AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 4

• Manufacturer Part Number: AN2434
• Description: Input/Output (I/O) Pin Drivers
• Manufacturer: Freescale Semiconductor / Motorola

Unique Pins
of the ports associated with the core are available for use as general-purpose input/output (GPIO) in the
non-expanded modes.
Because these ports are part of the core, their capabilities are grouped differently than the PIM ports. For
example, the core pullups can be turned on or off on a port-wide basis. Pullup/pulldown devices for the
PIM can be turned on and off based on an individual bit basis.
Unique Pins
Recent HCS12 MCUs have between 48 and 144 pins consisting of four major types: power/ground,
digital, analog, and special function. Special function pins can include oscillator pins, open drain, and
super-voltage functions that are enabled by applying a voltage above the normal operating voltage of 5.0
Vdc.
Unused Pins
Unused input pins on the M68HC12 and HCS12 Families of devices should be terminated to ensure
proper operation and reliability because these are CMOS devices. CMOS device pins consist primarily of
an n-channel and a p-channel field effect transistor (FET). Generally only one of these devices is on at
any given time; however, there is a very brief period of time during switching when both devices conduct
slightly while one device is turning off and the other is turning on. An un-terminated input can oscillate or
float to a mid-supply level, causing both of the FET devices to be in a partial on state, raising device
dissipation, increasing noise, and drawing additional supply current.
There are three types of inputs found on most of the M68HC12/HCS12 devices. These are primarily ATD
pins, input-only pins, and input/output pins. The best way to terminate input-only pins is to use a pullup or
pulldown resistor to tie each unused pin to V
terminated with a single resistor to V
this connection if an additional input is needed later in the design process.
Unused input pins can also be connected together and then pulled up or down with a common resistor.
While use of a common resistor may save component count and cost, it will reduce the flexibility if an
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Some pins have the pulldown disabled or the P-channel field effect
transistor (P-FET) disabled, such as the wire OR-ed pins on the
inter-integrated circuit (IIC) or serial peripheral interface (SPI) functions.
The device user guide for the 9S12DP256B states that the internal
structure for all I/O pins, the analog inputs, BKGD, and RESET have
identical internal structures. It states further that some of the functionality
may be disabled, for example the output drivers, pullup and pulldown
resistors are disabled permanently for the analog inputs. (This applies to
most of the HCS12 microcontrollers. Two exceptions are the MC9S12E128
and MC9S12C32 Families where the ATD (analog-to-digital converter) pins
can be configured as digital outputs as well as digital inputs. See the port
integration module (PIM) documentation and the ATD for details.)
DD
Input/Output (I/O) Pin Drivers, Rev. 0
or ground; however, it may be more difficult to separate a pin from
SS
or V
NOTE
DD
. Input-only pins can also be tied together and
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