Freescale Semiconductor, Inc. Data Sheet: Technica.
NXP
MK21DX128AVMC5 - 50MHz ARM Cortex-M4-based Microcontroller
Freescale Semiconductor, Inc. Data Sheet: Technical Data Document number: K21P121M50SF4V2 Rev 6, 04/2014 Kinetis K21D Sub-Family Data Sheet 50 MHz A.