Clock and Data Recovery Just three models are sufficient for all CDRs.
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The clock, still in use today, was completed in 1583 (and complies with the calendar reform by Pope Gregory XIII of the year before!). It is the masterpiece of Pietro Fanzago, a Clusone citizen (the town written history dates back to 774) and replaced a previous one (of which very little is known) that was in dire need of maintenance and upgrade. Besides the time of the day (shown on a 24 circular dial where the hand advances counteclockwise!), it indicates the month of the year, the Zodiac signs and the duration of the night, the day of the moon' month plus the phase of the moon and finally the trine, square, sextile and opposition for astrological purposes. The passing of each hour is marked by a bell, in cycles of one to twelve strikes, followed by a repetition after about 80 seconds. The “mostra”, i.e. the dials, appears on the outer south wall of the city hall that faces the town main square. The turret clock, entirely made of wrought iron, is located inside the tower of the city hall and drives the “mostra” via coaxial iron axes. The mechanical power is still today provided by the slow descent of heavy stones, that need recharging within 30 hours. Most of it is perfectly original, but the oscillating element which was a foliot, replaced during the 18th century by a pendulum, and the escapement (the original was almost certainly of the vertical verge type) that is today a pin pallet escapement.
The CDR function is a simple one and just three architectures are sufficient to model it. The actual implementation of the CDR may differ from the neat, simple analog structure of the mathematical models. Complex digital blocks, DLLs, DSPs may disguise the architecture, but the fundamental operation of the CDR will not differ. Knowledge of the three models is fundamental to imagine, specify, design, check, measure and interpret the behavior of the CDR. Reference to them is the best way to avoid confusion and errors.