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SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER with CLOSED CAPTION DECODER
and ON-SCREEN DISPLAY CONTROLLER
MITSUBISHI MICROCOMPUTERS
M37150M6/M8/MA/MC/MF-XXXFP, M37150EFFP
57
Rev. 1.0
8.10.7 Reference Voltage Generating Circuit
and Comparator
The composite video signal clamped by the clamping circuit is input
to the reference voltage generating circuit and the comparator.
(1) Reference voltage generating circuit
This circuit generates a reference voltage (slice voltage) by us-
ing the amplitude of the clock run-in pulse in the line specified by
the data slice line specification circuit. Connect a capacitor be-
tween the VHOLD pin and the VSS pin, and make the length of
wiring as short as possible to prevent a leakage current from be-
ing generated.
(2) Comparator
The comparator compares the voltage of the composite video
signal with the voltage (reference voltage) generated in the refer-
ence voltage generating circuit, and converts the composite video
signal into a digital value.
Fig. 8.10.10 Clock Run-in Detect Register
8.10.8 Start Bit Detecting Circuit
This circuit detects a start bit at the line decided in the data slice line
specification circuit.
The detection of a start bit is as follows:.
A sampling clock is generated by dividing the reference clock out-
put by the timing signal.
A clock run-in pulse is detected by the sampling clock.
After detection of the pulse, a start bit pattern is detected from the
comparator output.
8.10.9 Clock Run-in Determination Circuit
This circuit determinates clock run-in by counting the number of pulses
in a window of the composite video signal.
The reference clock count value in one pulse cycle is stored in bits 3
to 7 of the clock run-in detect register (address 00E416). Read out
these bits after the occurrence of a data slicer interrupt (refer to
“8.10.12 Interrupt Request Generating Circuit”).
Figure 8.10.10 shows the structure of the clock run-in detect register.
b7 b6 b5 b4 b3 b2 b1 b0
Clock run-in detect register (CRD) [Address 00E416]
R W
Clock Run-in Detect Register
0
to
2
0
R —
Test bits
3
to
7
Number of reference clocks to
be counted in one clock run-in
pulse period.
Clock run-in detection
bit(CRD3 to CRD7)
0
R —
Read-only
After reset
Functions
Nam
Be