C. PIA System Setup Considerations
143
Power
Supply
16C50A
PIA
ZT 8904
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
Figure 7. Computer and External Power Supply with Common Switch and Ground
Correct Power Supply Sequence, Correct Signal Level Match
PROTECTING CMOS INPUTS
The most common causes of damaged inputs are:
•
Slow rise times, resulting in a ground bounce within the chip
•
Inductive coupling on I/O lines causing noise to be coupled into the chip, resulting in
intermittent operation
Each of these conditions is covered in the following topics.
Rise Times
Slow rise times on a CMOS input can easily cause the transistor to bounce between Vil
and Vih. When this oscillation occurs, the operating current goes up, resulting in
"ground bounce." Ground bounce can cause internal latchup or can cause other system
components to malfunction. A pullup termination resistor is used to increase the rise
time.
Input rise times must be kept to less than 50 ns. Given a maximum chip capacitance of
10 pF, a 5k
Ω
resistor is the largest that could be used without additional cabling. As
cabling is added, the capacitance goes up, resulting in the use of a smaller pullup
resistor until the maximum sink current of the output is achieved.
If the 16C50A PIA device is driving the output, its maximum sink current at a Vol of .4 V
is 12 mA. This gives a lower limit of 420
Ω
for the pullup resistor, allowing a maximum
cabling capacitance of 110 pF. Note that while the input feature of the PIA may not be
used by your application (PIA used as an output only), the input circuitry remains in
parallel; therefore, the output rise time is still a critical parameter that the input still sees.
The output rise time must not exceed 50 ns.
Be wary of using low pass filters to remove electrical noise. The resulting capacitance is
typically too large to meet the 50 ns rise time requirement.