or constraints such as:
The main purpose of BIST is to reduce the complexity, and thereby decrease the cost and reduce reliance upon external (pattern-programmed) test equipment. BIST reduces cost in two ways:
Both lead to a reduction in hourly charges for automated test equipment (ATE) service.
The BIST name and concept originated with the idea of including a pseudorandom number generator (PRNG) and cyclic redundancy check (CRC) on the IC. If all the registers that hold state in an IC are on one or more internal scan chains, then the function of the registers and the combinational logic between them will generate a unique CRC signature over a large enough sample of random inputs. So all an IC need do is store the expected CRC signature and test for it after a large enough sample set from the PRNG. The CRC comparison with expected signature or the actual resultant CRC signature is typically accessed via the JTAG IEEE 1149.1 standard.
Automotive tests itself to enhance safety and reliability. For example, most vehicles with antilock brakes test them once per safety interval. If the antilock brake system has a broken wire or other fault, the brake system reverts to operating as a normal brake system. Most automotive engine controllers incorporate a "limp mode" for each sensor, so that the engine will continue to operate if the sensor or its wiring fails. Another, more trivial example of a limp mode is that some cars test door switches, and automatically turn lights on using seat-belt occupancy sensors if the door switches fail.
Almost all avionics now incorporate BIST. In avionics, the purpose is to isolate failing line-replaceable units, which are then removed and repaired elsewhere, usually in depots or at the manufacturer. Commercial aircraft only make money when they fly, so they use BIST to minimize the time on the ground needed for repair and to increase the level of safety of the system which contains BIST. Similar arguments apply to military aircraft. When BIST is used in flight, a fault causes the system to switch to an alternative mode or equipment that still operates. Critical flight equipment is normally duplicated, or redundant. Less critical flight equipment, such as entertainment systems, might have a "limp mode" that provides some functions.