Rev. 1.0, 02/00, page 793 of 1141
27.3.6
Automatic Frequency Controller (AFC)
The AFC averages the Hsync signal fluctuation of the video signal. Figure 27.20 shows the AFC
configuration. The AFC generates a reference clock having 576 or 448 times the frequency (576
×
fh or 448
×
fh) of the Hsync signal. From this clock, several clocks are generated, such as the
horizontal sync signal (AFCH signal), clock run-in detection window signal, start bit detection
window signal, V complement and mask counter clock when the V complement function is
selected, and the field detection window signal. The reference clock is also used as the dot clock
for the OSD; modifying the reference clock frequency can change the dot width of the character
display. To change the frequency, connect a circuit having a value suitable for the desired
frequency to the AFCosc and AFCpc terminals, and select the division ratio for the frequency-
dividing counter through the setting of the FRQSEL bit in SEPIMR. Note that the data slicer will
not operate when 448
×
fh is selected as the reference clock.
AFCLPF
R
C
VCO
AFCpc
AFCosc
HHK
HSEL
HCKSEL
Masking H
FSC
AFCH
AFC error output circuit
(comparator)
Internal Csync
generator
H complement
and mask
counter
External Hsync
Error signal
Switching
Switching
Reference clock
FRQSEL
Masking and
complementing H
Reference
Hsync signal
Internally generated
Hsync
External Hsync
Signals such
as dot clock
Low pass
filter
Frequency-
dividing counter
(Divided by
576 or 448)
R
Figure 27.20 AFC Configuration