Hitachi H8S/2199 Hardware Manual page 799

(1) AFC Oscillator
The AFCosc terminal, which is the oscillation signal terminal of the voltage controlled
oscillator (VCO), oscillates at 576 times the frequency (576 × fh) of the Hsync signal when the
Hsync signal is input at a certain phase and frequency. The difference in phase or frequency is
detected between the reference Hsync signal and the Hsync signal (AFCH signal) obtained by
dividing the 576 × fh signal, the error signal is converted to a voltage by a low pass filter
through the AFC error output circuit, and the voltage is used to control the VCO.
The VCO control voltage (the AFCLPF terminal voltage) is within a range from about 1.0 V to
4.0 V. The oscillating capacitance should be set so that the AFCosc oscillating frequency
becomes 576 × fh at the center (about 2.5 V) of the control voltage range. To set the oscillating
frequency to 448 × fh, change the values of the external circuits connected to the AFCpc and
AFCosc terminals and modify the FRQSEL bit in SEPIMR.
(2) AFCLPF
The AFC error output circuit detects the difference in phase or frequency between the
reference Hsync signal and the Hsync signal (AFCH signal) obtained by dividing the 576 × fh
or 448 × fh signal, and generates a pulse corresponding to the error. Connect a low pass filter
(LPF) to the AFCLPF terminal to average these error pulses. If the cut-off frequency is too
low, the oscillation stabilizing time (the pull-in time) needed to reach 576 × fh or 448 × fh
becomes long when a large error is detected or after the power is turned on; a high cut-off
frequency will cause jitter or an unstable display.
Connect a suitable LPF by referring to the external circuit examples shown in figures 27.21
and 27.22. When the Hsync signal includes a large disturbance, for example during special
playback operation, the AFC circuit may operate incorrectly.
(3) Reference Hsync Signal for AFC
The AFC reference clock is also used as the dot clock for the OSD. Accordingly, select the
reference Hsync signal depending on whether the OSD operates in the super-imposed mode or
text display mode. Refer to table 27.4, Reference Hsync Signal for AFC.
Rev. 1.0, 02/00, page 794 of 1141