MSF Clock Synchronisation for Microcontrollers – Part 6 – Decoding the signal

Decoding the signal

The specification for the NPL time signal (previously Rugby Time signal) is described at http://www.npl.co.uk/science-technology/time-frequency/products-and-services/time/msf-radio-time-signal with each second of transmission encoding two bits (A&B) except for the minute marker which encodes zero bits but instead marks the epoch.

Time information when transmitted normally comprises 59 sets of two bits except for minutes that include an extra leap second (60 sets of 2 bits) or a absent leap second (58 sets of 2 bits).   My interpretation is that during an additional leap second minute the Year code starts in the 18th second and an extra second is inserted after the DUT code; during a negative leap second minute the 16th second (containing 16B of the DUT code) is omitted and the year code starts in the 16th second.  Detection of the length of each minute is not-obvious but can be detected by inspecting the 01111110 pattern that occurs in the last eight seconds of each minute.

Below is an example of a decoding of the sample captured at the end of previous post.

fullMin_decoded

the protocol includes parity bits for the year, month/day, day of week and hour portions but is only resilient to a single bit error; in this example it indicates that all is intact.

The decoded time is 00:10 on Friday 19th June 2015 with BST in effect, no DST change imminent and DUT -700mS.  The message pertains to the following minute so the following minute marker indicates the epoch.

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About nivagswerdna

Professional Geek
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