Testing Automotive MCU – An Overview


There’s a rising share of electronics in a automobile, starting from infotainment, physique and engine-controls to superior driver-assistance modules. Right this moment, premier automobiles have as much as 70 MCUs (Microcontroller Management Items), interconnected by a number of system buses and trade 1000’s of alerts between themselves. To place that in enterprise perspective, analysts estimate autonomous chips annual income to develop from $11 billion (in 2019) to $29 billion in 2030, representing a income of $350 per automobile by 2030.

Growing semiconductor share in an automotive
Fig. 1: Rising semiconductor share in an automotive

The rising electronics in autos permits efficiency enhancement, higher security, safety together with different value-added options. With elevated complexity of electronics parts together with cameras, radars, sensors and so forth, it is very important put enough emphasis on their reliability. A single malfunction of an digital part can result in a life-threatening state of affairs.

Semiconductor firms supplying the digital parts topic them to rigorous testing for any useful or manufacturing defects.

System testing is a well-established course of, that requires particular design exercise as a way to insert correct check infrastructure within the die, with help of devoted EDA software program. It Is categorized as Design for Take a look at (DFT) or extra usually as DFx, to incorporate different manufacturability, reliability and yield elements. Nevertheless, automotive microcontroller unit (MCU) pose further challenges and constrains on the testing mechanism, in comparison with communication, networking or leisure domains.

On this article, we offer an outline of those distinctive challenges and testing options deployed.

Automotive Testing Challenges Overview

1. Mission crucial software

Automotive unit is a life-sensitive software, each for individuals inside in addition to outdoors the automobile, thus there is no such thing as a room for an error. We will very properly think about the influence, if the air baggage don’t get deployed on the proper time! The extent of acceptable defects is expressed by DPPM (Faulty Elements Per Million) and Automotive Security Integrity Ranges (ASIL) outlined below ISO 26262. Whereas for a shopper grade machine, a DPPM variety of ~300 could also be acceptable, for automotive it needs to be near zero!

Thus automotive MCU requires a really excessive test-coverage and it is not uncommon apply to check nearly all design nodes by way of structural stuck-at (SA) and transition delay(TD) exams. The necessities are barely relaxed for typical shopper grade units. It’s price mentioning right here that gaining simply the final 0.1% protection, takes important design efforts and a whopping variety of check patterns, thus including to the check time and check price. Additionally, as a way to cowl all sorts of attainable defects in automotive units, new fault fashions are constantly explored and added to the check fits, eg, cell-aware, bridging and small-delay-defect exams.

Along with the manufacturing facility testing, the units are usually screened for any defects that will have creeped-in in the course of the working lifecycle. Important logic and reminiscences are fitted with a self-test functionality utilizing LBIST and MBIST respectively, that will get triggered at machine booting, shutdown or at common intervals. The outcomes are monitored by software software program and any subject will get raised as an acceptable alarm within the system.

Self-test, nonetheless, brings its personal design overheads when isolating the test-logic from exterior interferences to make sure that the performance shouldn’t be disturbed, prevention of unknown states (X-sources) to keep away from corruption of signatures and test-point utilization to extend the controllability and observability of the design.

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Major purpose of any self-test method is to detect in-field failures, therefore the execution time requirement for such strategies will be very stringent. Any fault ought to be detected in a specified time referred to as DTI (Diagnostic Take a look at Interval) in any other case it could possibly show to be catastrophic for your complete system. This makes self-test implementation like LBIST an uphill process. Because of random nature of Logic Constructed-In-Self-Take a look at (LBIST) engine, generated by on-chip PRPG (Psuedo Random Sample Generator), it’s generally very difficult to get the required fault protection within the allotted time. This calls for large check level insertions within the design to enhance the controllability and observability for random resistant and laborious to detect faults. Whereas this step has been non-compulsory for regular ATPG testing, it’s an absolute important for LBIST. Testpoints are inserted for hard-to-detect faults, which often occur to be in logic with deep combo depths and therefore timing crucial paths, which pose its personal challenges in the course of the backend implementation

Fig. 2 reveals the rigorous train achieved to realize the specified run occasions for LBIST in two crucial IPs for an ST automotive chip. IP1 is a fancy design having very excessive combinational depths. A number of iterations with the CAD vendor to boost the check level insertion algorithms resulted in reaching the required check time and protection purpose. Nevertheless, few designs like IP2 which achieved the check time purpose with enhanced check level insertion circulation, created opposed impact on timing, as many management factors have been added on the crucial useful paths. Thus, offering self-test function in automotive chips will be very iterative and fascinating course of, with so many conflicting necessities for the DFT engineers.

LBIST implementation challenges for two of ST’s automotive chips
Fig. 2 : LBIST implementation challenges for 2 of ST’s automotive chips

2. Vast setting vary, -40 to +150C temp

A automobile is predicted to work seamlessly when driving from the snow-laden mountains proper into the scorching dessert or into the humid rain-forests. This places a whole lot of strain when signing-off the machine throughout temperature extremes. The testing additionally must cowl these excessive nook situations but preserve excessive production-yields. Automotive qualification contains testing the programs at places with excessive and opposed situations like Finland in Winter or Morocco in Summer time, and so forth. to validate the working vary.

Automotive DFT structure is designed to deal with die-to-die and on-chip variance ensuing from manufacturing course of parameter variations, along with excessive temperature vary. The resultant influence to setup and maintain timings on design paths, throughout shift in addition to seize part of scan primarily based testing, are dealt with by way of devoted and strong design buildings. That is usually not a necessity for shopper grade merchandise the place the ambient temperature vary is roughly 0 to 85C.

The library characterization, analog fashions and design sign-off additionally have to cater to those elevated variations and extra margins. That is additional aggravated with machine growing old. As an illustration, Fig 3 depicts how delays get impacted as a consequence of variations throughout PVT (Course of, Voltage, Temperature) and ageing. Excessive left on the determine is the reference delay with regular (typical) parameters and subsequent curves present how the delays get skewed with altering parameters.

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System qualification includes samples which can be particularly manufactured at completely different course of corners (referred to as matrix-lots) after which examined at each supply-temperature situation. Particular circuits, eg on-chip course of screens, are added on every unit to establish the machine habits and to tune (or trim) regulators, oscillators and different crucial parts accordingly. The info is collected over massive variety of samples to establish any course of drift and to fine-tune the manufacturing, as wanted. Scan strategies and yield analyzers are leveraged closely to extract, diagnose and course of such knowledge.

PVT variations for 300 paths on an ST device at 28FDSOI process
Fig. 3 : PVT variations for 300 paths on an ST machine at 28FDSOI course of
Change of failure rate λ over time
Fig 4: Change of failure price λ over time

3. Prolonged Lifetime – 15yrs

An MCU within the automobile is required to serve for total working lifetime of the automobile, usually 10-15yrs, with no need any service or substitute. Fig 4 reveals typical failure price change over time. The machine qualification must account for ageing, long-term reliability and early failure detection.

Each automotive unit is run by way of stress exams (BurnIN, HVST, VLV and so forth), in contrast to many shopper purposes the place solely few pattern items are topic to emphasize exams. The purpose of stress exams is to push any weak part to fail upfront, moderately than fail within the subject.
A few of the ageing manifestations are NBTI (Detrimental-bias temperature instability), Scorching Provider Injection (HCI) and Time-Dependent Dielectric Breakdown (TDDB) results.

These are usually screened by way of HTOL (Excessive Temp Working Life) stress and extra Vmin/Vmax margins throughout check. For brevity sake, we are going to skip delving into the main points. Nevertheless, these exams additional push the design and check limits. For instance, testing at Vmin of 0.9V, whereas additionally accounting for tester-equipment uncertainties and on-chip volt-drop, the top nodes of a path could finally get a voltage under the signoff Vmin. Couple this with PVT parameters and we could also be headed at throwing some in any other case good units (yield influence). We usually add sign-off guard-bands and extra robustness on scan-structure, particularly on hold-sensitive shift-paths, to keep away from such losses.

Silicon Lifecycle Administration (SLM) is one other rising paradigm, as a way to preserve the machine reliably accessible through-out the working lifecycle [4]. SLM leverages check infrastructure, along with different sensors like in-situ screens, to detect and handle points whereas in-field. Presence of those further buildings provides to check overheads and require distinctive answer at every layer. For instance, in-situ cells are personalized to completely scan-test the monitoring websites, along with the useful nodes.

Unnecessary to re-iterate that almost all shopper purposes are exempt from such rigorous exams.

4. Standby operation

Sure sensors and management domains stay powered-up through-out, even when the ignition is off.

These units draw energy from the battery within the automobile and therefore are required to maintain the facility consumption to reveal minimal. We will surely be upset to see the battery all drained and unable to self-start, after parking the automobile for two-weeks within the storage!
Many automotive units, particularly physique purposes, are designed with a number of power-domain islands; which generally have impartial voltage ranges as properly.

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The check structure is designed to deal with the isolation exams, power-controllers, standby operation and so forth. A number of provides additionally want consideration throughout low-pin-contact testing.
Networking, server and gaming purposes stay powered with an electrical energy supply, therefore donot require such low-power designing.

5. Safety and security hardening of check logic

Take a look at logic has been demonstrated as a useful gizmo to extract machine secrets and techniques by the adversaries. A automobile within the subject incorporates many secret keys and codes, from chip producer, OEMs, person in addition to third celebration distributors. Entry to those belongings imposes monetary losses in addition to threat on the roads (each for the person in addition to individuals across the automobile), if misused. A tool could comprise delicate knowledge from the person, chip vendor in addition to third celebration answer suppliers. Hacking or manipulating a rented automobile could put the subsequent person in danger or at ransom!

Structural logic, like scan chains, are proven as simple instruments to learn out machine secrets and techniques. Thus it is important that check logic is robustly disabled and can’t be used to launch an assault or learn any machine secrets and techniques [5], even below diagnostic or fail-return eventualities,

On the similar time, check logic will also be leveraged to establish any malicious logic or Trojans on the machine, inserted in the course of the design or manufacturing course of.

Along with safety, check alerts additionally must be security compliant. Any soft-error (SET/SUT) in check logic can’t be allowed to influence the machine performance and put it into an undesirable state. Varied obfuscation strategies in addition to redundancy logic (e.g Triple Module Redundancy) is positioned on the check logic and enablement paths to cater to safety and security necessities.

6. Quantity economics

Automotive qualification and certification is a protracted, rigorous and costly course of. So a tool, as soon as certified, is used for a number of years, earlier than being upgraded to a brand new model. Automotive producers would deploy a single certified product throughout a number of fashions for a few years. Automotive chip distributors have to maintain their design, fabrication and testing amenities for an extended interval for a single product. All amenities have to constantly carry out at similar parameters on which machine was certified, with none deviations, thus including to the upkeep prices.

This locations the automotive MCUs into high-volume, low-margin bracket in comparison with shopper markets. A lot in order that ‘Automotive grade’ units are generally referred as ‘military-spec merchandise at shopper costs’.

The ensuing income strain pushes larger multisite and low-cost-tester options, thus including additional complexity to the check structure and execution.

Conclusion

We talked about a number of the distinctive wants and challenges confronted by automotive chips and related complexities whereas testing these units. Testing neighborhood has put particular structure and strategies in place, and are consistently evolving, as a way to guarantee a protected, safe and dependable drive on the roads. It definitely impacts the machine and cycle-time prices, however as somebody stated – in case you discover testing costly, attempt with out it!


 



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