Jeff Epler's blog

6 March 2012, 1:41 UTC

WWVB CRT mock-up


The plan is to be able to watch the leap second on some manner of wwvb-controlled clock of my own making.

To that end, Chris gifted me with a compact B&W CRT from Goodwill, and I picked up a TellyMate, which can display 38x25 text on a TV.

Ignoring for the moment the difficulty of receiving the WWVB signal when all the interference from a CRT TV is right nearby, I've created a mock-up of what the display will look like (except that the font won't be nearly so good looking as this):


         0123456789
   0.. 9 M01100000M  Minute  = 30
  10..19 000000111M  Hour    = 07
  20..29 000000110M  YDay    = 66
  30..39 011000010M  DUT1    = +0.3
  40..49 001100000M  Year    = 2008
  50..59 100001000M_ nols LY nodst
   
  Local time:
  1:31:00.0 AM

  Thu Mar 6 2008
  UTC-0600 (CST)
   

At the top, the WWVB data is displayed, along with its interpretation. The previous minute and the current minute are mixed, with the cursor indicating the most recently read second of data. Whatever piece of data is currently being read or will next be read will be blanked out until all bits are received (for instance, during seconds 0..8, "minute" will be blank; from 9..18, "hour" will be blanked).

Most of the time the "_" shown after second 59 will be blank. During a leap minute it will be "_" and during the leap second it will read "M".

Below, in larger lettering, the local time will be displayed. I think I can achieve 10 updates per second, so I'll show tenths but not hundredths (or, say, thirtieths). The UTC offsets for standard and daylight saving time will be hardcoded (so if I move to Colorado to be closer to WWVB I'll have to fix my firmware).

The WWVB receiver and decoder is written and passes my tests. I have yet to write the display code (and optimize it--I can transmit fewer than 500 characters and control codes per 1/10 second to the tellymate), and I also have to work out how to get enough reception that having the WWVB display is not a big old waste of time. (or maybe I'll just display a simulated WWVB signal instead—with a disclaimer, of course. stop looking at me like that.)

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