Securing nonvolatile, nonresettable counters in embedded designs
It is sometimes desirable for warranty reasons to count and record certain events such as power cycles, operating time, hard (pushbutton) resets, and timeouts – and do so securely.
The traditional electronic counters for this purpose are built from flip-flops, using a binary code such as the one shown in Figure 1 below. The maximum count is reached when all flip flops are set, so the size of the counter is determined by the maximum number of events to be counted during the interval of interest.
Figure 1: In natural binary code, the place value doubles from one bit to the next.
Addressing the requirements
Although a counter based on flip-flops is easily built, it has major drawbacks. It rolls over to zero (resets itself) when it reaches the counting limit. Its operation is also volatile—it requires power to maintain the state of its count.
The first issue can be addressed by freezing the counter when it reaches its limit; you can resolve the second issue by attaching a battery to maintain power on the counter. But these measures may not be acceptable, because they add cost and work only for a limited time.
There is an alternative. You can back up the count value in EEPROM or some other form of nonvolatile (NV) storage. On the next power-up, the counter can be preloaded with the value stored in this NV memory.
Unless the NV memory is embedded in another chip like a microcontroller or FPGA, however, the memory contents are not secure because the memory chip can easily be removed, reprogrammed (reset), and replaced on the circuit board. This method, therefore, does not meet the requirement of being nonresettable.
A trick from the past
EPROM is another type of nonvolatile memory that requires no battery. EPROM became popular in the 1970s with the advent of the microprocessor. Fresh from the factory, all EPROM bytes read FFh.
Data is stored by programming individual bits from 1 (erased) to 0 (programmed); the programming requires pulses of 12V to 13V. Before loading new data, you must erase the entire memory by exposing the chip to high-intensity ultraviolet light through a window built into the package for that purpose.
One-time-programmable (OTP) EPROMs do not have a window and, therefore, cannot be erased. Because of these inconveniencies, the popularity of EPROMs has dwindled in favor of EEPROMs and high-density FLASH memories, both of which can be operated and programmed at 5V or less.
Nonetheless, by combining the one-way programming (1 to 0) and lack of erasability of an OTP EPROM with modern EEPROM technology, you can attain a new feature called EPROM-emulation mode. EPROM-emulation mode is a key technique for realizing nonvolatile, nonresettable counters.
Loading comments... Write a comment