Are more bits better in an MCU? That’s similar to asking: are more cylinders better in an internal combustion engine (ICE)? Peripherals and other features matter, not just bits (or cylinders). For example, in the 1980s, Volvo offered a station wagon with a turbo-V4 engine that was faster off the line than Chevrolet Corvette with a很多larger V8. Of course, the station wagon was toast once the V8 was wound up after a hundred yards. But the first hundred yards made a great visual for the Volvo commercials. In the case of MCUs, total power consumption, availability of peripherals, latency demands, and other factors are important, not just the number of bits.
MCUs are everywhere. They are found in cell phones, toys, vehicles, white goods, renewable energy systems, Internet of things (IoT) devices, and on and on. MCUs are essential for controlling hardware functions from motors to flashing LEDs. The basic architecture of MCUs includes a central processing unit, some memory, and peripheral functions(Figure 1)。The MCU gets inputs from sensors, switches, and other devices and controls the function of peripherals such as drivers for displays, actuators, and motors, as prescribed with preprogrammed instructions. Depending on the complexity of the functions being controlled and the amount of processing needed by the various inputs, MCUs are available with 8-bits, 16-bits, and 32-bits. In general, the more bits, the more processing potential.
“位”是“二进制数字”。在8位MCU中，数据存储在8位，称为字节（或在某些情况下是八位字节）。8位内存寄存器可以存储可能的2个8值。根据所使用的整数表示，实际值的范围有所不同。例如，使用未签名的二进制数，范围为0到255（28负1），使用两个的补体表示，范围为-128（-1 x 2）7）至127（27——1)。在一个8位单片机8位数据总线wide, the address bus widths can vary. Address bus widths of 12- to 16-bits are common. A 16-bit wide address bus, for example, results in a directly addressable memory space of 65,536 (216）字节（称为64KB）。但是这些分类只会开始暗示特定的8位MCUS的实际功能(Figure 2)。
More power with 32 bits
While 8-bit processors are versatile, they can be equally limited in performance. When 8-bits is insufficient, designers can turn to 32-bit MCUs that can pack quite a performance punch. 32-bit MCUs are used across a wide range of applications, and they can support high-end algebraic operations and floating-point mathematics. And while 8-bit MCUs are typically limited to several MHz of processing speed, 32-bit units are available with clock rates into the GHz range, although several hundred MHz units are more common.
The higher clock speeds generally associated with 32-bit MCUs can also lead to increased energy consumption. In general, 32-bit MCUs can support more intense computation by trading off higher energy consumption. But the choice between 8-bits and 32-bits is not quite that simple. For example, a 32-bit MCU can complete a series of computations faster than an 8-bit MCU and then enter sleep mode for a longer time. So, is the 32-bit MCU more energy efficient? That’s possible, but not necessarily true. An 8-bit MCU can have lower overall energy consumption and contribute to longer battery life in portable devices. The choice is not simple, and the optimal energy consumption solution must be determined on a case-by-case basis.
That’s where 16-bit MCUs can come in; they can combine the simplicity of 8-bit units while providing enhanced performance without an excessive power penalty. Especially in applications that don’t need massive multi-threading and moderate memory needs, 16-bit MCUs can provide an optimal middle ground solution(Figure 3)。
外围设备的关键。有16位单片机math co-processors that support intensive mathematical calculations, similar to some 32-bit units at the same clock speed. And, there are numerous communications stacks that support 16-bit MCUs. For example, with the correct implementation, Ethernet, controller area network (CAN), universal serial bus (USB), and Zigbee are among the communications protocols that can run efficiently on 16-bit MCUs. And the use of 16-bit MCUs can provide for simpler circuit board implementations compared with many 32-bit solutions.
The availability of peripherals and where they reside can be important factors driving the choice of 8-bits, 16-bits, or 32-bits. Adding an external communications peripheral to an 8-bit MCU can be an option. But, the addition of a peripheral chip can often eliminate the cost-benefit of using an 8-bit MCU. It can be less expensive to use a 32-bit unit with the communications functions already integrated. Moving up the scale finds 32-bit MCUs often include more features and can handle multiple peripheral functions more efficiently than 8-bit and 16-bit devices.
8-bit MCUs are available that can support some cryptographic functions for increased security. But those functions are often implemented in hardware that increases costs or software that places an increased burden on a relatively low-power processor. If higher-level security functions are important, it may be wise to use a 16-bit or 32-bit MCU.
总结一切 - 哪个MCU最好？
So, are more bits better? No, not necessarily. It’s a complex decision process(Figure 4)。Successful designs identify the MCU architecture that delivers the needed performance in the most cost-effective manner. And for battery-powered devices, in the most energy-efficient manner.
8-bit microcontroller summary，，，，Microchip
8-bit vs. 32-bit MCU: Choosing the Right Microcontroller for Your PCB Design，，，，Altium
IoT: choosing 8-bit vs. 32-bit MCUs, Lynnette Reese
MCU Basic Structure/Operation, Renesas
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