User`s manual

2–10 Hardware Reference RabbitCore RCM2100 Series
2.4 Other Hardware
2.4.1 Clock Doubler
The RabbitCore RCM2100 series takes advantage
of the Rabbit 2000 microprocessor’s internal clock
doubler. A built-in clock doubler allows half-fre-
quency crystals to be used to reduce radiated emis-
sions. The 22.1 MHz frequency is generated using
an 11.05 MHz crystal. The clock doubler is disabled
automatically in the BIOS for crystals with a fre-
quency above 12.9 MHz.
The clock doubler may be disabled if 22.1 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 sim-
ple change to the BIOS as described below.
2.4.2 Backup Battery Circuit
The RabbitCore RCM2100 does not have a battery,
but there is provision for a customer-supplied bat-
tery to back up SRAM and keep the internal Rabbit
2000 real-time clock running.
Header J2, shown in Figure 2–7, allows access to
the external battery. This header makes it possible to
connect an external 3 V power supply. This allows
the SRAM and the internal Rabbit 2000 real-time
clock to retain data with the RabbitCore RCM2100
powered down.
A lithium battery with a nominal voltage of 3 V and
a minimum capacity of 165
mA·h is recommended.
A lithium battery is strongly recommended because
of its nearly constant nominal voltage over most of
its life.
The drain on the battery by the RabbitCore RCM2100
is typically 16 µA when no other power is supplied.
If a 950 mA·h battery is used, the battery can last
more than 6 years:
The actual life in your application will depend on
the current drawn by components not on the Rabbit-
Core RCM2100 and the storage capacity of the bat-
tery. Note that the shelf life of a lithium battery is
ultimately 10 years.
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 RCM2100 series is operated at
11.05 MHz.
4. Save the change using File > Save.
Figure 2–7: External Battery Connections
at Header J2
VCC
VRAM
26
29
27
28
GND
VBAT
External
Battery
950 mA·h
16 µA
------------------------
6.8 years.=