C. Digital I/O ASIC System Setup Considerations
ÛZIATECH
131
Figure 6. Computer-Switched External Power Supply, Common Ground
Correct Power Supply Sequence, Correct Signal Level Match
Power
Supply
ZT 8907
Interface Cable
External
Power
Supply
Custom
Application
Vcc
1Amp
S
24
ZT8907
Digital
16C50A
I/O
ASIC
Figure 7. Computer and External Power Supply with Common Switch and Ground
Correct Power Supply Sequence, Correct Signal Level Match
Power
Supply
ZT 8907
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
ZT8907
Digital
16C50A
I/O
ASIC
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 5 k
Ω
resistor is the largest that could be used without additional cabling. As