AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 8

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

Input Characteristics
Input Characteristics
Specifying the input levels as 65% and 35% of V
tells the user that by the time a rising voltage reaches
DD
65% of V
or above, it will be recognized as a logic-high level. When a voltage falls to 35% of V
or
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below, it will be recognized as a logic-low level. Voltages between these levels are generally undefined
because it is not possible to predict which logic level the MCU will interpret.
Measuring one sample on the bench showed port A0 changed from 0 to 1 when the voltage exceeded
2.582 V with respect to ground. Port A0 changed from 1 to 0 when the voltage was equal or less than
2.440 V. This shows a small hysteresis of 2.582 – 2.440 = 0.142 V (or 142 mV). This is for one sample at
5.0 Vdc and room temperature of approximately 75°F (or 24°C). See
Figure
5.
HIGH
LOGIC
LOW
2.440
2.582
0
5.0
VOLTAGE
Figure 5. Hysteresis Graph at 25°C, +5.0 Vdc
Note that automated test equipment often measures rise and fall times in terms of percent of V
. For
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example, on a common oscilloscope, the rise time is measured as the time a signal takes to move from
10% to 90% or 20% to 80% of V
. This is not to be confused with the logic recognition levels in the
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previous paragraph.
Pullups
Pullup resistors help protect high-impedance inputs from oscillation while unconnected and bias pins to
an active or inactive state. To ease the integration of an MCU into a user design, pullups are often
incorporated into the MCU, itself. The HC(S)12 Families provide internal pull devices. These devices are
called pull devices because they can (with some exceptions) be configured to pullup or pulldown. The pull
devices are modeled as current sources that can be enabled under user control. In most cases, the user
can select whether the pull devices are enabled or disabled. The user can also select the appropriate
polarity in some cases.
The device electrical specification states that the pullup and pulldown currents are 10 µA to 130 µA and
–10 µA to –130 µA. Some manufacturers list these as impedance (ohms) and others list them as current
sources or sinks (µA or mA). In the HCS12 Family, these devices are weak N-channel and P-channel
transistors rather than resistors so it would not be appropriate to specify an impedance in ohms.
Input/Output (I/O) Pin Drivers, Rev. 0
8
Freescale Semiconductor