9: Processor Board
PRC1099A-MS
9-3
GPI1 through GPI3 are for external interrupt sources while address lines A1
through A20 and data lines D0 through D15 communicate with the external
EPROM, RAM, and analog converter system.
9.1.2
EPROM Program Storage
EPROM chips U2 and U3 provide storage for the main software operating
program.
9.1.3
RAM Program Storage
RAM chips U4 and U5 provide storage for all the channel frequency data,
display data, and memory tuner information.
9.1.4
Clock Oscillator Circuit
The clock oscillator is internal within the processor, except for the external
crystal Y1, and capacitors C48 and C49.
9.1.5
Processor Controller
Controller U9 provides three functions. As a reset controller it has a voltage
detector that monitors the +5V line. It generates a reset output to hold the
processor’s reset line low whenever the +5V line is below 4.65 VDC. An
internal monostable multivibrator holds the reset line low for
200 milliseconds after the +5V line rises above 4.65 VDC. This prevents
repeated toggling of the reset line.
U9 is also a power failure detector that issues a non-maskable interrupt to the
processor whenever a power failure occurs. This lets the processor power
down in the correct state before data is lost.
U9 also functions as memory backup preserving the contents of RAM
memory storage. This is storage for frequency program information and must
be preserved at all times. When a power failure is detected, the lithium battery
is automatically switched in to preserve the RAM memory data.
9.1.6
Memory Backup Circuit
The memory backup power circuit includes memory backup battery BT1,
current limiting resistor R22, and reverse protection diode D3. This battery
provides backup power for the RAM chip to prevent settings loss.
9.1.7
RS-232 I/O Circuit
RS-232 chip U8 communicates with the front panel communications port. U8
has an on-board capacitor charge pump for generating the -10 VDC and
+10 VDC necessary for RS232 communications. This circuit can easily
handle baud rates up to 9600 baud. It also can be put into a low power idle
mode for low power consumption.
