X98021

FN8219 Rev 3.00

Page 21 of 29

March 8, 2006

HSYNC and VSYNC Activity Detect

Activity on these bits always indicates valid sync pulses, so

they should have the highest priority and be used even if the

SOG activity bit is also set.

SOG Activity Detect

The SOG activity detect bit monitors the output of the SOG

slicer, looking for 64 consecutive pulses with the same period

and duty cycle. If there is no signal on the Green (or Y)

channel, the SOG slicer will clamp the video to a DC level and

will reject any sporadic noise. There should be no false positive

SOG detects if there is no video on Green (or Y).

If there is video on Green (or Y) with no valid SOG signal, the

SOG activity detect bit may sometimes report false positives (it

will detect SOG when no SOG is actually present). This is due

to the presence of video with a repetitive pattern that creates a

waveform similar to SOG. For example, the desktop of a PC

operating system is black during the front porch, horizontal

sync, and back porch, then increases to a larger value for the

visible portion of the screen. This creates a repetitive video

waveform very similar to SOG that may falsely trigger the SOG

Activity detect bit. However, in these cases where there is

active video without SOG, the SYNC information will be

Active bit should only be considered valid if HSYNC Activity

Detect = 0. Note: Some pattern generators can output HSYNC

and SOG simultaneously, in which case both the HSYNC and

the SOG activity bits will be set, and valid. Even in this case,

however, the monitor should still choose HSYNC over SOG.

TriLevel Sync Detect

Unlike SOG detect, the TriLevel Sync detect function does not

check for 64 consecutive trilevel pulses in a row, and is

therefore less robust than the SOG detect function. It will report

false positives for SOG-less video for the same reasons the

SOG activity detect does, and should therefore be qualified

with both HSYNC and SOG. TriLevel Sync Detect should only

be considered valid if HSYNC Activity Detect = 0 and SOG

Activity Detect = 1.

If there is a SOG signal, the TriLevel Detect bit will operate

correctly for standard trilevel sync levels (600mVp-p). In some

real-world situations, the peak-to-peak sync amplitude may be

significantly smaller, sometimes 300mVp-p or less. In these

cases the sync slicer will continue to operate correctly, but the

TriLevel Detect bit may not be set. Trilevel detection accuracy

can be enhanced by polling the trilevel bit multiple times. If

HSYNC is inactive, SOG is present, and the TriLevel Sync

Detect bit is read as a 1, there is a high likelihood there is

trilevel sync.

CSYNC Present

If a composite sync source (either CSYNC on HSYNC or SOG)

is selected through bits 3 and 4 of register 0x05, the CSYNC

Present bit in register 0x01 should be set. CSYNC Present

detects the presence of a low frequency, repetitive signal

inside HSYNC, which indicates a VSYNC signal. The CSYNC

Present bit should be used to confirm that the signal being

received is a reliable composite sync source.

SYNC Output Signals

The X98021 has 2 pairs of HSYNC and VSYNC output signals,

incoming sync signals; no synchronization is done. These

signals should be used for mode detection.

are synchronized to the output DATACLK and the digital pixel

applications.

separator function) remain active in power-down mode. This

allows them to be used in conjunction with the Sync Status

registers to detect valid video without powering up the X98021.

incoming signal’s period, polarity, and width to aid in mode

sync signal: either horizontal or composite sync. If a SOG input

TABLE 6. SYNC SOURCE DETECTION TABLE

HSYNC

DETECT

VSYNC

DETECT

SOG

DETECT

TRILEVEL

DETECT

RESULT

1

1

X

X

Sync is on HSYNC and VSYNC

1

0

X

X

Sync is composite sync on HSYNC. Set Input configuration register to CSYNC on HSYNC and

confirm that CSYNC detect bit is set.

0

0

1

0

Sync is composite sync on SOG. It is possible that trilevel sync is present but amplitude is too

low to set trilevel detect bit. Use video mode table to determine if this video mode is likely to

have trilevel sync, and set clamp start, width values appropriately if it is.

0

0

1

1

Sync is composite sync on SOG. Sync is likely to be trilevel.

0

0

0

X

No valid sync sources on any input.