User`s manual

User’s Manual 13
2.5 Clock Doubler
The RabbitCore 2000 takes advantage of the Rabbit 2000 microprocessor’s internal clock
doubler. A built-in clock doubler allows half-frequency crystals to be used to reduce radi-
ated emissions. The 25.8 MHz (RCM 2000 and RCM2010) and 18.4 MHz (RCM 2020)
frequencies are generated using 12.9 MHz and 9.2 MHz crystals. The clock doubler is
disabled automatically in the BIOS for crystals with a frequency above 12.9 MHz.
The clock doubler can be disabled if 25.8 MHz or 18.4 MHz clock speeds are not required.
Disabling the Rabbit 2000 microprocessor’s internal clock will reduce power consumption
and further reduce radiated emissions. The clock doubler is disabled with a simple change
to the BIOS as described below.
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 the clock if XTAL<=12.9MHz,
to read as follows.
#define CLOCK_DOUBLED 0 // set to 1 to double the clock if XTAL<=12.9MHz,
3. Change the serial baud rate to 57,600 bps when the RabbitCore 2000 is operated at 12.9
MHz or 9.2 MHz.
4. Save the change using
File > Save.