Jameco Electronics Rabbit 3000 - Manual (Page 27)

18 Rabbit 3000 Microprocessor
length of the pulses. When the duty cycle is greater then 1/1024 the pulses are spread into
groups distributed 256 counts apart in the 1024 frame. The pulse width modulation outputs
can be passed through a filter and used as a 10-bit D/A converter. The outputs can also be
used to directly drive devices that have intrinsic filtering such as motors or solenoids.
2.2.12 Spread Spectrum Clock
The main system clock, which is generated by the crystal oscillator or input from an exter-
nal oscillator, can be modified by a clock spectrum spreader internal to the Rabbit 3000
chip. When the spectrum spreader is engaged, the clock is alternately speeded up and
slowed down, thus spreading the spectrum of the clock harmonics in the frequency
domain. This reduces EMI and improves the results of official radiated-emissions tests
typically by 15–20 dB at critical frequencies. The spectrum spreader has 3 modes of oper-
ation: off, normal, and strong. Slightly faster memory access time is required when the
spectrum spreader is used: 2–3 ns for the normal setting when the clock doubler is
enabled, and 6–9 ns for the strong setting when the clock doubler is used. The spreader
slightly influences baud rates and other timings because it introduces clock jitter, but the
effect is usually small enough to be negligible.
2.2.13 Separate Core and I/O Power Pins
The silicon die that constitutes the Rabbit 3000 processor is divided into the core logic and
the I/O ring. The I/O ring located on the 4 edges of the die holds the bonding pads and the
large transistors used to create the I/O buffers that drive signals to the external world. The
core section, inside the I/O ring contains the main processor and peripheral logic. The
clock and clock edges in the core are very fast with large transient currents that create a lot
of noise that is communicated to the outside of the package via the power pins. The I/O
buffers have slower switching times and mostly operate at much lower frequencies than
the core logic. The Rabbit has separate power and ground pins for the core and I/O ring.
This allows the designer to feed clean power to the I/O ring filtered to be free of the noise
generated by the core switching. This minimizes high frequency noise that would other-
wise appear on output pins driven by buffers in the I/O ring. The result is lower EMI.
2.3 Design Standards
The same functionality can often be accomplished in more than one way with the Rabbit
3000. By publishing design standards, or standard ways to accomplish common objec-
tives, software and hardware support become easier.
Refer to the Rabbit 3000 Microprocessor Designer’s Handbook for additional information.
2.3.1 Programming Port
Rabbit Semiconductor publishes a specification for a standard programming port (see
Appendix A, “The Rabbit Programming Port”) and provides a converter cable that may be
used to connect a PC serial port to the standard programming interface. The interface is
implemented using a 10-pin connector with two rows of pins on 2 mm centers. The port is
connected to Rabbit Serial Port A, to the startup mode pins on the Rabbit, to the Rabbit