NEC PD17062 - Manual (Page 274)

274
µ
PD17062
21. HORIZONTAL SYNC SIGNAL COUNTER
21.1 HORIZONTAL SYNC SIGNAL COUNTER CONFIGURATION
The horizontal sync signal counter counts the frequency of a horizontal sync signal for TV or similar
equipment. When a TV broadcasting signal is received, a prescribed horizontal sync signal is output. Using
this fact, the horizontal sync signal counter checks whether there is a broadcast station at a particular
frequency.
The horizontal sync signal counter consists of a 6-bit HSYNC counter (HSC), gate clock generator, gate
control register (HSCGT), gate input amplifier, and test gate open register (HSCGOSTT).
A signal supplied to the P0B
3
/HSCNT pin is amplified by the self-biased input amplifier. The output of the
amplifier passes through a gate which opens for a specific time interval specified by the gate control register.
After passing through the gate, the amplifier output is counted in the 6-bit HSYNC counter. When the gate
is closed, the HSYNC counter stops counting and sets 1 in the test gate open register. The HSYNC counter
is a read-only register. Reading the HSYNC counter finds out how many pulses are counted when the gate
is open. Dividing the number of pulses by the time during which the gate is open (1.69 ms) can obtain the
frequency of the horizontal sync signal. The P0B
3
/HSCNT pin is also used as an I/O port. It is assigned to the
P0B
3
port. When it is used as a horizontal sync signal counter, the P0B
3
must be set as an input port. When
it is used as a port, the HSCGT must be set with 0000B. When the P0B 3 is used as an input to the horizontal
sync signal counter, it is read always as 0.
Fig. 21-1 Horizontal Sync Signal Counter Block Diagram
HSCGOSTT
RF92Hb 3
(R/W)
HSCGT
RF91Hb 1 ,b 0
(R/W)
Gate clock generator
To a port
Gate input
amplifier
Gate HSYNC counter
Selector
Peripheral address 04H (R)