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Electronic device fingerprints, unique bit vectors extracted from device's physical properties, are used to differentiate between instances of functionally identical devices. This article introduces a new technique that extracts fingerprints from unique properties of partially erased NOR flash memory cells in modern microcontrollers. NOR flash memories integrated in modern systems-on-a-chip typically hold firmware and read-only data, but they are increasingly in-system-programmable, allowing designers to erase and program them during normal operation. The proposed technique leverages partial erase operations of flash memory segments that bring them into the state that exposes physical properties of the flash memory cells through a digital interface. These properties reflect semiconductor process variations and defects that are unique to each microcontroller or a flash memory segment within a microcontroller. The article explores threshold voltage variation in NOR flash memory cells for generating fingerprints and describes an algorithm for extracting fingerprints. The experimental evaluation utilizing a family of commercial microcontrollers demonstrates that the proposed technique is cost-effective, robust, and resilient to changes in voltage and temperature as well as to aging effects.