Real-time clock
Real-time Clock Terminology and Purpose:
The term ‘real-time clock’ distinguishes it from ordinary hardware clocks, focusing on maintaining time in human units rather than the time of day. Real-time clocks (RTCs) are crucial for accurate timekeeping through system states, allowing the main system to prioritize time-critical tasks. They often operate with low power consumption to run from alternative power sources, ensuring continuous timekeeping even during power outages. Some RTCs can be more accurate than other methods and may incorporate a GPS receiver for faster startup time.
RTC Power Source and Timing:
RTC devices typically feature an alternate power source such as lithium batteries or supercapacitors to sustain timekeeping during power outages. These power sources can also supply energy to battery-backed RAM for data retention. Most RTCs utilize a crystal oscillator with a frequency of 32.768kHz, which is convenient for binary counter circuits. For enhanced accuracy, temperature-compensated RTC ICs are available, while chip-scale atomic clocks offer high precision at the expense of increased power consumption.
Examples and Historical Context of RTCs:
Various manufacturers produce RTCs, with notable examples including Dallas Semiconductor DS1387 and Maxim Integrated DS1307. RTCs were introduced to PC compatibles by the IBM PC/AT in 1984 and have since become integral components in modern systems. These devices have historical significance, having been used in computer designs such as smaller IBM System/360s, PDP-8s, and Novas. Some regions align AC mains frequency adjustments with national standards for precise timekeeping, a practice less relevant for portable computers.