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Chapter 26 Low-Power Operation
273
26.2 Operation
26.2.1 Unused Pins
Input (or bidirectional) pins that are unused in a design can pick up noise that may cause
the transistors in the input buffer to switch states quickly, causing unnecessary current
draw. To avoid this, all unused pins should be connected to a weak pullup or pulldown
resistor (approximately 100 k
Ω
) and left as inputs. This provides protection from noise
when the pin is an input, but also limits the current draw if the pin gets inadvertently
enabled as an output.
26.2.2 Clock Rates
The processor and peripheral clocks in the Rabbit 4000 can be run in six different modes
using the main oscillator: full speed; divided by 2, 4, 6, or 8; and the processor clock divided
by 8 with the peripheral clock at full speed. If the clock doubler is enabled, the options
also include twice the main oscillator frequency and the main oscillator divided by 3.
In addition, the 32 kHz clock can be used for the processor and peripheral clocks; the 32 kHz
clock can also be divided by 2, 4, 8, or 16, which provides dramatically lower power
consumption.
Table 26-1 lists the options for the clock modes and the processor clock frequency.
Table 26-1. Clock Modes
Main Oscillator
GCSR Setting
Clock
Doubler
32 kHz
Divider
Processor Clock
Frequency
Full
On
N/A
2 × Main Oscillator
Full
Off
Main Oscillator
Divided by 2
On
Divided by 2
Off
Main Oscillator / 2
Divided by 4
On
Divided by 6
On
Main Oscillator / 3
Divided by 4
Off
Main Oscillator / 4
Divided by 8
On
Divided by 6
Off
Main Oscillator / 6
Divided by 8
Off
Main Oscillator / 8
Off
(32 kHz divider
used)
N/A
Disabled
32.768 kHz
/ 2
16.384 kHz
/ 4
8.192 kHz
/ 8
4.096 kHz
/ 16
2.048 kHz
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