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LH75400/01/10/11 (Preliminary) User's Guide
25.2.3 Unused Input Signal Conditioning
Floating input signals can cause excessive power consumption. Unused inputs which do
not include internal pull-up or pull-down resistors should be pulled up or down externally,
to tie the signal to its inactive state.
Some GPIO signals may default to inputs. If the pins which carry these signals are unused,
software can program these signals as outputs, to eliminate the need for pull-ups or
pull-downs. Power consumption may be higher than expected until such software executes.
Some inputs have internal pull-ups or pull-downs. If unused, these inputs do not require
external conditioning.
Consider all signals that are Inputs at reset (see Chapter 1, Section 1.8).
25.2.4 Other Circuit Board Layout Practices
All output pins have fast rise and fall times. Printed circuit trace interconnection length
must therefore be reduced to minimize overshoot, undershoot and reflections caused by
transmission line effects of these fast output switching times. This recommendation partic-
ularly applies to the address and data buses.
When considering capacitance, calculations must consider all device loads and capaci-
tances due to the circuit board traces. Capacitance due to the traces will depend upon a
number of factors, including the trace width, dielectric material the circuit board is made
from and proximity to ground and power planes.
Attention to power supply decoupling and printed circuit board layout becomes more criti-
cal in systems with higher capacitive loads. As these capacitive loads increase, transient
currents in the power supply and ground return paths also increase.
Recommended Layout Practices
6/17/03
25-3
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