Refers to the width of the data processed by the CPU, the data length of the registers participating in the operation.
If the bus width is the same as the data width processed by the CPU at a time, this width is the number of bits of the microcontroller.
If the bus width is different from the data width processed by the CPU at one time:
1) If the bus width is smaller than the data width processed by the CPU at a time, the number of bits of the microcontroller is defined by the data width of the CPU, but it is called the number of bits. For example, the famous Intel 8088, the CPU is 16 bits but the bus is 8 bits, so it is a quasi 16 bit.
2) If the bus width is smaller than the data width processed by the CPU at a time, the number of bits of the microcontroller is defined by the data width of the CPU.
Less bit width is not the bus width, nor the width of the memory. The address bus like the 51 MCU is 16 bits, but it is an 8-bit machine. Memory like ARM also has eight bits, but it is a 32-bit machine. It refers to the width of the data processed by the CPU, that is, the throughput of the CPU once. For example, the same instruction: MOV R0 R2 
In the 51 MCU, R0 and R2 are both 8-bit, so the CPU of 51 can only process 8 bits of data at a time.
In ARM, R0 and R2 are 32-bit, so the ARM CPU can process 32-bit data at a time. This is the difference.
There is a complicated, one sentence: the data length of the registers participating in the operation.
The 8-bit MCU has a data bus width of 8 bits and usually can only process 8 bits of data directly;
The 16-bit microcontroller has a data bus width of 16 bits and can usually process 8-bit or 16-bit data directly.
The most essential difference is that the internal CPU has different word lengths, that is, the maximum number of bits processed by the CPU is different. There are 8 and 16-bit CPU differences, which you can think of as the ALU, the word length of the register, and so on. Some 32-bit DSP chips have 16-bit external interface data bus. According to this, it is not acceptable.
There is a difference in speed, depending on the word length of the CPU and the register. The 8-bit MCU cannot directly process 16-bit data, and it must be processed according to 8-bit data. It must be completed in several processes. The 16-bit MCU can directly process 16 data, because its ALU, registers, etc. are all 16 bits, which can complete the operation of 8-bit MCU in multiple steps at a time, especially for data processing, 16-bit MCU has its advantages. . Moreover, most of the 16-bit MCU data interfaces are 16 bits.
To put it simply, the 16-bit is faster than the 8-bit, and the 8-bit microcontroller has a long development time and the price is low. The most important thing is to be competent in development tasks in many applications. The 16-bit MCU is much more powerful than the 8-bit, and is only used when needed.
You know the binary, do you know that the microcontroller is all binary calculations when doing calculations. Therefore, the most fundamental difference between an 8-bit MCU and a 16-bit MCU is that an 8-bit MCU can simultaneously add two 8-bit binary numbers, while a 16-bit MCU is twice as large, and can simultaneously make two 16-bit 2s. Add the hexadecimal numbers.
The CPU can process the number of bits in a binary number at the same time, which is called the number of computers. There are also cases where the number of bits of the data bus is half of the number of CPU bits, which is called the number of computers. For example, the CPU is 16 bits, the data bus is 16 bits, it is a 16-bit computer; the CPU is 16 bits, and the data bus is also 8 bits, which is a quasi 16-bit computer.
It turns out that it is ok to replace the 8-bit MCU with an 8-bit MCU, but the circuit and the program have to be changed accordingly, and after changing to an 8-bit machine, it is necessary to meet the needs of your work in terms of function and speed.
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