As designers of digital signage, we want to use computers, but can’t. The reason is because computers (generally) rely on Random Access Memory. In RAM, individual bits of data can be accessed and changed. This makes RAM very fast, since the computer can directly access the data it wants to manipulate. Computers can tweak small parts of the dataset stored in RAM and not disturb the rest. But most RAM has a major limitation – it’s volatile, meaning the data lasts only as long as there is a power supply. Unplug the RAM and it’s gone. Not good for digital signage where power outage is a fact of life. So, we use flash memory. Flash memory is nonvolatile, so data can be stored even when the device is powered down. Some types of flash can read individual bits of data, but none of them can write data by the bit. In order to make a change, flash memory must rewrite the whole file. Flash also has issues with scalability – the ability to work properly at all sizes. The smaller flash memory chips get, the fewer electrons distinguish a charged memory cell from an unchanged one, which makes errors more likely. A few stray electrons could switch a bit from a 1 to a 0. An ideal mix of these two – nonvolatile memory that can be written in bits at small sizes – would be HUGE!
Introducing Phase-change memory (PCM)!
You are probably already aware that electronic memory stores data by collecting electrical charges – groups of electrons – in memory cells on a computer chip. One memory cell equals one bit of of memory, and the presence or absence of a charge determines if that bit is a 0 or a 1. Information is coded in this binary form – it’s the order of the 0s and 1s in each file that differentiates your MP3s from your photos, for example. PCM works a lot like RAM and flash – a collection of memory cells, each equal to one bit, are set to either 0 or 1. But PCM sets those bits in chalcogenides – materials (like tellurium) which can be either highly ordered crystalline solids or non-ordered amorphous solids. To write a bit, the amorphous solid is zapped with an electric current that heats the material to just below its melting point and held there long enough for the atoms to line up. This crystalline form can be switched back by heating it just long enough to melt it. When the current is cut off, the atoms “freeze” into an amorphous state. Because the amorphous state’s electrical resistance is high and the crystal state’s electrical resistance low, a computer can read the state of the glass as bits by measuring its ability to conduct electricity. PCM combines some of the best features of flash and RAM – nonvolatility and the ability to write to individual bits – and it doesn’t rely on collections of captured electrons, which means scalability isn’t a problem.
Several companies, including Samsung and Micron Technology, are already producing PCM chips, mostly for handheld devices. If PCM can supplant silicon as the semiconductor memory of choice, then we will be able to build cost-effective digital signage devices that have the power, speed and features of computer-based signage, but without the heat or power interruption concerns.
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