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the battery and the VRECHARGE terminals are mechanically connected. This applies the battery
voltage to VRECHARGE, pulling BAT_TYPE high. R100 and CR27 are a current-limiting resistor
and a voltage-clamping diode that are used to protect the input port from excessive battery voltage. If
disposable batteries are used, VRECHARGE is electrically isolated, which allows R101 to pull
BAT_TYPE input low.
The nominal voltages and voltage discharge curves are significantly different between rechargeable
and disposable batteries. In order for the CPU to predict how much "battery life" remains, the nominal
voltage and discharge curves must be known; the BAT_TYPE signal provides that information.
9.10.21 Printer Control
Printer circuitry is divided into two subsections: the printer interface and the printer flex circuit.
Printer interface circuitry is present on both models, but is disabled by software in the N-20.
•
Printer interface circuit (auxiliary PCB)—This circuit detects the presence of the flex circuit,
and supplies power to the print heads and paper-advance motor. Noise generated by the printer
motor is filtered. The circuitry is protected from excessive battery currents by a fuse. The printer
interface circuit is illustrated in Figure 9-31 (at the end of this section).
•
Printer flex circuit (N-20P only)—The printer flex circuit is added when the printer is present.
The printer generates a timing signal that is read by the CPU and sent to the flex circuit. This
circuit signals the CPU that a printer is present by connecting one CPU input to ground. Power
and power control signals from the auxiliary PCB generate an output load for a resistor array; heat
from this process produces a dot matrix pattern on thermal paper. The printer flex circuit is
illustrated in Figure 9-32.
Technical Supplement
9-27
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