Physically Unclonable Functions (PUFs) are potential security blocks to generate unique and more secure keys in low-cost cryptographic applications. Memories have been popular candidates for PUFs because of their prevalence in the modern electronic systems. However, the existing techniques of generating device signatures from DRAM is very slow, destructive (destroy the current data), and disruptive to system operation. In this paper, we propose latency-based (precharge) PUF which exploits DRAM precharge latency to generate signatures. Our proposed methodology for key generation is fast, robust, least disruptive, and non-destructive. The silicon results from DDR3 chips show that the proposed key generation technique is at least ~4,300X faster than the existing approaches, while reliably reproducing the key in extreme operating conditions.