Other Hardware; Clock Doubler; Spectrum Spreader - Rabbit RCM3000 User Manual

4.4 Other Hardware

4.4.1 Clock Doubler

The RCM3000 takes advantage of the Rabbit 3000 microprocessor's internal clock dou-
bler. A built-in clock doubler allows half-frequency crystals to be used to reduce radiated
emissions. The 29.4 MHz frequency specified for the RCM3000 is generated using a
14.7456 MHz crystal. The clock doubler will not work for crystals with a frequency
above 26.7264 MHz.
The clock doubler may be disabled if 29.4 MHz clock speeds are not required. Disabling
the Rabbit 3000 microprocessor's internal clock doubler will reduce power consumption
and further reduce radiated emissions. The clock doubler is disabled with a simple config-
uration macro as shown below.
1. Select the "Defines" tab from the Dynamic C
2. Add the line
The clock doubler is enabled by default, and usually no entry is needed. If you need to
specify that the clock doubler is always enabled, add the line
always enable the clock doubler.
3. Click
OK
in the project file where you defined the macro.

4.4.2 Spectrum Spreader

The Rabbit 3000 features a spectrum spreader, which helps to mitigate EMI problems. By
default, the spectrum spreader is on automatically, but it may also be turned off or set to a
stronger setting. The means for doing so is through a simple configuration macro as shown
below.
1. Select the "Defines" tab from the Dynamic C Options > Project Options menu.
2. Normal spreading is the default, and usually no entry is needed. If you need to specify nor-
mal spreading, add the line
ENABLE_SPREADER=1
For strong spreading, add the line
ENABLE_SPREADER=2
To disable the spectrum spreader, add the line
ENABLE_SPREADER=0
NOTE: The strong spectrum-spreading setting is not recommended since it may limit
3. Click OK to save the macro. The spectrum spreader will now remain off whenever you are
in the project file where you defined the macro.
NOTE: Refer to the Rabbit 3000 Microprocessor User's Manual for more information
26
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CLOCK_DOUBLED=0
to save the macro. The clock doubler will now remain off whenever you are
the maximum clock speed or the maximum baud rate. It is unlikely that the strong set-
ting will be used in a real application.
on the spectrum-spreading setting and the maximum clock speed.
Options > Project Options
to always disable the clock doubler.
menu.
to
CLOCK_DOUBLED=1
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