AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 27

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

Managing Unused Pins
If a general purpose input does not have a pull device enabled or is not driven externally, as the input
approaches mid-rail (i.e. ~ 2.5 V), a ‘cross-over current’ of ~ 2.5 mA can flow in the I/O stage. This
‘crossover current’ is on a the main I/O drive supply. This is not a concern for the device itself but can
directly impact the power supply demand and low power mode currents.
ATD inputs have a slightly different input stage and can be left open-circuit, although it is preferable to
ground unused analog inputs to minimize pick-up of unwanted noise.
There are two approaches to managing unused GPIO: configuring them as inputs, and configuring them
as outputs; a combination of the two may be appropriate. In the event of code runaway, a GPIO port could
be reconfigured unintentionally; protective software can reduce the impact of this, for either configuration.
Unused GPIO Configured as Outputs
This is a good solution as the pins will have a low impedance to rail. To minimize the impact of the ports
being accidentally reconfigured as inputs, enable the internal pull-down resistors (these will only be active
when configured as inputs).
Unused GPIO Configured as Inputs
Tie unused inputs to the supply rails, preferably with pull devices. (Where there is no internal pull device,
an external pull device will be required.). In some cases, it may be an application requirement for unused
inputs to be defined during reset; in this case, all unused pins that default to high impedance must be
pulled externally to a supply rail.
Several optional strategies for tying input pins are discussed below. Consideration should be given to the
the possibility of I/O conflict occurring, if unused input ports are reconfigured unintentionally as outputs,
where:
Freescale Semiconductor
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two ports are connected together and might be driven with opposing polarities,
Leaving unused GPIO undefined is a common cause of unexpectedly high
levels of STOP or WAIT IDD. This is not always obvious on every device or
assembled module as this is dependent on small variations in the
manufacturing process, operating temperature and voltage. Consequently,
this is often not detected during development or qualification, but shows up
in the application production test or in use where a greater number and
range of operating conditions is encountered.
HCS12 devices are often available in different packages (mostly 112-pin
LQFP and 80-pin QFP). When using a lower pin count variant, it is
important to be aware that the I/O available on the largest pin count device
is still present but unbonded and must be configured for low power modes.
Either enable the internal pull devices or configure as outputs.
Designing Hardware for the HCS12 D-Family, Rev. 0
CAUTION
NOTE
Managing Unused Pins
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