For correct operation of the SoftDevice, it is a requirement that the 16 MHz crystal oscillator (16 MHz XOSC) startup time is less than 1.5 ms.
The external clock crystal and other related components must be chosen accordingly. Data for the crystal oscillator input can be found in the product specification (nRF51822 Product Specification) for the SoC.
When the SoftDevice is enabled the SEVONPEND flag in the SCR register of the CPU shall only be changed from main or low interrupt level (priority not higher than 2), otherwise the behavior of the SoftDevice is undefined and the SoftDevice might malfunction.