STM32 gotchas
167.'H7 longevity revisited

In g57, we've discussed the possible lifetime shortening of 'H7 devices operated at elevated temperatures. In 'H74x errata (ES0392 rev.9) there's an interesting erratum, "Possible drift of USB PHY pull-up resistor", which may shed some light at the mechanism which causes the 'H7 lifetime shortening:

When the VDDUSB33 equals 3.6 V and the USB Idle resistor (pull-up 1 connected between 3.6 V and ground) remains activated for a long period of time, the pull-up resistor value might drift, reaching the maximum value defined in USB Specification.

Let's put forward, that this erratum is pertinent only to the older, rev.Y version of the chip.

Also, the real-world effect is less dramatic than it may sound at first reading - the erratum follows with:

The degradation can be observed after USB PHY pull-up continuous activation in the following conditions:

The proportion of applications where USB is in Transmit (let alone USB-Reset) state any significant portion of its lifetime is probably miniscule. However, why these strange conditions?

The original USB specification requires a relatively precise, 1.5kΩ±5% resistor as pullup on D+ indicating Full-speed device to the upstream hub. This kind of precision, while trivial with todays discrete resistor, is hard to achieve in integrated circuits. So, an Engineering Change was added to the USB standard, allowing to use two less-precise resistors of different values together with switches, which select the appropriate resistor in various modes of the USB transceiver (this EC is part of the .zip file which forms the current USB2.0 standard, and contains a detailed rationale for this solution). And, so it happens, that the resistor which is used during Transmit, is of the higher value, 1475Ω..3090Ω. To reduce silicon area, instead of using a long resistor, a shorter and thinner resistor has been probably designed.

The nature of this erratum again appears to point to electromigration, effects of which are more pronounced on extremely narrow structures (here: the USB pullup resistor) allowed by the 45nm technology used to build the 'H7. This structure was probably modified to be somewhat wider in the subsequent revisions of the chip, to remove the limitation in question.

This erratum highlights the challenges associated with shrinking features on integrated circuits. While the pressure to produce microcontrollers cheaper but with more features, can still be felt, it is time to realize that we are probably hitting the physical, economical and rational limits of this process. Further development in microcontrollers lies probably in making thing smarter, rather than just making them more complex and adding more memory.

In memory of Gordon Moore - IC pioneer, co-founder of Intel, and author of Moore's Law.