Posts Tagged ‘discover mcu ic software data’

PostHeaderIcon RC element with a characteristic time delay

Every transistor and its connection paths acts like an RC element with a characteristic time delay. The maximum usable clock frequency of a processor is determined by the maximum delay among its elements. Similarly, every flip-flop has a characteristic time window (of a few picoseconds) during which it samples its input voltage and changes its output accordingly. This window can be anywhere inside the specified setup cycle of the flip-flop, but is quite fixed for an individual device at a given voltage and temperature. So if we apply a clock glitch (a clock pulse much shorter than normal) or a power glitch (a rapid transient in supply voltage) this will affect only some transistors in the chip and cause one or more flip-flops to adopt the wrong state. By varying the parameters, the CPU can be made to execute a number of completely different wrong instructions, sometimes including instructions that are not even supported by the microcode. Although we do not know in advance which glitch will cause which wrong instruction in which chip, it can be fairly simple to conduct a systematic search.