AN202 Datasheet, PDF (1/5 Pages) List of Unclassifed Manufacturers – Testing and specifying FAST logic

Philips Semiconductors
Testing and specifying FAST logic
Application note
AN202
INTRODUCTION
FAST™ is a second generation Schottky logic family that utilizes
advanced oxide-isolation techniques to increase the speed and
decrease the power dissipation beyond the levels achievable with
conventional junction isolated families. The improved performance
of the family is exhibited in two ways — first, the speed and power
characteristics of the devices are improved, and second, the
conditions under which speed and power are specified are much
tighter. For instance, LS and S TTL families offer AC limits only at a
nominal +5.00V V_CC supply voltage and at room temperature, 25°C.
By contrast, FAST guarantees improved AC performance and
specifies that performance over a supply variation of +5.00V ±10%
and at temperatures from 0°C to 70°C. Thus the designer no longer
needs to derate his propagation delays from the data sheet limits to
compensate for speed degradation over the temperature range.
With every advance of this magnitude, there arise new
considerations that must be kept in mind both by the system
designer and the user setting up test procedures. FAST is no
exception, and it is these considerations that will be addressed in
this application note. This paper represents an attempt to describe
the way the FAST logic parts are specified, why they are specified in
the way they are, and how the parts may be tested in the
qualification lab and at incoming inspection to verify their
performance.
THE FAST DATA SHEET PHILOSOPHY
Philips Semiconductors FAST data sheets have been configured
with an eye to quick usability … they are self contained and should
require no reference to other sections for information. The typical
propagation delays listed at the top of the page are the average
between t_PLH and t_PHL for the most significant data path through the
part. In the case of clocked products, this is sometimes the
maximum frequency of operation, but in any event, this number is a
5.00V – 25°C typical specification. The I_CC typical current is shown
in that same specification block is the average current (in the case
of a gate, this will be the average of the I_CCH and I_CCL currents) at
room temperature and V_CC = 5.00V. It represents the total current
through the package, not the current through individual
functions.
Other considerations are the Fanout and Loading tables. Some
manufacturers relate these numbers in therms of 7400 gate loads …
Philips Semiconductors feels that FAST is unlikely to be mixed with
other logic families and so gives the loading factors in terms of FAST
unit loads. A FAST unit load is defined to be 0.6mA in the Low state
and 20µA in the High state. Thus in the case of the 74F00 gate, the
inputs are specified as 1 Ful (FAST unit load) each … the outputs
need a little explanation. The standard FAST output is specified with
an I_OL sink current of 20mA and an I_OH of +1.0mA. Thus the fanout
of this gate in the Low state is 20mA/0.6mA or 33 FAST unit loads.
In the High state the fanout is 1mA/20µA or 50 FAST unit loads. In
each case, the Fanout and Loading Table on the Philips data sheets
states the High/Low fanout numbers … thus the 74F00 output fanout
is specified as 50/33 Ful.
ABSOLUTE MAXIMUM RATINGS
The Absolute Maximum Ratings table carries the maximum limits to
which the part can be subjected without damaging it … there is no
implication that the part will function at these extreme conditions.
Thus, specifications such as the most negative voltage that may be
applied to the outputs only guarantees that if less than –0.5V is
applied to the output pin, after that voltage is removed the part will
still be functional and its useful life will not have been shortened — it
is difficult to imagine the meaning of the term “functionality” WHILE
that voltage is applied to the output.
Input voltage and output voltage specification in this table reflect the
device breakdown voltages in the positive direction (+7.0V) and the
effect of the clamping diodes in the negative direction (–0.5V).
RECOMMENDED OPERATING CONDITIONS
The Recommended Operating Conditions table has a dual purpose.
In one sense, it sets some environmental conditions (operating
free-air temperature), and in another, it sets the conditions under
which the limits set forth in the DC Electrical Characteristics table
and AC Electrical Characteristics table will be met. Another way of
looking at this table is to think of it, not as a set of limits guaranteed
by Philips Semiconductors, but as the conditions Philips
Semiconductors uses to test the parts and guarantee that they will
then meet the limits set forth in the DC and AC Electrical
Characteristics tables.
Some care must be used in interpreting the numbers in this table.
Philips Semiconductors feels strongly that the specifications set
forth in a data sheet should reflect as accurately as possible the
operation of the part in an actual system. In particular, the input
threshold values of V_IH and V_IL can be tested by the user with
parametric test equipment … if V_IH and V_IL are applied to the inputs,
the outputs will be at the voltages guaranteed by the DC Electrical
Characteristics table providing that there is adequate grounding and
the input voltages are free from noise, otherwise a guardbanded V_IH
and V_IL should be used, i.e., 2.5V instead of 2.0V and 0.5V instead
of 0.8V. There is a tendency on the part of some users to use V_IH
and V_IL as conditions applied to the inputs to test the part for
functionality in a “truth-table exerciser” mode. This frequently causes
problems because of the noise present at the test head of
automated test equipment. Parametric tests, such as those used for
the output levels under the V_IH and V_IL conditions are done fairly
slowly, on the order of milliseconds, and any noise present at the
inputs has settled out before the outputs are measured. (This is not
the case with clocked or enabled parts and poor or moderate
fixturing may induce oscillations or severe ground bounce if noise is
present.) But in functionality testing, the outputs are examined much
faster, before the noise on the inputs is settled out and the part has
assumed its final and correct output state. Since these are unloaded
outputs, having faster edge rates, this causes more noise, If the
outputs are loaded, the 50pF per output pin can cause substantial
ground bounce. Thus V_IH and V_IL should never be used in testing
the functionality of any TTL part including FAST. For these types of
tests input voltages of +4.5V and 0.0V should be used for the High
and Low states respectively.
In no way does this imply that the devices are noise sensitive in the
final system. The use of “hard” Highs and Lows during functional
testing is done primarily to (1) reduce the effects of the large
amounts of noise typically present at the test heads of automated
test equipment with cables that may at times reach several feet and
(2) deal with testing parts exhibiting fast edge rates and 50pF per
output pin. The situation in a system on a PC board is less severe
than in a noisy production environment.
FAST is a trademark of Fairchild Camera and Instrument Corporation.
June 1987
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