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236 Rabbit 3000 Microprocessor
16.10 Reduced-Power External Main Oscillator
The circuit in Figure 16-13 can be used to generate the main clock using less power than
with the built-in oscillator buffer. The power consumption is less because of the current-
limiting resistors that cannot be used with the built-in buffer. The 2.2 kΩ series resistor
must be reduced as the clock frequency increases, as must be the current-limiting resistors.
Figure 16-13. Reduced-Power External Main Oscillator
Table 16-8 lists results for the reduced-power external oscillator with no current-limiting
resistors.
Design Recommendations
• Add current-limiting resistors to reduce current without inhibiting oscillator start-up
• Increase the 1 MΩ resistor to improve gain
• Minimize loop area to reduce EMI
Table 16-8. Current Draw Using Reduced-Power External Oscillator
(0 Ω current-limiting resistors)
Voltage
(V)
Current (incl built-in buffer)
(mA)
3.3 0.635
2.5 0.380
1.8 0.252
1 MW
2.2 kW
33 pF
33 pF
3.68 MHz
(C
L
= 20 pF)
To Rabbit 3000
XTALA1
+3.3 V
Optional current-reducing
resistors
SN74HCT1G04DBVR