Rabbit 3000, User Manual

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User’s Manual 195
power and ground planes, noise from the core propagates less into the power and ground
planes. Using a small, 1 nF, decoupling capacitor gives better high-frequency response, up
to the 100 MHz region, than a larger 10 or 100 nF capacitor would. 10 nF and 100 nF
capacitors can be distributed between the ground and power planes to provide bulk decou-
pling for the power supply.
Figure 15-2. Recommended Core and I/O Power-Supply Connections
Noise on the core power rails can only exit the package via the core power-supply pins. It
will not be transmitted through the buffers in the I/O ring because these buffers are satu-
rated when in a steady on or off state. The frequency components of the noise that are
most troublesome for creating unintentional radiation are generally frequencies in the
range of 100–300 MHz. Lower frequencies are more easily attenuated by decoupling
capacitors and do not radiate well because the effective antennas are too small relative to
the wavelength. Higher frequencies are attenuated by the finite rise time of the edges, and
government emission limits are more relaxed at the higher frequencies. A 1 nF surface-
mount capacitor has its minimum impedance of about 1
Ω
at around 200 MHz. A larger
capacitor, such as 10 nF, will be less effective in the critical frequency range. At 200 MHz
the impedance of the bond wires and lead frame path is about 5
Ω
.
I/O
Ring
CPU
Core
+3.3 V
Rabbit
3000
CPU Core
Power Pins
I/O
Power Pins
10 nF
Make these traces
short and wide for
low inductance.
Make these traces
longer (several mm)
and minimum width
VCC
GND
Feedthrough
to power lines
Decoupling
Capacitor
1 nF