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It's a pretty interesting question, even if a bit tangential to the article! Their main advantage is the comparatively low transistor count. The chips can be m
by PumpkinSpice 3y ago
It's a pretty interesting question, even if a bit tangential to the article!
Their main advantage is the comparatively low transistor count. The chips can be made cheaply and don't need cutting-edge fabs. This also makes them exceptionally power-efficient and tolerant of a wide range of supply voltages, so they're easier to integrate into many designs.
Just as important, they are far easier to master. Single-cycle instruction execution, single chip-wide clock, simple interfaces to all peripherals. If you don't have an OS to abstract it all away, most of the contemporary 32-bit platforms are a serious pain. Adjusting clock speed on a typical Cortex-M chip requires following a carefully-choreographed sequence of steps, possibly fiddling with flash wait states, various bus clocks, PLLs, and so on. On an 8-bit MCU, you just flip a bit or two.
It's a weird misconception that "8-bit" means antiquated or slow. Modern 8-bit MCUs run at high speeds, have plenty of fast memory, and integrate all kinds of cool on-die peripherals - DACs, ADCs, op-amps, DMA controllers, USB controllers, and so on. And in a good majority of embedded applications, they are enough.
Of course, there's nothing sacred about 8-bit ALUs, so I wouldn't be surprised if 32-bit ALUs eventually take hold in this market segment. But the replacements would need to preserve that "I can learn this in a weekend" property that isn't preserved in most of the high-performance 32-bit chips available today.
- sweetjuly 3y ago> This also makes them exceptionally power-efficient ... > Single-cycle instruction execution For what it's worth, single cycle execution is really a detriment to power efficiency. There is an optimal logic level per pipeline stage for CMOS processes which will minimize power (and it's usually quite small!). Extremely deep pipelining lets you hit a higher Fmax, but if you don't crank up the frequency you will often save quite a bit of power.
- KingLancelot 3y agoI appreciate your thorough answer, USB is pretty surprising, but just 256 byte address space, and that needs to take into account NULL (I assume?), and various hardware devices represented in the address space, I just don’t get how it’s possible. Honestly even 16 bit is tiny.
- jhgb 3y ago8-bit CPUs typically have a 16-bit address space though.
- j16sdiz 3y ago8bit MCU does not meant it only have 8bit address space. STM8S007xx operates on 8bit words, but have 8/16/24bit address space
- PumpkinSpice 3y agoAs others are saying, "8-bit" doesn't mean 8-bit address space. It refers to the ALU "word" size instead - i.e., how many bits you can add, subtract, multiply, or divide in a single step. (Larger types are possible, you just need to do the operations in steps, just like 64-bit math on 32-bit platforms.) Even in the 1980s, most 8-bit microcomputers had at least 64 kB memory. Today, many AVR chips will have 145 kB of memory (flash + SRAM + EEPROM), and pretty much arbitrarily more can be installed externally.
- RetroTechie 3y agoAnd many 8-bit architectures allow some registers to be used in pairs. Or have some >=16-bit registers for specific operations (eg. indirect memory addressing).
- nxobject 3y agoI'm curious, and knowing nothing about the determinants of power draw and node process: wouldn't a "nearer-to-cutting-edge" fab process, though, still end up producing processors that draw even less energy? Or has that not been a goal of recent semiconductor progress?
- RetroTechie 3y agoThe silicon that drives IO pins & oscillator(s) requires some physical size (for one, due to ESD protection reasons), and requires X amount of power. A complex, modern cpu can consume many times that, so cutting edge process is useful to minimize overall power use. With small/simple parts like a uC, not so much. So the cost/part, other considerations, and whatever vendor or silicon process provides that, is what matters. Here, power savings come from sleep modes, disabled peripherals & (maybe) reduced or even stopped clocks. Not so much reduced feature sizes in the IC.
- contingencies 3y agoAlso the supply chain doesn't vanish for multiple years every time a single vendor fab burns down.