Fault injection attacks against embedded security devices have attracted much attention in recent years. As a highly efficient injection solution, EM fault injection (EMFI) outperforms other injection means owing to its outstanding penetration capability in incurring local faults into security ICs. In this paper, we present an all digital countermeasure for detecting the on-the-fly EMFI attempts in silicon chips. The proposed logic consists of a watchdog ring-oscillator (RO), and a Hogge Phase Detector (PD) for sensing the frequency turbulence induced by the on-going EMFIs. Experimental validation on Xilinx FPGA Virtex-5 FPGA reports a fault detection rate of 93.15% and a failure rate of 0.0069, with negligible overhead. And the provided security margin for alerting the injection attempt is also noticeable. The technique is transplantable to any digital IC scenarios for its lightweight and all digital architecture, especially for the security-critical scenarios, such as the endpoints of Internet-of-Things (IoTs).