Freescale's tamper-resistant MCU powerful smart meter solution

Stealing energy and tampering with electricity meters is a worldwide problem that can lead to lost revenue. Consumers find that they can control their own meters, make them pause or not be recorded, change or stop the real-time timers in the meter, and even skip the meter to use the power directly without paying. Freescale's latest MCF51EM256 microcontroller in the meter market integrates several tamper-proof features into one chip, greatly reducing the cost of billing (BOM) for customers and effectively preventing tampering.

Introduction - Tamper-resistant feature requirements

With the increase in electricity costs, theft has become a major concern for government agencies (public utilities). This is a global problem, especially in populous countries such as India and China. In utility measurement applications, criminals will try to steal information or change internal settings. A common practice among these methods is to fine tune the time to trick the system.

Distribution companies may implement peak-to-peak tariffs based on factors such as time of day, maximum demand, and load. This requires a real-time timer (RTC) to provide an accurate time reference. Some criminals will tamper with the timer or manipulate the time spoofing system in order to change the billing result, such as changing the PM to AM, so that the meter firmware will charge because the off-peak load price is used during the changed time period. Less cost. The real-time timer (RTC) usually relies on an external crystal oscillator of 32.768 kHz. The illegal molecules change the RTC crystal to slow down the oscillations and achieve less counting, which will make the measurement and billing different from the actual power consumption. .

Most of these lost revenues can be solved by installing electronic meters because they can detect tampering to ensure proper billing, which is different from electromechanical meters. In addition, these meters can be connected using advanced metrology technologies such as AMR or smart grid, and utility companies can benefit from automatic learning of any remote tampering.

About MCF51EM256

The MCF51EM256 is based on a ColdFire V1-based microcontroller with 256K Flash and four high-precision 16-bit SAR ADCs that measure both current and voltage, as well as neutral current, in all phases. The ADC is automatically activated by an internally programmable delay module to make accurate, in-phase readings of current and voltage. The microcontroller includes an accurate 1.2V output voltage reference that can be used by external ADCs and external components such as gain amplifiers for accurate measurements. The microcontroller includes a separate RTC (iRTC) that, like the clock, has its own power domain and is therefore separate from other system components. Most tamper-proof features are implemented in iRTC. The MCF51EM256 also includes an LCD driver module and a range of communication peripherals such as IIC, SCI, SPI, programmable comparators, and AMR SPI/SCI modules.

Tamper-proof characteristics of MCF51EM256

RTC with independent power domain

When the main power supply is available, the iRTC runs on the main power supply (VDD) at full time, and when the main power supply fails, it automatically switches to battery power (VBAT). This behavioral process is shown in Figure 1.

Smart meter

Figure 1 - Independent RTC

The iRTC can detect a drop in the main supply voltage (VDD) at a certain threshold. If it is below this threshold, it will automatically switch to battery power. Below this threshold, only the RTC and oscillator will remain active while the other microcontrollers will shut down.

This allows all relevant logic to work with tamper detection (discussed later) in the event of a power outage unless the battery is removed or the power is exhausted.

Protection under battery removal or power failure

The general method of tampering with the system is to remove the battery when the mains power is not available. This allows the hacker to have time to manipulate the system, and then replace the battery as if nothing had happened. The independence of the iRTC includes its reset, clock and power. When the independent power supply is removed when the device is powered off, if the battery is connected again, the iRTC will be reset and then falsified by default.

It is worth noting that iRTC has a separate "power-on reset (POR)" relative to SoC POR. The iRTC POR only works when the main power and battery power are removed and one of them is reconnected. The iRTC has the ability to detect the removal of the battery and internally generate a tamper-interfering CPU. This tampering can be ignored during the initial metrology phase because the system is in analysis mode.

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