clock. If this happens, even momentarily, the only way the
encoder can recover is for the data master to pin RESET* the CX25870. The
encoder will then be re-enabled as a timing master and respond again to serial
commands transmitted by the data master.
Several other registers must be reprogrammed to make this special type of
interface work properly. Consult your local Conexant representative for technical
assistance.
100381B
Conexant
1-21
CONEXANT [ CONEXANT SYSTEMS, INC ]
