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ARM Cortex M0+

Cortex M family processor specification by ARM.

User Guide: https://developer.arm.com/documentation/dui0662/b/

Tools: https://en.wikipedia.org/wiki/List_of_ARM_Cortex-M_development_tools

Common Microcontroller Software Interface Standard (CMSIS)

The ARM Cortex MCU abstraction layer permitting programmers to build portable software.

CMSIS-Core for Cortex-M0+: /wiki/arm-cortex-m0-plus/DUI0662B_cortex_m0p_r0p1_dgug.pdf