The following text is focused on describing in detail the prepayment solution based on the Near Field Communication technology (NFC). NFC is a special, low range (a few centimeters), wireless technology that enables devices such as smartphones to securely connect with other NFC-enabled devices, such as prepaid meters. This technology has already been proven in the banking industry, combining it with a secure microcontroller. A typical prepaid meter reference design, based on this technology, has been jointly introduced by two companies, Freescale Semiconductor (U.S.) and Inside Secure (France). This reference design, which has been developed thanks to the cooperation between both companies, provides the NFC-based prepaid meter with the ability to securely reload energy balances. This reference design uses three key parts: the Freescale MK30X microcontroller (MCU), a part of the recently introduced Kinetis series, and the secure element, the ATVaultIC460, together with the NFC (Microread 3.4) chipset, both from the Freescale partner company, Inside Secure.
The meter is driven by the Freescale MK30X256 32-bit MCU, which is the heart of the metering engine. This MCU is built on the popular ARM?Cortex-M4 core. The main function of this MCU, coupled with an integrated Analog Front-End (AFE), is to periodically read data from external voltage and current sensors and compute other values consecutively, mainly the powers and accumulated energies. As the main computing technique is based on the Fast Fourier Transform (FFT), the meter can measure energy precisely in all four quadrants (import/export energy, active/reactive energy). Thanks to this, the meter can perform a complete frequency analysis of the mains. The next MCU function is to communicate with on-board AMR communication interfaces, such as an optically-isolated RS232 interface, energy LEDs pulse output interface, an infrared IEC1107 interface, and an I2C/SPI interface for communication with RF/ZigBee daughter cards. The MCU also cooperates with on-board human-machine interfaces (HMI), such as an LCD, which is used for showing the demand values, and with a built-in push-button used for menu item selection.
The next important part of this reference design is a stand-alone RF daughter card, which incorporates two main functions: security and NFC communication. The secure element is the basis for implementing end-to-end security between the prepaid meter and the utility and its distributors who sell energy credits. The referred to RF daughter card features Inside Secure’s ATVaultIC460 and an NFC (Microread 3.4) chipset. The ATVaultIC460 security module provides comprehensive security functions, such as mutual authentication, verification and generation of certificates, encryption/decryption and managing the secure storage of cryptographic keys. Additionally, the controller is EAL4+ ready and can also support FIPS 140-2 Level 3 and other certification and standards. Combining all these cryptographic services and an 8/16-bit RISC processor, the ATVaultIC460 delivers a secure control solution not only for metering applications. The NFC connectivity integrated in this design allows users to upload energy credits using contactless smart card technology or through an NFC phone. NFC connectivity on this daughter card is implemented using the Microread 3.4 controller and the Open NFC protocol stack. Both the RF/NFC daughter card with security functions and the MK30 MCU are connected through an I2C interface, with communication based on the standard Host Communication Interface (HCI) protocol.