MSF Clock Synchronisation for Microcontrollers – Part 2

Looking at the MSF received signals…

Having established a receiver away from the computer equipment that was causing all my noise issues it was time to investigate the signals so wired up the trusty Rigol oscilloscope and see what we are getting… The MSF Signal is divided into 1 second long segments with normally 60 segments per minute but where leap seconds are present this may change to 59 or 61 seconds. Each minute has a single minute marker segment. The actual signal is encoded as is described here http://en.wikipedia.org/wiki/Time_from_NPL with the encoding being based on either the presence or absence of the 60kHz carrrier, however presence of carrier is considered to be a 0 and absence of carrier is a 1 (i.e. Negative Polarity Bit Signalling). The receiver module I am using helpfully produces an inverted output so on the traces below a hi signal is a 1 and a lo is 0 which reduces confusion considerably!

The minute marker

Once per minute a special minute marker signal is sent.  This comprises, according to the specification 500mS carrier off (i.e. a one) followed by 500mS carrier on (i.e. a zero).  The actual receiver has to determine when the tone is present or absent and therefore introduces a certain amount of stretching/hysteresis to the received signal.

MinuteMarker_Full

The observed, by the receiver signal, is presented on its output as a 520mS hi followed by a 480mS lo.

MinuteMarker

The minute epoch is, I believe, on the initial lo to hi transition although the specification is slightly unclear on that point.

The AB Bits in the other segments

Each second other than the minute marker encodes a pair of bits in the form of a 100mS carrier off and then two bits each of 100mS followed by 700mS of carrier on. Bit A is encoded between 100mS and 200mS and Bit B is encoded between 200mS and 300mS of the initial carrier off (i.e. high output from receiver’s inverted output).

AB=00

00Full 00

The 100mS carrier off has been stretched in the receiver and presents as 136mS of hi.

AB=01

The encoding of A=0, B=1 causes the signal to create a two hump display.  The intial hump is the 100mS initial carrier off, the valley is the A bit and the next hump is the B bit.   01

The 300mS transmitted signal presents as a 312mS stretched combination which is worth looking at in detail…

01_1

The initial carrier off is approx 124mS being a stretched 100mS.

The A bit is in the next 100mS but the receiver only holds the line low for 90mS at most and in some cases even less…

01_2

with the A=0 be encoded in only 76mS of low!  That’s a good deal less than 100mS and we will need to take that into account when decoding the signal.

01_3

The B bit follows and is slightly stretched at around 106mS

AB=10

As per each segment the A=1, B=0 encoding fits exactly into 1.00sec

10Full

with the leading carrier off and the A=1 (remember it’s an inverted polarity bit encoding) combining to make a long pulse of 200mS but stretched to 224mS by the receiver.

10

AB=11

Finally the coding of A=1, B=1, which again fits exactly into the 1.00 second interval

11Full

Here the initial carrier off and A and B combine to a single pulse of 300mS from the transmitter but stretched to 320mS at the receiver.

11

Final observations

As we can see from the traces the signals are crystal clear now that I have moved away from my computer monitors, they demonstrate the stretching of the theoretical pulses by the receiver and in particular the compression of the A=0 bit in the A=0,B=1 segments to have only 70mS of low for the A bit in some cases.  Armed with this knowledge we can embark on decoding the signal using a controller and do this in a way that hopes to be resilient to some noise.

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About nivagswerdna

Professional Geek
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