Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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ADSP-BF535 Blackfin Processor Hardware Reference 19-15
System Design
You can add a series termination resistor near the pin for point-to-point
connections. Typically, serial port applications use this termination
method when distances are greater than six inches. For details, see the ref-
erence source in
“Recommended Reading” on page 19-17 for suggestions
on transmission-line termination. Also, see ADSP-BF535 Blackfin Embed-
ded Processor Data Sheet for output drivers’ rise and fall time data.
Signal Integrity
The capacitive loading on high-speed signals should be reduced as much
as possible. Loading of buses can be reduced by using a buffer for devices
that operate with wait states (for example, DRAMs). This reduces the
capacitance on signals tied to the zero-wait-state devices, allowing these
signals to switch faster and reducing noise-producing current spikes.
Signal run length (inductance) should also be minimized to reduce ring-
ing. Extra care should be taken with certain signals such as external
memory, read, write and acknowledge strobes.
Other recommendations and suggestions to promote signal integrity:
• Use more than one ground plane on the PCB to reduce crosstalk.
Be sure to use lots of vias between the ground planes. These planes
should be in the center of the PCB.
• Keep critical signals such as clocks, strobes, and bus requests on a
signal layer next to a ground plane and away from or laid out per-
pendicular to other non-critical signals to reduce crosstalk.
• Design for lower transmission line impedances to reduce crosstalk
and to allow better control of impedance and delay.
• Experiment with the board and isolate crosstalk and noise issues
from reflection issues. This can be done by driving a signal wire
from a pulse generator and studying the reflections while other
components and signals are passive.