为何要选择Cortex-M4内核

　　supports all single-precision data-processing instructions and data types described in the ARM Architecture Reference Manual. • Satisfying the Requirements of Next-Generation Products The ARM Cortex-M family is aimed at the areas such as commercial electronics and low-cost industrial control including motor control， power management， automotive electronics， and audio processing. The increasing computational loads in these areas consume an unacceptable portion of the CPU resources if all the digital signal processing tasks are handled by software. The ARM Cortex-M4 solves this issue by integrating a single-cycle multiply-accumulate （MAC） unit and an instruction set of single-cycle SIMD operations， as well as an optional FPU to satisfy the digital signal processing requirements of next-generation products. A Cortex-M4 can be regarded as a Cortex-M3 with integrated DSP extensions， which means the software from the Cortex-M3 can also function in the M4 and it is easy to implement migration from M3 to M4 without too much effort. The figures shown below illustrate the relation between these two processors