6.14 Power Supply
In:
9 Volt Alkaline Battery, 650 mah
Out:
+5 Volts
Current
Consumption:
Sleep Mode - Approximately 25
µ
A
Wake Mode - Approximately 7.5 mA
Battery Life:
6 to 8 months (average 600 temps / month)
6.15 The +5 Volt Supply
1. U8, a three-terminal linear regulator is used to produce a 5 volt output from a 9 volt input.
2. The +5 volt output always supplies current to the CPU.
3. When in sleep (low-power) mode, the CPU holds the gate of Q1 high, thus shutting off the +5 volts from the
rest of the circuitry.
4. When the CPU is caused to wake-up (INTO), it puts a "low" on the gate of Q1, allowing Q1 to conduct
current to the rest of the circuitry.
6.16 The -5 Volt Supply
The circuit uses a DC-to-DC converter (U9), to produce a -5 volt output from a +5 volt input. CPU Reset.
1. A high-to-low transition of a specified duration is required at pin 76 of the CPU for a power-on reset.
2. The first instant the battery is installed, C6 is discharged, and pin 76 of the CPU, being connected to the
negative side of C6, is allowed to go high.
3. As C6 charges, pin 76 of the CPU drops low. This high-to-low transition, caused the CPU to generate an
internal "reset".
6.17 Liquid Crystal Display
The drive circuitry and power to the LCD is internal to the CPU. The drive "duty-cycle" and "frequency" is a
function of the 32 khz crystal, X1.
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