8-bit MSI 16-bit Logic Products with Unused or Floating Logic Inputs

Standard Products
Application Note
8-bit MSI & 16-bit Logic Products
with Unused or Floating Logic Inputs
The most important thing we build is trust
1
Overview
To avoid system-level problems in designs that use 8-bit and 16-bit logic devices, it is important that unused or floating
CMOS, or TTL inputs and bi-directional signals are properly managed. Since CMOS inputs are inherently high impedance
(high-Z), when inputs are left unconnected, or otherwise not properly driven, the voltage potential at the input can float to
most any value between VSS and VDD. This is because the floating input is effectively an isolated capacitor with one terminal
unconnected, and so it can easily pick up noise or stray charges. See Figure 1. Ensuring that unused inputs and bi-directional
signals are properly managed will reduce the chance of current and future system malfunction or failure.
Figure 1. CMOS Inverter Schematic and Symbol
There are two common conditions that occur in practice and that can lead to floating inputs for Cobham Semiconductor
Solutions16-bit or 8-bit MSI CMOS Logic products. The first case can arise when some logic inputs are not needed, or
unused during logic design. The second results from a high impedance (High-Z) logic state of the driving circuit or bus
connected to the 16-bit or 8-bit MSI CMOS logic input. A Tri-State output driver or data bus connection to the input is an
example of this condition.
Since CMOS inputs are High-Z, any condition where the logic input is not driven to either a logic HI or logic LO state can
result in indeterminate logic levels. The results of this condition are: 1) Undesired or anomalous output behavior, such as
unknown logic state, or oscillation and 2) High current consumption can result if the input logic level is at or near the
mid-point of the input voltage range, or between LO and HI logic levels. This is the region of maximum switching current
(IDDQ) for CMOS logic circuits. The Vout vs. Vin and IDDQ vs. Vin transfer curves for a CMOS inverter are shown in Figure 2.
950004-001
Version 1.0.0
-1 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Technical Background
Figure 2. CMOS Inverter DC Voltage and Current Transfer Curves
The question then arises as to what to do with these unused, High-Z, or otherwise floating logic inputs. This Application Note
describes the methods and techniques recommended by Cobham to mitigate potential problems due to floating inputs for
Cobham 8-bit MSI and 16-bit Logic products.
2
Technical Background
Cobham 8-bit and 16-bit logic devices include both CMOS and TTL input/output (I/O) types. If the device name includes
“ACS” in the part name, then the device has CMOS I/O logic buffers and operates with CMOS logic levels. If the device name
includes “ACT” in the part name, then the device has TTL I/O buffers and operates with TTL logic levels. This is noted
because the input voltage levels (VIL, VIH) can be different between the two I/O logic standards and there can also be minor
differences in the response to noise or the floating level of the unused input between parts based on these two standards.
The focus of the Application Note will be on CMOS devices, but the findings, methods, and recommendations are also
directly applicable to TTL circuits as well. Some of Cobham’s 8-bit and 16-bit logic devices also include Schmitt Trigger
inputs. These are provided for applications requiring additional noise immunity and tolerance for slow input rise and fall
times (tr, tf). Additional details of Schmitt Trigger inputs are provided below in Section 3.0.
In some cases in application, not all gates or inputs of digital logic devices are used. For example, there may be a
configuration where only two inputs of a three input AND gate are used. When this situation occurs, the unused inputs
should not be left unconnected because the indeterminate voltages at the external connections will result in undefined
operational states. This is a general practice that must be observed for digital logic design under all circumstances.
All unused inputs of digital logic devices must be connected to a logic LO or HI voltage level to prevent them from floating.
The logic level (LO or HI) that should be applied to the particular unused input depends on the function of the device. CMOS
inputs are High-Z and so even the smallest change in voltage or charge on the open input can result in undesired logic levels.
A small change in the charge at the unconnected input, as caused by proximity to another charged object (triboelectric
effect), for example, can dramatically change the voltage and logic state of the input buffer and so too the logic output.
Additionally, and for the same reasons, unconnected inputs may be influenced by noise, either radiated, or coupled from
nearby traces or circuit devices. As a result of this coupled voltage, the behavior at the output of the logic circuit can no
longer be predicted for unconnected inputs.
950004-001
Version 1.0.0
-2 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Technical Background
2.1
Definition of Threshold Voltages
The threshold voltage definitions for standard CMOS inputs and Schmitt Trigger inputs are given here and in Figure2 and
Figures 3, 4.
Vt+/Vt- are the threshold voltages for standard CMOS input buffers, which are determined by the properties of the P and N
MOSFET devices in an inverter, or logic element, and as defined in Figure 2.
VT+/VT- are the threshold voltages for CMOS Schmitt Trigger input buffers, which determine the hysteresis properties of the
buffer, and as defined in Figures 3, 4.
Figure 3. CMOS Non-Inverting Schmitt Trigger DC Voltage Transfer Curve
Figure 4. CMOS Inverting Schmitt Trigger DC Voltage Transfer Curve
950004-001
Version 1.0.0
-3 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Unused or Floating Logic Inputs
3
Unused or Floating Logic Inputs
The question often arises in practical application of 8-bit MSI and 16-bit Logic products as to whether or not floating inputs
are allowed in circuit design. Owing to the High-Z nature of CMOS logic gate inputs, as described previously in Sections 1.0
and 2.0, above, some considerations of floating inputs are presented here as general design practices.
A floating input can be most any voltage value. This is quasi-Analog operation where a continuous range of voltages is
allowed. Digital logic, however, is binary valued (i.e. logic LO (0) or logic HI (1) by definition. These two types of signals (i.e.
Analog/Digital) are not directly compatible without ADC/DAC conversion.
This consideration applies to both standard CMOS inputs and Schmitt Trigger inputs. Schmitt Trigger inputs include
hysteresis for improved noise immunity and a greater tolerance to slow input rise and fall times (tr, tf), but are otherwise
functionally equivalent to standard CMOS inputs. Hysteresis enables a Schmitt Trigger input to switch at different trigger
voltages (VT+, VT-) for a HI to LO transition vs. a LO to HI transition. The difference between VT+ and VT- is the hysteresis
voltage, as shown in Figures 3 and 4.
Figure 3. General “best practices” for digital design dictates that all I/O signals are maintained at known logic values. Pull-up
and pull-down resistors are frequently used for this purpose. Since the CMOS logic inputs are High-Z, and do not require
current to set the input voltage, these pull-up/pull-down resistors can be high-valued (e.g. ~100kΩ), resulting in a weak
pull-up/pull-down action. This is all that is necessary. After the initial charge adjustment, no DC current is required.
Figure 4. Space applications of electronics result in exposure to charging of unconnected signals, metal surfaces and traces,
etc. Without a sufficiently low impedance discharge path, this charging effect can be significant due to various space
radiation and electric fields. Device charging can result in ESD damage if there is an uncontrolled discharge event. Including
pull-up/pull-down resistors (e.g. ~100kΩ) would mitigate charging on the (un-driven) input nodes by draining off any static
change in a slow and controlled manner.
The following three cases are circuit operating modes to consider for “floating” inputs.
1.
In the best case situation, the ‘floating input’ voltage potential is well below Vt- or well above Vt+ and there is little or no
noise on the floating logic gate input pin. In this case the output remains stable.
2.
In the intermediate case, input voltage potential is between Vt- and Vt+ and there is little or no noise at the floating
input. In this case with the input voltage at approximately (VDD-VSS)/2, the input buffer now dissipates high DC current
(IDDQ), which is normally only flowing during high-speed switching transients. This current dissipates excess power and
may damage the logic circuits over time. See Figure 2 for DC voltage and current transfer curves for a CMOS inverter.
As is seen from the curves in Figure 3, if the inverter input voltage is held at an indeterminate value (i.e. between VIL and
VIH), then the output voltage will also be indeterminate in accordance with the Vout-Vin relationship of the inverter’s
transfer curve. This condition not only results in very high static IDDQ current, but the indeterminate inverter output voltage
will also propagate to the next stage logic gate operation, resulting in similar anomalous operation for the downstream
electronics.
This high current and indeterminate output conditions apply equally to both standard CMOS inputs and Schmitt Trigger
inputs, since both types of inputs have similar DC voltage transfer curves and are based on the CMOS inverter circuit. This
means that both device types can experience high IDDQ current when inputs are near (VDD-VSS)/2. The peak IDDQ current
for a Schmitt Trigger input will be at a different input voltage value than for a standard CMOS input. This is due to the
differences between Vt+/Vt- threshold voltages for the standard CMOS inputs (see Figure 2) and VT+/VT- Schmitt Trigger
threshold voltages (see Figures 3, 4). The peak IDDQ current will, however, still occur close to the midpoint of the VDD power
supply voltage as in the case of the standard CMOS inverter circuit.
3) In the worst-case situation, the floating input voltage potential is between Vt- and Vt+, and with enough noise to cross
both thresholds causing not only high DC switching current, but also an output signal that bounces around between logic LO,
logic HI, and indeterminate logic states (i.e. above VOL max., but below VOH min.). One generally applies “worst-case”
analysis for HiRel applications, since the condition can happen, and would result in the poorest outcome.
If the Designer wants a guarantee that the outputs won’t bounce around, then they must add pull-ups or pull-downs to the
unused, or otherwise floating input pins so that they will always be in a known (determinate) state. A large value resistor
(~100kΩ), providing a weak pull-up/down mechanism is sufficient.
950004-001
Version 1.0.0
-4 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Summary and Conclusion
Another possible solution for situations where a single floating logic gate arises is to connect the unused logic input directly
to one of the other logic inputs of the same logic gate that is in use. For standard Boolean logic gates, the logic function of
the device is unaffected. This circuit arrangement can be used equally well with AND (NAND) or OR (NOR) gates. However,
this configuration would result in a 2x increase in load capacitance for the driving circuit since it is now driving two logic
inputs. Analysis or simulation would be required to determine if switching speed is still acceptable or not.
4
Summary and Conclusion
When using Cobham’s 8-bit and 16-bit Logic products, the following two general recommendations apply for reliable and
deterministic operation: 1) No inputs are left floating or otherwise unconnected, and 2) Resistive pull-ups or pull-downs are
connected to those input signal pins that are not physically connected (floating), or can be driven by a Tri-State signal, so
that they will always be in a known, or determinate state. In this way, the voltage on the outputs won’t bounce around
indeterminately, or causing device static power supply current (IDDQ) to exceed specified (low) limits. A large value resistor
(~100k) provides a weak pull-up/pull-down function, and is sufficient for this purpose.
950004-001
Version 1.0.0
-5 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
Appendix A: Lists of Applicable Products
Table 1. List of Applicable Products -8-bit MSI Logic
Logic Function Product Description
Logic quadruple 2-input NAND gates
Logic quadruple 2-input NOR gates
Hex inverters
Logic quadruple 2-input AND gates
Logic triple 3-input NAND gates
Logic triple 3-input AND gates
Hex inverting Schmitt trigger
Logic dual 4-input NAND gates
Logic triple 3-input NOR gates
950004-001
Version 1.0.0
Manufacturer
Part Number
SMD
Number
Device Internal PIC
Type
Number:
UT54ACS00
5962-96512
1
CA000
UT54ACS00E*
5962-96512
02,03
CE000
UT54ACTS00
5962-96513
1
LA000
UT54ACTS00E*
5962-96513
02,03
LE000
UT54ACS02
5962-96514
1
CA002
UT54ACS02E*
5962-96514
02,03
CE002
UT54ACTS02
5962-96515
1
LA002
UT54ACTS02E*
5962-96515
02,03
LE002
UT54ACS04
5962-96516
1
CA004
UT54ACS04E*
5962-96516
02,03
CE004
UT54ACTS04
5962-96517
1
L004
UT54ACTS04E*
5962-96517
02,03
LE004
UT54ACS08
5962-96518
1
CA008
UT54ACS08E*
5962-96518
02,03
CE008
UT54ACTS08
5962-96519
1
LA008
UT54ACTS08E*
5962-96519
02,03
LE008
UT54ACS10
5962-96520
1
CA010
UT54ACTS10
5962-96521
1
LA010
UT54ACS11
5962-96522
1
CA011
UT54ACTS11
5962-96523
1
LA011
UT54ACS14
5962-96524
1
CA014
UT54ACS14E*
5962-96524
02,03
CE014
UT54ACTS14
5962-96525
1
LA014
UT54ACTS14E*
5962-96525
02,03
LE014
UT54ACS20
5962-96526
1
CA020
UT54ACTS20
5962-96527
1
LA020
UT54ACTS20E*
5962-96527
02,03
LE020
UT54ACS27
5962-96528
1
CA027
UT54ACTS27
5962-96529
1
LA027
-6 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
Table 1. (Continued)List of Applicable Products -8-bit MSI Logic
Logic Function Product Description
Hex non-inverting buffers
Logic 4-wide AND-OR-INVERT gates
Manufacturer
Part Number
Device Internal PIC
Type
Number:
UT54ACS34
5962-96530
1
CA034
UT54ACTS34
5962-96531
1
LA034
UT54ACS54
5962-96532
1
CA054
UT54ACTS54
5962-96533
1
LA054
5962-96534
1
CA074
UT54ACS74E
5962-96534
02,03
CE074
UT54ACTS74
5962-96535
1
LA074
UT54ACTS74E*
5962-96535
02,03
LE074
FLIP-FLOPs dual D with clear and preset UT54ACS74
Comparators 4-bit
SMD
Number
UT54ACS85
5962-96536
1
CA085
UT54ACTS85
5962-96537
1
LA085
UT54ACS86
5962-96538
1
CA086
UT54ACS86E*
5962-96538
02,03
CE086
UT54ACTS86
5962-96539
1
LA086
UT54ACS109
5962-96540
1
CA109
UT54ACS109E*
5962-96540
02,03
CE109
UT54ACTS109
5962-96541
1
LA109
UT54ACS132
5962-96542
1
CA132
UT54ACS132E*
5962-96542
02,03
CE132
UT54ACTS132
5962-96543
1
LA132
5962-96544
1
CA138
UT54ACS138E*
5962-96544
02,03
CE138
UT54ACTS138
5962-96545
1
LA138
Decoders/demultiplexers dual 2-line to
4-line
UT54ACS139
5962-96546
1
CA139
UT54ACTS139
5962-96547
1
LA139
Data selectors/multiplexers 1 to 8
UT54ACS151
5962-96548
1
CA151
UT54ACTS151
5962-96549
1
LA151
UT54ACS153
5962-96550
1
CA153
UT54ACTS153
5962-96551
1
LA153
UT54ACTS153E*
5962-96551
02,03
LE153
Logic quadruple 2-input XOR gates
FLIP-FLOPs dual J-K
Logic quadruple 2-Input NAND with
Schmitt triggers
Decoders/demultiplexers 3-line to 8-line UT54ACS138
Multiplexers 4-input dual
950004-001
Version 1.0.0
-7 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
Table 1. (Continued)List of Applicable Products -8-bit MSI Logic
Logic Function Product Description
Multiplexers quadruple 2 to 1
Counters 4-bit synchronous
Registers 8-bit shift
Registers 8-bit shift parallel
Counters 4-bit binary up-down
Manufacturer
Part Number
SMD
Number
Device Internal PIC
Type
Number:
UT54ACS157
5962-96552
1
CA157
UT54ACTS157
5962-96553
1
LA157
UT54ACTS157E*
5962-96553
02,03
LE157
UT54ACS163
5962-96554
1
CA163
UT54ACTS163
5962-96555
1
LA163
UT54ACS164
5962-96556
1
CA164
UT54ACS164E*
5962-96556
02,03
CE164
UT54ACTS164
5962-96557
1
LA164
UT54ACTS164E*
5962-96557
02,03
LE164
UT54ACS165
5962-96558
1
CA165
UT54ACS165E*
5962-96558
02,03
CE165
UT54ACTS165
5962-96559
1
LA165
UT54ACS169
5962-96560
1
CA169
UT54ACTS169
5962-96561
1
LA169
Counters up-down BCD synchronous
4-bit
UT54ACS190
5962-96562
1
CA190
UT54ACTS190
5962-96563
1
LA190
Counters up-down binary synchronous
4-bit
UT54ACS191
5962-96564
1
CA191
UT54ACS191E*
5962-96564
02,03
CE191
UT54ACTS191
5962-96565
1
LA191
UT54ACS193
5962-96566
1
CA193
UT54ACS193E*
5962-96566
02,03
CE193
UT54ACTS193
5962-96567
1
LA193
Clock and wait-state generation circuit
UT54ACTS220
5962-96753
1
LA220
Buffers octal with inverted 3-state
outputs
UT54ACS240
5962-96568
1
CA240
UT54ACTS240
5962-96569
1
LA240
Buffers/line drivers octal with 3-state
outputs
UT54ACS244
5962-96570
1
CA244
UT54ACS244E*
5962-96570
02,03
CE244
UT54ACTS244
5962-96571
1
LA244
Clocks up-down synchronous 4-bit
950004-001
Version 1.0.0
-8 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
Table 1. (Continued)List of Applicable Products -8-bit MSI Logic
Logic Function Product Description
Transceivers octal bus with 3-state
outputs
Manufacturer
Part Number
SMD
Number
Device Internal PIC
Type
Number:
UT54ACS245
5962-96572
1
CA245
UT54ACTS245
5962-96573
1
LA245
UT54ACTS245E*
5962-96573
02,03
LE245
Transceivers octal bus Schmitt trigger
with 3-state outputs
UT54ACS245S
5962-96572
1
CA245
Multiplexers 4-input dual with 3-state
outputs
UT54ACS253
5962-96574
1
CA253
UT54ACTS253
5962-96575
1
LA253
5962-96576
1
CA264
UT54ACTS264
5962-96577
1
LA264
UT54ACS273
5962-96578
1
CA273
UT54ACS273E*
5962-96578
02,03
CE273
UT54ACTS273
5962-96579
1
LA273
UT54ACS279
5962-96580
1
CA279
UT54ACTS279
5962-96581
1
LA279
Look-ahead carry generator for counters UT54ACS264
FLIP-FLOPs octal D with clear
Latches quadruple S-R
9-bit parity generators/checkers
UT54ACS280
5962-96582
1
CA280
UT54ACTS280
5962-96583
1
LA280
UT54ACS283
5962-96584
1
CA283
UT54ACS283E*
5962-96584
02,03
CE283
UT54ACTS283
5962-96585
1
LA283
Registers universal shift/storage
UT54ACS299E*
5962-06238
2.03
CE299
Buffers/line drivers hex with 3-state
outputs
UT54ACS365
5962-96586
1
CA365
UT54ACTS365
5962-96587
1
LA365
Latches octal transparent with 3-state
outputs
UT54ACS373
5962-96588
1
CA373
UT54ACTS373
5962-96589
1
LA373
5962-96590
1
CA374
UT54ACTS374
5962-96591
1
LA374
UT54ACS540
5962-96592
1
CA540
UT54ACTS540
5962-96593
1
LA540
Adders 4-bit parity binary full
FLIP-FLOPS octal D with 3-state outputs UT54ACS374
Octal Buffers and Line Drivers with
inverted 3-state outputs
950004-001
Version 1.0.0
-9 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
Table 1. (Continued)List of Applicable Products -8-bit MSI Logic
Logic Function Product Description
Octal Buffers and Line Drivers with
3-state outputs
Manufacturer
Part Number
SMD
Number
Device Internal PIC
Type
Number:
UT54ACS541
5962-96594
1
CA541
UT54ACTS541
5962-96595
1
LA541
UT54ACTS541E*
5962-96595
02,03
LE541
EDACs
UT54ACTS630
5962-06239
1
LA630
Transceivers latchable with >parity
generator/checker
UT54ACS899*
5962-06240
1
CA899
Logic dual 4-input NOR gates
UT54ACS4002
5962-96596
1
CA4002
UT54ACTS4002
5962-96597
1
LA4002
Table 2.
Logic FunctionProduct
Description
SMD Number
Device
Type
UT54ACTQ16244
5962-06243
1
KN05AA
16-bit Bidirectional Transceiver, TTL
UT54ACTQ16245
Inputs, and Three-State Quiet Outputs
5962-06244
1
KN04AA
16-bit D Flip-Flop TTL Inputs, and
Three-State Quiet Outputs
UT54ACTQ16374
5962-06245
1
KN06AA
Schmitt CMOS 16-bit Bidirectional
MultiPurpose Registered Transceiver,
with cold/warm spare
UT54ACS164646S
5962-06234
1
KE01BA
Schmitt CMOS 16-bit Bidirectional
UT54ACS162245SLV
MultiPurpose Low Voltage Transceiver,
with cold/warm spare
5962-02543
1
WA04BA
16-bit Bidirectional MultiPurpose
Transceiver with cold spare
5962-98580
01,02,0 JM03EA,
3,04,05
JM03EW
16-bit Buffer/Line Driver,TTL Inputs,
and Three-State Quiet Outputs
Manufacturer Part
Number
UT54ACS164245S/SE
Internal PIC
Number:
JM04EA
JM04EW
16-bit Bidirectional MultiPurpose
Transceiver with cold/warm spare
950004-001
Version 1.0.0
UT54ACS164245SEI
5962-98580
06,07
JM06AA
JM06AW
- 10 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
May 2015
Appendix A: Lists of Applicable Products
Summary and Conclusion
This product is controlled for export under the U.S. Department of Commerce (DoC). A license may be
required prior to the export of this product from the United States.
Cobham Semiconductor Solutions
4350 Centennial Blvd
Colorado Springs, CO 80907
E: [email protected]
T: 800 645 8862
Aeroflex Colorado Springs Inc., DBA Cobham Semiconductor Solutions, reserves the right to make changes to any products and services described
herein at any time without notice. Consult Aeroflex or an authorized sales representative to verify that the information in this data sheet is current
before using this product. Aeroflex does not assume any responsibility or liability arising out of the application or use of any product or service
described herein, except as expressly agreed to in writing by Aeroflex; nor does the purchase, lease, or use of a product or service from Aeroflex
convey a license under any patent rights, copyrights, trademark rights, or any other of the intellectual rights of Aeroflex or of third parties.
950004-001
Version 1.0.0
- 11 -
Cobham Semiconductor Solutions
Cobham.com/HiRel
Similar pages