AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 14

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

Capacitive Loading
Capacitive Loading
Device electrical 7specifications typically show parameters such as V
maximum electrical signal levels and timing criteria based on a specified loading on the I/O pin (in terms
of impedance and capacitance). For the HCS12 Family of MCUs, this capacitive loading is specified as
50 pF (pico-Farads or 10
Signals with low capacitive loading have fast expected transition times. Increased capacitance on a pin
will cause the signal to have a longer transition time. The impedance of the internal circuitry restricts the
available current that can flow into or out of a pin. To charge or discharge a capacitive load, the current
and the capacitance form a time constant that increases with reduced current or increased capacitance.
Device specifications are based on design targets and are validated after testing many parts across
temperature, voltage, and a normal range of process variation. This testing is performed on a tester which
also puts a capacitive load on the I/O pins. The electrical specification generally includes an input
capacitance specification that accounts for pin capacitance.
When bench testing, the capacitance of oscilloscope probes, printed circuit board traces, and parasitic
capacitance can increase rise and fall times of signal transitions. One way to determine the maximum
transition times is to add a large capacitor between an output pin and ground and determine the effects
on signal timing. To minimize the effects of stray impedance, component leads and scope probe leads
must be very short (or low-capacitance probes must be used) when measuring timing.
Figure 13
other. This is the result of poor measurement techniques.
14
shows an output trace with multiple reflections of the signal transition superimposed on each
3.5
2.5
1.5
0.5
3
2
1
0
SPECIFIED I
0
Figure 12. V
-12
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Farads).
2
MC9S12DP256, 25°C, V
Input/Output (I/O) Pin Drivers, Rev. 0
OL
versus Sink Current — Reduced Drive
4
I
OL
(mA)
DD
6
= 5 V
OH
0.8 V = V
, V
OL
8
, I
OL
OH
, and I
Freescale Semiconductor
OL
10
plus absolute