CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
Since the CX25870/871 contains this new 3:2 Clocking Mode, the designer
does not face this constraint any longer. By choosing an appropriate
autoconfiguration mode, setting the PLL_32CLK bit to 1, and altering the values
for various timing registers within the controller and encoder (e.g.,
H_CLKI = HTOTAL, VLINES_I = VTOTAL, H_BLANKI, V_BLANKI, etc.),
the encoder switches into the 3:2 Clock mode. While in this operational state,
additional solutions in the overscan-compensation-pairs domain for higher
resolutions now exist. In addition, the encoder now allows the data master (e.g.,
graphics controller) to send digital data to it at a faster rate than is clocked out of
the encoder. Specifically, the CX25870/871 begins to transfer pixels out at a rate
of [2/3] that of the CLKI input frequency. In other words, the pixel input
frequency clocks in data at a ratio of [3:2] or 1½ times faster than the
CX25870/871 outputs the analog pixel data. In this mode, the encoder's expansive
on-chip FIFO bridges the frequency difference that now exists between the
digital-timing input and mixed-signal encoder output blocks of the CX25870/871.
The result is a much closer match in the available overscan percentages in the
horizontal and vertical direction for the higher resolutions. This ensures the TV
Out picture appears more orthogonal where the amount of blanking is nearly
equal on all sides of the image.
Since the Horizontal Blanking Time only becomes a critical issue at higher
resolutions, the user should use a 3:2 Clocking Mode only when necessary at
800x600, and always at 1024x768. For software programming ease, some of the
autoconfiguration modes for 800x600 and all for the 1024x768 resolution are 3:2
solutions already. The specific modes that use the 3:2 clock feature are contained
in Appendix C and summarized in Table 1-6 below.
Table 1-6. Autoconfiguration Solutions that Utilize 3:2 Clocking Mode
Autoconfiguration
Mode #
Active Resolution
Type of Digital Input
Overscan Ratio
Standard
Video Output Type
10
11
14
15
18
22
26
30
34
40
42
43
1024x768
1024x768
1024x768
1024x768
800x600
800x600
1024x768
1024x768
800x600
800x600
1024x768
1024x768
RGB
NTSC
RGB
Standard
Standard
Standard
Lower
PAL-BDGHI
NTSC
YCrCb
YCrCb
RGB
PAL-BDGHI
NTSC
YCrCb
RGB
Lower
NTSC
Lower
NTSC
YCrCb
RGB
Lower
NTSC
Higher
NTSC
RGB
Alternate
Higher
NTSC
RGB
NTSC
RGB
Higher
PAL-BDGHI
100381B
Conexant
1-17
CONEXANT [ CONEXANT SYSTEMS, INC ]
