Circuit Cellar - GIFT

CC-2015-06-Issue 299

Circuit Cellar - GIFT

Issue link: http://read.uberflip.com/i/206133

Contents of this Issue

Navigation

Page 39 of 83

CIRCUIT CELLAR • JUNE 2015 #299 38 FEATURES F rom a technical standpoint, radio- controlled clocks with digital displays are not very interesting. When powered-up they turn on their radio, listen for a broadcast that tells them the correct time, and instantly set their digital display to agree. Except for waiting to get broadcast data, it's all over in the blink of an eye. On the other hand, radio-controlled analog clocks are fascinating because of the mechanical nature of their display. Their hands are driven by motors that move relatively slowly in only one direction (you guessed it, clockwise), and nothing happens instantly. This article was written for those who are curious about how RC clocks work. We'll examine a few different types of clock movements to see how they set themselves initially to the correct time, how they maintain near-perfect accuracy, and how they automatically adjust for daylight savings time. We treat only lightly on details of the radio broadcast that an RC clock relies on. That subject is extremely well documented by the National Institute of Standards and Technology. You'll find many interesting references by searching nist.time.gov. DATE/TIME INFORMATION A message containing exact date, time, and other relevant information is broadcast every minute at 60 kHz from the National Institute of Standards and Technology (NIST) radio station WWVB near Boulder, CO. The message consists of 60 binary data pulses at 1-s intervals (see Figure 1). Note that 0.5-s pulses (colored red) are "1" data bits, 0.2-s pulses (green) are "0" data bits, and 0.8-s pulses (blue) are "position markers" to help synchronize decoding. When the clock radio is turned on, it looks for an adjacent pair of position markers (P6 and P0) indicating start of a frame, and then begins counting seconds and decoding pulses. Data is checked for accuracy by comparing successive messages. Clocks everywhere in the United States receive the same broadcast, with time given for the Greenwich meridian. So the clock usually has some sort of switch for selecting how many hours the processor must subtract to adjust for a local time zone. Only the most important pulses are labeled in Figure 1. This example was broadcast during the 42 nd minute of the 18 th hour of the 258 th day of the year in 2012. Some digital clocks convert day-of-year data, using the leap year flag (pulse 55), to month and day of month. Simple analog clocks learn everything they need from the first 20 pulses of a frame, plus a peek at daylight savings time status (pulses 57 and 58). Those two data pulses tell the processor when to move the clock ahead or back one hour. "00" means standard time is in effect. "01" means DST starts today. "11" means DST is in effect. "10" means DST ends Radio-Controlled Clocks Curious about how RC clocks work? Tommy examines a few different types of clock movements to see how they set themselves initially to the correct time, how they maintain near-perfect accuracy, and how they automatically adjust for daylight savings time. By Tommy Tyler (US)

Articles in this issue

Archives of this issue

view archives of Circuit Cellar - GIFT - CC-2015-06-Issue 299