We demonstrate a neuromorphic core with 64k-cell phase change memory (PCM) synaptic array (256 axons by 256 dendrites) with in-situ learning capability. 256 configurable on-chip neuron circuits perform leaky integrate and fire (LIF) and synaptic weight update based on spike-timing dependent plasticity (STDP). 2T-1R PCM unit cell design separates LIF and STDP learning paths, minimizing neuron circuit size. The circuit implementation of STDP learning algorithm along with 2T-1R structure enables both LIF and STDP learning to operate asynchronously and simultaneously within the array, avoiding additional complication and power consumption associated with timing schemes. We show hardware demonstration of in-situ learning with large representational capacity, enabled by large array size and analog synaptic weights of PCM cells.