The Tech Blog

I recently read about the STM32V8 launch and it has pretty great specs. When you see that it has a 800 MHz on a Cortex-M with a 5,072 CoreMark, you know you are getting close to compute you normally see on embedded microprocessors(MPU) with an external DDR memory. ST positions STM32V8 as its flagship high-performance STM32 MCU.

The chip is built on 18nm FD-SOI plus embedded phase-change memory. FD-SOI is a chip process that reduces leakage current and helps push frequency without wasting as much power. It also integrates up to 4 MB of non-volatile memory and is specified up to an extreme 140°C junction temperature. The key part is the PCM.

Embedded phase-change memory (PCM) is a way to store bits without the floating gate used in Flash. The bit lives as resistance in a special material. When the material is in one phase it reads high resistance, and in the other phase it reads low resistance. Because the phase stays put when power is removed, it is non-volatile memory. You write to it by sending a short, shaped current pulse that heats the cell into the wanted phase.

This matters because embedded Flash does not scale nicely into newer chip process nodes. Resistive memories like PCM is the route most vendors take from around 28nm and below, mainly to avoid extra process complexity and cost. RRAM was another, I wrote about that one earlier in a post.

For firmware, PCM is not an infinite-write storage. The chip separates code and data profiles. Code can be around 1,000 write cycles with 10-year retention. Data can be up to 1,00,000 cycles with about 1-year retention at 140°C after cycling. This is actually great as realistically you wont have firmware changes that frequent in production. The practical benefit is you can keep more code, tables, and even small ML model weights on chip, and then buffer hot data in SRAM, the fast RAM that forgets its contents when power is off.

The new V8 chip seems to be in sampling mode currently by ST with elite customers like SpaceX because of the temp profile and radiation immunity claims.

Last week in the lab I was working on a PCB mounted on a metal surface powered by an AC to DC adapter. When I touched the painted metal surface, I felt faint buzzing/roughness on my fingers. With the power off, the same surface felt perfectly smooth. Initially, I had no idea what that feeling was. It made me go down a rabbit hole to find out what the issue was. Let’s discuss that today.

The feeling was not what you would attribute to while getting a shock. It was the same feeling when you run your finger over a rough surface. My first thought was that there was something wrong with the AC supply. I switched my switchboard to a new one at a different location in the lab, and the problem was gone. So the conclusion was it was something related to earthing. I took out a multimeter and measured the mains at the plug. Line to neutral was about 240V, which is normal. Line to earth was about 187V and neutral to earth about 53V. That told me the earth at this outlet was floating between line and neutral instead of being solidly tied to real ground.

Next, I measured between the metal body and a good earth point. Through the paint coating, I saw only about 15-20V AC, meaning a small AC voltage on the metal from the supply and wiring. That leakage current would be zero with a proper earth and you would never notice it. On this bad outlet, my body was the earth, closing the circuit through my fingers and feet and creating that electro-vibration feeling. Technically a mild shock.

In haptics, this effect is called electro-vibration or electro-adhesion. Its when a conductive surface at AC voltage with a thin insulator like paint turns your finger and the metal into a tiny capacitor. The changing electric field slightly changes friction between skin and surface. This creates the microtexture feeling. Googled it and found that this tech is used for creating cool new touch interfaces.

Although I feel like I’m glorifying a shock in this post, if you ever feel something like this, don’t experiment further. Turn OFF the AC and get your hand off it. Please keep this in mind and don’t be stupid like me.