Jameco Electronics Rabbit 2000, User Manual

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4 Rabbit 3000 Microprocessor
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A built-in clock doubler allows ½-frequency crystals to be used.
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The built-in main clock oscillator uses an external crystal or a ceramic resonator. Typical
crystal or resonator frequencies are in the range of 1.8 MHz to 30 MHz. Since precision
timing is available from the separate 32.768 kHz oscillator, a low-cost ceramic resonator
with ½ percent error is generally satisfactory. The clock can be doubled or divided down
to modify speed and power dynamically. The I/O clock, which clocks the serial ports, is
divided separately so as not to affect baud rates and timers when the processor clock is
divided or multiplied. For ultra low power operation, the processor clock can be driven
from the separate 32.768 kHz oscillator and the main oscillator can be powered down.
This allows the processor to operate at approximately between 20 and 100 µA and still
execute instructions at the rate of up to 10,000 instructions per second. The 32.768 kHz
clock can also be divided by 2, 4, 8 or 16 to reduce power. This “sleepy mode” is a pow-
erful alternative to sleep modes of operation used by other processors.
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Processor current requirement is approximately 65 mA at 30 MHz and 3.3 V. The cur-
rent is proportional to voltage and clock speed—at 1.8 V and 3.84 MHz the current
would be about 5 mA, and at 1 MHz the current is reduced to about 1 mA.
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To allow extreme low power operation there are options to reduce the duty cycle of
memories when running at low clock speeds by only enabling the chip select for a brief
period, long enough to complete a read. This greatly reduces the power used by flash
memory when operating at low clock speeds.
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The excellent floating-point performance is due to a tightly coded library and powerful
processing capability. For example, a 50 MHz clock takes 7 µs for a floating add, 7 µs
for a multiply, and 20 µs for a square root. In comparison, a 386EX processor running
with an 8-bit bus at 25 MHz and using Borland C is about 20 times slower.
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There is a built-in watchdog timer.
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The standard 10-pin programming port eliminates the need for in-circuit emulators. A
very simple 10-pin connector can be used to download and debug software using
Z-World’s Dynamic C and a simple connection to a PC serial port. The incremental cost
of the programming port is extremely small.
Figure 1-1 shows a block diagram of the Rabbit.