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Technical Supplement
Whenever SPST analog switches U11A and U11D are closed by HSO0 (active low), the DC portions
of the IR and red signals create a charge, which is stored on C29 and C89, respectively. These
capacitors hold this charge even after the switches are opened and the resulting voltage is subtracted
from the combined signal— leaving only the AC modulation output. This AC signal is superimposed
on the baseline voltage output by U7C and U7D. The IRDC and REDDC are then filtered and input
to the CPU, and can be measured at TP58 and TP54, respectively.
9.9.8.2 AC Variable Gain Control Circuits
The AC variable gain control circuit is illustrated in Figure 9-13 (at the end of this section).
The AC modulations are amplified by U7A (red) and U7B (IR) and superimposed on the baseline
voltages present at the output of U7D (IR) and U7C (red). The amplification is handled by means of
the SPDT analog multiplexing switch U3 within the feedback loop, which increases gain as PWM0 is
increased. The IRAC and REDAC are then filtered and input to the CPU, and can be measured at
TP55 and TP59, respectively.
9.10
Digital Circuitry
The digital hardware and related circuitry, which is illustrated in the following block diagram (Figure
9-4), includes the following subsystems:
Measure
Check Battery
To analog section
9.10.1 CPU
A 16-bit microcontroller that includes a serial port, watchdog timer, A/D converter with an 8-input
analog multiplexer, 3-pulse width modulators, and a high-speed I/O subsystem.
9.10.2 System Memory
External to the CPU and consists of an 8K × 8 static RAM and a 64K × 16 EPROM.
9-12
N-20/N-20P Control buttons
Power supply
AUX
& control
PCB
Ambient
light sensor
CPU
Main PCB
Memory
&
software
Real-time
Ambient
AUX
clock
PCB
temp. sensor
Figure 9-4: Digital Circuitry Block Diagram
Main
PCB
AUX PCB
Display
drivers
Audio beeper
AUX PCB
(N-20 only)
Printer
Main
Printer Control button
ON
ADV
D/D
PCB
Display
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