Digi RCM4000 - Manual (Page 44)

38 RabbitCore RCM4000
4.5 Other Hardware
4.5.1 Clock Doubler
The RCM4000 takes advantage of the Rabbit 4000 microprocessor’s internal clock doubler.
A built-in clock doubler allows half-frequency crystals to be used to reduce radiated emis-
sions. The 58.98 MHz frequency specified for the RCM4000 is generated using a
29.49 MHz crystal.
The clock doubler may be disabled if 58.98 MHz clock speeds are not required. Disabling
the Rabbit 4000 microprocessor’s internal clock doubler will reduce power consumption
and further reduce radiated emissions. The clock doubler is disabled with a simple global
macro as shown below.
4.5.2 Spectrum Spreader
The Rabbit 4000 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 global macro as shown below.
NOTE: Refer to the Rabbit 4000 Microprocessor User’s Manual for more information
on the spectrum-spreading setting and the maximum clock speed.
1. Select the “Defines” tab from the Dynamic C Options > Project Options menu.
2. Add the line
CLOCK_DOUBLED=0
to always disable the clock doubler.
3. Click OK to save the macro. The clock doubler will now remain off whenever you are
in the project file where you defined the macro.
1. Select the “Defines” tab from the Dynamic C Options > Project Options menu.
2. For normal spreading, add the line
ENABLE_SPREADER=1
For strong spreading, add the line
ENABLE_SPREADER=2
NOTE: The strong spectrum-spreading setting is not recommended since it may limit
the maximum clock speed or the maximum baud rate. It is unlikely that the strong set-
ting will be used in a real application.
3. Click OK to save the macro. The clock doubler will now remain off whenever you are
in the project file where you defined the macro.