Download Test Philosophy

How do ABI test
Digital ICs?
Test principles
ABI digital tests are designed to find faults
on boards. To do this the following
principles are used:
• Confirm that the IC is correctly wired,
correctly driven and correctly powered
• Confirm that the input and output pins
are not damaged and that the IC is not
overloaded
• Confirm that the IC functions according
to its truth table
1. High voltage test
• Output drivers and BDO signals
isolated by relays
• 5V power supply switched on
• Check all pins voltages within
-0.5V to +5.5V range
2. Auto clip positioning
• Output isolation relays switched on
•
•
•
•
BDO signals enabled
VCC voltage must be >4.6V
Ground voltage must be <0.6V
Locates position of IC using
supplies
3. Pin impedance test
• Checks how pins of IC are driven
• Drives pin with various voltages
through 10K resistor
• Pins classified as:
• Output (displays blank)
• Undriven TTL input FLOT
• High impedance CMOS input
OPCT
4. Shorted pin test
• Shows pins connected to supply
rails (5V or 0V)
• Drives pins to opposite levels
• >4.4V indicates short to 5V
• <0.6V indicates short to 0V
5. Link detection
• Identifies links between pins on
same IC
• Shorted pins are ignored
• Pin pairs analysed to identify
matching change when driven
• Procedure repeated for all possible
pairs in both directions
• Results stored for use by circuit
compensation feature later
6. Input mid level test
• Measures IC input voltages
• Compares with threshold voltages
• Voltages above the low threshold
and below the switching voltage
are displayed as INPUT MID LOW
IPML
• Voltages below the high threshold
and above switch are displayed as
INPUT MID HIGH IPMH
7. Input signal detection
• Detects changing signals by
checking levels on each pin
• Checks each pin 256 times
asynchronously
• Different levels show as SIGNAL
• Detects signals below 4MHz
8. Input backdrive check
• Checks for valid low and high logic
levels to allow truth table test
• Checks each output by driving low
and high
• In-circuit low drive should be <0.9V
and high drive >1.9V
• Out-of-circuit low drive should be
<0.5V and high drive >2.5V
• Ignores pins shorted to rails
9. Output short check
• Checks IC outputs for shorts
• Displays SH0V for short to 0V
• Displays SH5V for short to 5V
• Open-collector and open-emitter in
wired-OR design are logical
therefore will not fail outputs
10. Output configuration
• Enables input channels
• Shorted pins not enabled
• For bi-directional or multiple gates,
pins enabled only as required to
reduce power dissipation
• Output pin position given by auto
clip positioning
11. Output conflict test
• In-circuit testing of tri-state ICs
• Disables IC
• Measures impedance of output
pins (high impedance expected)
• Displays CFLT on driven pins if not
high impedance
12. Truth table test
• Stimulates IC whilst backdriving
other signals in-circuit
• Compares output voltages with
programmed logic thresholds
• Invalid logic levels displayed
• Automatic circuit compensation
allows ‘as wired’ testing by altering
IC test program
13. Voltage test
• Measures voltages on each pin
• Pins connected to 0V via 10K
resistor
• Displays results
• Compares to programmed
thresholds for logic levels
14. Thermal test
• Power-off test
• Performs modified digital V-I test
• Uses forward junction voltage as
function of temperature
• Indicates overloaded ICs
15. Digital V-I test
• Power-off test
• Examines current against voltage
for each pin
• Pins driven through 10K resistor
• Linear voltage sweep between 10V and +10V
• Input leakage voltage breakdown
and shorts identified
• Optimised for digital ICs
Device fails
• Loop or fail loop selected?
• Open circuits present?
• Signal indicated on pins?
• Are conflicts identified?
•
••
•
••
•
Stop clock or processor to stabilise BUS.
Does device
have
a clock
pin?and check
Locate
other
devices
on BUS
Select
Suspect
LOOP
clip
connection.
test.
enable
Is signalpins.
present?
Repeat
Move
clip
test.
to remove
invalid open circuits.
Are
BUSand
devices
enabled?
Is origin
IC gate?
Use BDO to disable devices.
Summary
Truth table testing is just a small part
of the ABI test philosophy.
To get the best results all of the test
types should be utilised as much as
possible.
Understanding how the equipment
gets the information speeds up the
success in fault-finding.