User`s manual

14 RabbitCore RCM3000
2.3 Other Hardware
2.3.1 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 change
to the BIOS as described below.
2.3.2 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 change to the following BIOS
line in a way that is similar to the clock doubler described above.
#define ENABLE_SPREADER 1 // Set to 0 to disable spectrum spreader.
#define SPREADER_SETTING 0 // 0 = normal spreading, 1 = strong spreading
NOTE: The strong spectrum-spreading setting is not recommended since it may limit the
maximum clock speed or the maximum baud rate.
1. Open the BIOS source code file, RABBITBIOS.C in the BIOS directory.
2. Change the line
#define CLOCK_DOUBLED 1 // set to 1 to double clock if
// Rabbit 2000: crystal <= 12.9024 MHz,
// Rabbit 3000: crystal <= 26.7264 MHz,
// or to 0 to always disable clock doubler
to read as follows.
#define CLOCK_DOUBLED 0
3. Save the change using File > Save.