cx25870 Conexant Systems, Inc., cx25870 Datasheet - Page 142

• Manufacturer Part Number: cx25870
• Description: Video Encoder With Adaptive Flicker Filtering And Hdtv Output
• Manufacturer: Conexant Systems, Inc.

3.0 PC Board Considerations
3.6 Applications Information
Table 3-3. Hardware Modifications to Bt868/869-based PCB Required to Accommodate the CX25870/871 (1 of 3)
3-12
Pin
#
1
2
3
3.6.1.2 Hardware
AGND
N/C
N/C
Pin Name
Bt868/9
VDD
VDD
XTL_BFO
CX25870/1
Pin Name
routing through the on-chip DACs. On power-up, the Bt868 transmitted Video[0]
= Composite from all three of its DACs. Due to the popularity of S-Video out, the
CX25871, on power-up, broadcasts Video[0] = Composite from DAC_A,
Video[1] = Luminance from DAC_B, Video[2] = Chrominance from DAC_C,
and Video[3] = Delayed Luminance from DAC_D. For Bt868 drivers that did not
program register 0xCE, this step may be necessary to re-route the Video outputs
with the CX25870.
source code are required to enable the same features that exist within both
Conexant VGA encoders. Of course, to exploit the new features within the
CX25870/871, such as display of 1024x768 resolution on a TV , HDTV output,
SECAM output, and others, some software changes and new register sets will be
necessary. This usually equates to the release of a new driver and/or graphics
BIOS for support of the CX25870.
Similarly, the Bt868/869 is pin-for-pin backward compatible with the newest
Conexant encoder. Both devices are housed in exactly the same compact 80-pin,
[14 mm x 14 mm x 2.4 mm] plastic PQFP package. Furthermore, aside from pins
2, 3, 65, 66, and 67, which were no connects within the Bt868/869, the
CX25870/871 is identical in its pinout to the previous generation.
for the pins listed as N/C on the Bt868/869, then no PC board changes are needed
except for some passive component stuffing changes when upgrading to the
CX25870/871. However, if the Bt868/869 N/C pins were actually grounded or
utilization of the new external features within the CX25870/871 is desired, then a
few changes to a customer’s Bt868/869-based PC Board are definitely required to
accommodate the new CX25870/871.
alterations that need to be performed to existing designs.
Another difference between the two encoders is the default video output
As a result of software register compatibility, no modifications to a customer’s
Consequently, if the customer’s Bt868-designed PCB actually has no connects
This pin should be tied to VDD (3.3V) for both the CX25870/1 and Bt868/9, so the
encoder’s output video levels match the IRE levels that it was designed to transmit.
Conexant has seen 2-3 IRE excursions away from the correct color bar and other test
pattern IRE levels and have verified that either encoder's pin #1 being tied to GND to be
the root cause.
An output video difference of 2-3 IRE is a very small amplitude AND would only be
noticeable if you used a VM700T from Tektronix or some other advanced piece of video
measuring equipment. Visually, it is quite difficult to even detect a 2-3 IRE excursion.
In conclusion, tie Pin #1 which was the Bt868/9’s ‘AGND’ to VDD/VAA = 3.3V for both
the CX25870/1 and Bt868/9 for best operation. Rename this pin (#1) on any
schematics so it says 'VDD.’
The digital power pin needs to be tied to 3.3V. This was a no connect for the Bt868/9.
The buffered crystal clock output pin should be floated if not used. This was a no
connect for the Bt868/9.
For CX25870/1-designs, a small (e.g. 33 ohm) series resistor should be added in series
to XTL_BFO as close as possible to the signal source device. This reduces overshoot
and undershoot on this signal as it changes states.
Conexant
Flicker-Free Video Encoder with Ultrascale Technology
Table 3-3
Comment
summarizes all the likely
CX25870/871
100381B