668-0003-C Rabbit Semiconductor, 668-0003-C Datasheet - Page 215

• Manufacturer Part Number: 668-0003-C
• Description: IC CPU RABBIT2000 30MHZ 100PQFP
• Manufacturer: Rabbit Semiconductor

Specifications of 668-0003-C

• Rohs Status: RoHS non-compliant
• Processor Type: Rabbit 2000 8-Bit
• Speed: 30MHz
• Voltage: 2.7V, 3V, 3.3V, 5V
• Mounting Type: Surface Mount
• Package / Case: 100-MQFP, 100-PQFP
• Features: -
• Other names: 316-1004; 668-0003

B.2.4 Improved Battery-Backup Circuit
Improvements were made in revisions A–C to reduce the internal power consumption of
the RTC circuit. In addition, external circuitry was designed to further reduce power con-
sumption by the overall oscillator circuit in board-level products based on the Rabbit
2000.
Low-Power Oscillator Design
An external low-current oscillator can be built using an inexpensive single-gate (tiny logic)
unbuffered inverter. The current consumption of this circuit is about 4 µA with a 2 V sup-
ply. Using this circuit, oscillation continues even when the voltage drops to 0.8 V, and
oscillation is still very strong at 1.2 V. The oscillator should have its exposed circuit traces
conformally coated to prevent the possibility of loading the circuit by conduction on the
PC board surface in a moist atmosphere. (Rabbit Semiconductor has published an applica-
tion note on conformal coating, Technical Note TN303, Conformal Coatings.)
The capacitors on either side of the crystal provide the load capacitance, which is specified
by the crystal manufacturer. Typically the load capacitance is about 12 pF. This is the
capacitance that should be in parallel with the crystal for it to operate at the specified fre-
quency. C1 and C2 provide this load capacitance. The formula for the load capacitance is
This is just the formula the capacitance of two capacitors in series plus any stray capaci-
tance in the board layout, perhaps 2 pF. Note that the input capacitance of the gate (C
must also be taken into account. The gate input capacitance is not constant, but is a func-
tion of frequency. Thus if it is measured, it should be done with a sine-wave generator
operating at 32 kHz. The output capacitance is not relevant because the 330 kΩ resistor
isolates it from the crystal. If C2 is made smaller, this will increase the voltage swing on
Appendix B Rabbit 2000 Revisions
C
L
=
(
-------------------------------------- -
C1
C1
+
) ∗ C2
(
(
C2
Figure B-1. Low-Power 32.768 kHz Oscillator Circuit
W
+
+
C
C
i
i
)
)
+
W
C
stray
W
W
in
)
209