Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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High Frequency Design Considerations
19-16 ADSP-BF535 Blackfin Processor Hardware Reference
Decoupling Capacitors and Ground Planes
Ground planes must be used for the ground and power supplies. The
capacitors should be placed very close to the
VDDEXT
and
VDDINT
pins of the
package as shown in Figure 19-7 . Use short and fat traces for this. The
ground end of the capacitors should be tied directly to the ground plane
inside the package footprint of the processor (underneath it, on the bot-
tom of the board), not outside the footprint. A surface-mount capacitor is
recommended because of its lower series inductance.
Connect the power plane to the power supply pins directly with minimum
trace length. The ground planes must not be densely perforated with vias
or traces as their effectiveness is reduced. In addition, there should be sev-
eral large tantalum capacitors on the board.

Designs can use either bypass placement case shown in Figure 19-6
or combinations of the two. Designs should try to minimize signal
feedthroughs that perforate the ground plane.
Oscilloscope Probes
When making high-speed measurements, be sure to use a “bayonet” type
or similarly short (< 0.5 inch) ground clip, attached to the tip of the oscil-
loscope probe. The probe should be a low-capacitance active probe
with 3 pF or less of loading. The use of a standard ground clip with
4 inches of ground lead causes ringing to be seen on the displayed trace
and makes the signal appear to have excessive overshoot and undershoot.
To see the signals accurately, a 1 GHz or better sampling oscilloscope is
needed.