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Rabbit 3000 Microprocessor
A more extreme approach would be to place several decoupling capacitors in parallel or
use special low inductance capacitors. In addition, a series ferrite can be placed between
VCC and the decoupling capacitor at each pin. However, these measures are probably
overkill for the Rabbit 3000, especially when the spectrum spreader is enabled.
15.1.1 Noise Generated in the I/O Ring
Any noise on the I/O ring power rails will propagate to all output pins, thus spreading
potential EMI. By keeping core noise off the power planes, as described above, the
amount of noise fed into the I/O ring power pins can be minimized. However noise is also
generated by activity of the buffers in the I/O ring. The worst potential offender the buffer
that optionally drives the clock (CLK, pin 2). This output may be disabled or set to output
1/2 the clock frequency or the full clock frequency. For EMI it is best to disable this pin. If
it must be enabled it is much better to drive 1/2 the clock frequency rather than the full fre-
quency. If the clock does exit the package then loading on the buffer should be minimized
to reduce the noise referred to the I/O ring power rails. This can be done by placing a
series resistor in the clock line near the package or at least minimizing the load capac-
tance. This will reduce the current switched at the clock frequency or at 1/2 the clock fre-
quency. Of course a series resistor will slow rise time, possibly to an unacceptable extent.
The next item to consider is the oscillator buffer. This buffer will generally operate at 1/2
the clock frequency, assuming that the clock doubler is used. Since the oscillator buffer
has fairly high impedance and slow rise times, the power supply noise should be minimal.
Be sure to decouple the power pins adjacent to the oscillator buffer pins. The current loop
made by the external components of the oscillator circuit should be laid out with minimal
area and be adjacent to a power plane. The series resistor in the oscillator circuit should be
as large as can be tolerated and still have reliable oscillator operation.
Of the remaining output lines address line zero may switch at up to 1/2 the clock fre-
quency although the periodicity is broken up by wait states and instructions that do not
perform 2 clock memory accesses. To minimize EMI generated by this line keep the
capacitance driven to a minimum, avoid loops and make sure the line runs over a power
plane or is buried between power planes. Consider not using address zero to address I/O
devices other than memory, so as to reduce the capacitance driven. Generally address line
one (A1) will generate 6 dB less EMI because it operates at 1/2 the frequency.
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