Single stuck at line
Single stuck line is a fault model used in digital circuits. It is used for post manufacturing testing, not design testing. The model assumes one line or node in the digital circuit is stuck at logic high or logic low. When a line is stuck, it is called a fault.
Digital circuits can be divided into:
- Gate level or combinational circuits which contain no storage (latches and/or flip flops) but only gates like NAND, OR, XOR, etc.
- Sequential circuits which contain storage.
This fault model applies to gate level circuits, or a block of a sequential circuit which can be separated from the storage elements. Ideally a gate-level circuit would be completely tested by applying all possible inputs and checking that they gave the right outputs, but this is completely impractical: an adder to add two 32-bit numbers would require 264 = 1.8*1019 tests, taking 58 years at 0.1 ns/test. The stuck at fault model assumes that only one input on one gate will be faulty at a time, assuming that if more are faulty, a test that can detect any single fault, should easily find multiple faults.
To use this fault model, each input pin on each gate in turn, is assumed to be grounded, and a test vector is developed to indicate the circuit is faulty. The test vector is a collection of bits to apply to the circuit's inputs, and a collection of bits expected at the circuit's output. If the gate pin under consideration is grounded, and this test vector is applied to the circuit, at least one of the output bits will not agree with the corresponding output bit in the test vector. After obtaining the test vectors for grounded pins, each pin is connected in turn to a logic one and another set of test vectors is used to find faults occurring under these conditions. Each of these faults is called a single stuck-at-0 (s-a-0) or a single stuck-at-1 (s-a-1) fault, respectively.
This model worked so well for transistor-transistor logic (TTL), which was the logic of choice during the 1970s and 1980s, that manufacturers advertised how well they tested their circuits by a number called "stuck-at fault coverage", which represented the percentage of all possible stuck-at faults that their testing process could find. While the same testing model works moderately well for CMOS, it is not able to detect all possible CMOS faults. This is because CMOS may experience a failure mode known as a stuck-open fault, which cannot be reliably detected with one test vector and requires that two vectors be applied sequentially. The model also fails to detect bridging faults between adjacent signal lines, occurring in pins that drive bus connections and array structures. Nevertheless, the concept of single stuck-at faults is widely used, and with some additional tests has allowed industry to ship an acceptable low number of bad circuits.
The testing based on this model is aided by several things:
- A test developed for a single stuck-at fault often finds a large number of other stuck-at faults.
- A series of tests for stuck-at faults will often, purely by serendipity, find a large number of other faults, such as stuck-open faults. This is sometimes called "windfall" fault coverage.
- Another type of testing called IDDQ testing measures the way the power supply current of a CMOS integrated circuit changes when a small number of slowly changing test vectors are applied. Since CMOS draws a very low current when its inputs are static, any increase in that current indicates a potential problem.
A logic gate is a device that performs a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has, for instance, zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device.
Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics which work primarily with analog signals. Despite the name, digital electronics designs includes important analog design considerations.
Transistor–transistor logic (TTL) is a logic family built from bipolar junction transistors. Its name signifies that transistors perform both the logic function and the amplifying function, as opposed to earlier resistor–transistor logic (RTL) and diode–transistor logic (DTL).
NMOS or nMOS logic uses n-type (-) MOSFETs to implement logic gates and other digital circuits.
In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. It outputs a bit opposite of the bit that is put into it. The bits are typically implemented as two differing voltage levels.
The 7400 series is a popular logic family of transistor–transistor logic (TTL) integrated circuits (ICs).
ATPG is an electronic design automation method or technology used to find an input sequence that, when applied to a digital circuit, enables automatic test equipment to distinguish between the correct circuit behavior and the faulty circuit behavior caused by defects. The generated patterns are used to test semiconductor devices after manufacture, or to assist with determining the cause of failure. The effectiveness of ATPG is measured by the number of modeled defects, or fault models, detectable and by the number of generated patterns. These metrics generally indicate test quality and test application time. ATPG efficiency is another important consideration that is influenced by the fault model under consideration, the type of circuit under test, the level of abstraction used to represent the circuit under test, and the required test quality.
A fault model is an engineering model of something that could go wrong in the construction or operation of a piece of equipment. From the model, the designer or user can then predict the consequences of this particular fault. Fault models can be used in almost all branches of engineering.
Reading is an action performed by computers, to acquire data from a source and place it into their volatile memory for processing. Computers may read information from a variety of sources, such as magnetic storage, the Internet, or audio and video input ports. Reading is one of the core functions of a Turing machine.
The OR gate is a digital logic gate that implements logical disjunction. The OR gate outputs "true" if any of its inputs is "true"; otherwise it outputs "false". The input and output states are normally represented by different voltage levels.
XOR gate is a digital logic gate that gives a true output when the number of true inputs is odd. An XOR gate implements an exclusive or from mathematical logic; that is, a true output results if one, and only one, of the inputs to the gate is true. If both inputs are false (0/LOW) or both are true, a false output results. XOR represents the inequality function, i.e., the output is true if the inputs are not alike otherwise the output is false. A way to remember XOR is "must have one or the other but not both".
The NAND Boolean function has the property of functional completeness. This means that any Boolean expression can be re-expressed by an equivalent expression utilizing only NAND operations. For example, the function NOT(x) may be equivalently expressed as NAND(x,x). In the field of digital electronic circuits, this implies that it is possible to implement any Boolean function using just NAND gates.
The XNOR gate is a digital logic gate whose function is the logical complement of the Exclusive OR (XOR) gate. It is equivalent to the logical connective from mathematical logic, also known as the material biconditional. The two-input version implements logical equality, behaving according to the truth table to the right, and hence the gate is sometimes called an "equivalence gate". A high output (1) results if both of the inputs to the gate are the same. If one but not both inputs are high (1), a low output (0) results.
Scan chain is a technique used in design for testing. The objective is to make testing easier by providing a simple way to set and observe every flip-flop in an IC.The basic structure of scan include the following set of signals in order to control and observe the scan mechanism.
- Scan_in and scan_out define the input and output of a scan chain. In a full scan mode usually each input drives only one chain and scan out observe one as well.
- A scan enable pin is a special signal that is added to a design. When this signal is asserted, every flip-flop in the design is connected into a long shift register.
- Clock signal which is used for controlling all the flip-flops in the chain during shift phase and the capture phase. An arbitrary pattern can be entered into the chain of flip-flops, and the state of every flip-flop can be read out.
Iddq testing is a method for testing CMOS integrated circuits for the presence of manufacturing faults. It relies on measuring the supply current (Idd) in the quiescent state. The current consumed in the state is commonly called Iddq for Idd (quiescent) and hence the name.
Diode logic constructs AND and OR logic gates with diodes and resistors.
In integrated circuit design, dynamic logic is a design methodology in combinational logic circuits, particularly those implemented in metal–oxide–semiconductor (MOS) technology. It is distinguished from the so-called static logic by exploiting temporary storage of information in stray and gate capacitances. It was popular in the 1970s and has seen a recent resurgence in the design of high-speed digital electronics, particularly central processing units (CPUs). Dynamic logic circuits are usually faster than static counterparts and require less surface area, but are more difficult to design. Dynamic logic has a higher average rate of voltage transitions than static logic, but the capacitive loads being transitioned are smaller so the overall power consumption of dynamic logic may be higher or lower depending on various tradeoffs. When referring to a particular logic family, the dynamic adjective usually suffices to distinguish the design methodology, e.g. dynamic CMOS or dynamic SOI design.
A logic probe is a low-cost hand-held test probe used for analyzing and troubleshooting the logical states of a digital circuit. When many signals need to be observed or recorded simultaneously, a logic analyzer is used instead.
Transistor Fault model is a Fault model used to describe faults for CMOS logic gates. At transistor level, a transistor may be stuck-short or stuck-open. In stuck-short, a transistor behaves as it is always conducts, and stuck-open is when a transistor never conducts current. Stuck-short will usually produce a short between VDD and VSS.
The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 and reset to store a logic 0. Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.
