3.0 PC Board Considerations
CX25870/871
3.6 Applications Information
Flicker-Free Video Encoder with Ultrascale Technology
Another difference between the two encoders is the default video output
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.
As a result of software register compatibility, no modifications to a customer’s
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.
3.6.1.2 Hardware
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.
Consequently, if the customer’s Bt868-designed PCB actually has no connects
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. Table 3-3 summarizes all the likely
alterations that need to be performed to existing designs.
Table 3-3. Hardware Modifications to Bt868/869-based PCB Required to Accommodate the CX25870/871 (1 of 3)
Pin
#
Bt868/9
Pin Name
CX25870/1
Pin Name
Comment
1
AGND
VDD
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.’
2
3
N/C
N/C
VDD
The digital power pin needs to be tied to 3.3V. This was a no connect for the Bt868/9.
XTL_BFO
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.
3-12
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
