One way this can be done is with a silicon real-time-clock, or RTC. Although the intended use for such products is to measure time of day, they can be used as elapsed time counters with alarming capability. The accuracy of such products is very good, on the order of 50 ppm as dictated by the 32.768 kHz crystal. Many products of this type feature low-power non-volatization of the timekeeping data with the use of an external battery or supercapacitor. While this may be acceptable in some equipment, the requirement of a battery and a crystal may not be acceptable in equipment in high shock, vibration, dust, humidity, etc. environments. Also, care must be taken in the PCB layout of a crystal-based RTC, resulting in startup failures, oscillator lockup, or timekeeping inaccuracy. This function can also be realized with a low-cost microcontroller. If the timebase is a crystal, the timekeeping accuracy will also be very good. However, as in the case with the RTC, the presence of the crystal is often undesirable in harsh environments. As a programmable device, it has the benefit of flexibility to the designer. Many equipment types, particularly military, find it undesirable to have to track microcontroller code revisions. If the microcontroller features flash memory, that solves the non-volatility requirement, but introduces a wearout mechanism. Otherwise, a battery would be required.