what is 1st RTC and 2nd RTC
Answers
Answer:
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Explanation:
The following system is well-known to embedded systems programmers, who sometimes must construct RTCs in systems that lack them. Most computers have one or more hardware timers from quartz crystals or ceramic resonators with inaccurate absolute timing (more than 100 parts per million) that is very repeatable (often less than 1ppm). Software can do the math to make these into RTCs. The hardware timer can produce a periodic interrupt, e.g. 50Hz, to mimic a historic RTC (see below). However, it uses math to adjust it for accuracy:
time = time + rate.
When the "time" variable exceeds a constant, usually a power of two, the nominal, calculated clock time (say, for 1/50 of a second) is subtracted from "time", and the clock's timing-chain software is invoked to count fractions of seconds, seconds, etc. With 32-bit variables for time and rate, the mathematical resolution of "rate" can exceed one part per billion. The clock remains accurate because it will occasionally skip a fraction of a second, or increment by two fractions. The tiny skip is imperceptible for almost all real uses of an RTC.
The complexity with this system is determining the instantaneous corrected value for the variable "rate". The simplest system tracks RTC seconds and reference seconds between two settings of the clock, and divides reference seconds by RTC seconds to find "rate". Internet time is often accurate to less than 20 milliseconds, so 8000 or more seconds (2.2 or more hours) of separation between settings can usually divide the forty milliseconds (or less) of error to less than 5 parts per million to get chronometer-like accuracy. The main complexity with this system is converting dates and times to counts of seconds, but methods are well known.[17]
If the RTC runs when a unit is off, usually the RTC will run at two rates, one when the unit is on and another when off. This is because the temperature and power-supply voltage in each state is consistent. To adjust for these states, the software calculates two rates. First, software records the RTC time, reference time, on seconds and off seconds for the two intervals between the last three times that the clock is set. Using this, it can measure the accuracy of the two intervals, with each interval having a different distribution of on and off seconds. The rate math solves two linear equations to calculate two rates, one for on and the other for off.
Another approach measures the temperature of the crystal with an electronic thermometer, (e.g. a thermistor and ADC) and uses a polynomial to calculate "rate" about once per minute. The most common quartz crystals in a system are SC-cut crystals, and their rates over temperature can be characterized with a 3rd-degree polynomial. So, for these, the rate is measured at four temperatures. The common tuning-fork-style crystals used in watches and many RTC components have parabolic (2nd-degree) equations of temperature, and can be characterized with only three measurements. Then a linear regression can find the equation of temperature. Something like this approach might be used in commercial RTC ICs, but the actual methods of efficient high-speed manufacturing are proprietary
Answer:
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