Triple Voting Flops (TVF)

I was hearing this term for a long time in my current assignment. I couldn't understand what it is. So, finally I decided to read about this in detail and share the knowledge with you. The basic idea is to introduce fault tolerant mechanism in the design to reduce the chances of design failure.

In the recent times, the percentage of electronic components in the vehicles have increased manifolds changing these vehicles from simple transportation system to a mobile electronic hub. With time, electric and hybrid-electric vehicles are getting popular, more and more companies are investing in these modern vehicles. To add advanced safety feature mechanisms to the vehicles, the percentage of electronics used is likely to increase. This increase the chances of the electronic components failure. Some mechanisms are developed and introduced to minimize the occurrence of failure. Triple Modular Redundancy (TMR) is one such technique.

Triple Modular Redundancy (TMR):

In this case, the logic (memory cell) sensitive to single-event-upsets is tripled and voters are placed at the outputs to identify the correct value. TVF architecture ensures the safety of a design in the sense that If one system fails, there is another system to back it up. If the second system fails, there is a third system to back it up. The idea is that if system fails, the design would continue to work. Consider a circuit shown below:

In the above figure, the system is triplicated, which is formally termed as Triple Modular Redundancy. In order for the system to fail, atleast two Flip flops shown above should fail which very less likely to happen. The above system is designed in such a way that each one of the three Flip flops rarely fail, the probability of all three failing is calculated to be extraordinarily small. All the three flops are getting the same input and as the same clock is used for all the three flops, the output of all the flops should ideally be same.

The truth table for the above circuit is shown below:

In the truth table, ID1 and ID8 represents the case when all the three flops are working perfectly.

ID2, ID3, ID5 represents the cases in which one of the flops become faulty. Still, the correct intended output is propagated to Z. It means even if one of the three flops fail, the design will work as intended.

ID4, ID6 and ID7 represents the case when two out of three flops become faulty, in that case the faulty output is propagated to Z. But this case is very less likely to arise in the real scenario. In case of two failures, the result is wrong but still is not ambiguous

The purpose of TVF flops is to reduce the fault tolerance to a certain minimum level but not 0. The probability of occurrence of the third scenario is negligible.

One shortcoming of using TVFs, according to me is that It will increase the area overhead significantly.

Applications:

1. Used as a safety mechanisms in the manned aircrafts.
2. Electric and Hybrid Electric Vehicles
3. Military applications where robustness and safety of the design is priority.

Acknowledgements:

T Bharathwaj: https://www.dhirubhai.net/in/bharathwaj-t-a-3b1594a/

References:

https://hackaday.com/2019/01/25/circuit-vr-redundant-flip-flops-and-voting-logic/

https://www.eenewseurope.com/Learning-center/method-safety-compliant-triple-voting-flop-implementation

https://www.cadence.com/content/dam/cadence-www/global/en_US/documents/solutions/automotive-functional-safety-lbist-wp.pdf