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User’s Manual 31
4.5 Other Hardware
4.5.1 Clock Doubler
The RCM3600 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 22.1 MHz frequency specified for the RCM3600 is generated using a
11.06 MHz resonator.
The clock doubler may be disabled if 22.1 MHz clock speeds are not required. This will
reduce power consumption and further reduce radiated emissions. The clock doubler is
disabled with a simple change to the BIOS as described below.
4.5.2 Spectrum Spreader
The Rabbit 3000 features a spectrum spreader, which helps to mitigate EMI problems. The
spectrum spreader is on by default, but it may also be turned off or set to a stronger setting
by changing the following macro in the BIOS.
#define ENABLE_SPREADER 1 // Set to 0 to disable spectrum spreader,
// 1 to enable normal spreading, or
// 2 to enable strong spreading.
NOTE: Refer to the Rabbit 3000 Microprocessor User’s Manual for more information
on the spectrum-spreading setting and the maximum clock speed.
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
.