AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 15

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

Capacitive Loading
Figure 13. Effect of Long Leads and Stray (or Parasitic) Impedances
By minimizing the length of component leads, PCB traces, and wire used to connect the scope probe,
many of these effects can be reduced.
Figure 14
shows the effect of capacitive loading at 0 pF, 200 pF,
and 470 pF with very short leads. (The capacitor value is that of bulk capacitance added to the I/O pin.
The scope probe adds approximately 10 pF to the existing pin capacitance—which is approximately 2 to
3 pF—and any existing PCB parasitics. PCB parasitics are approximately 7 pF on a 4-layer evaluation
board.) Minimizing the parasitic or stray component effects makes it clear that the effect of increased
capacitive loading results in longer rise and fall times. It now becomes obvious that exceeding the
capacitive load for an output pin will result in longer transition times. Adhering to the loading limits in the
device electrical specifications will help ensure that the timing specifications can be met in practice.
Input/Output (I/O) Pin Drivers, Rev. 0
Freescale Semiconductor
15