[time-nuts] M12+T ASCII interface - I'm confused?

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Nov 20 06:48:40 EST 2008


Hi Stephan,

I have been asking my question because I wanted to learn whether an FPGA
was really needed. Don't understand me wrong, I am a dedicated user of
XILINX and ALTERA FPGAs and CPLDs myself, but sometimes a $2
microcontroller will do the job better than any FPGA. It is correct that
you may put a 32 bit micro + numerous peripherals say in an XC3S400 and
still it is half-empty for other purposes. I also use technology like
that. However it is not ALWAYS better. Even the bigger FPGAs do not have
enough RAM as that you could hold the embedded firmware + the RAM needed
for the application in it. That calls for external RAM. Also be prepared
NOT to fit the complete firmware into the FPGA's configuration device.
One must be lucky if a kind of bootloader fits into the FPGA
configuration that enables the FPGA (once that it is configured) to load
the "real" firmware from an external memory device like a serial EEPROM.
You see that there are a number of trade offs and one has to think very
carefully which solution is the best overall compromise.

Best regards
Ulrich Bangert  

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Stephan Sandenbergh
> Gesendet: Donnerstag, 20. November 2008 11:48
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] M12+T ASCII interface - I'm confused?
> 
> 
> Hi Ulrich,
> 
> Sure - among other things the FPGA keeps a real-time clock. 
> It ticks away on a disciplined clock, but the time and date 
> value is constantly kept in sync with what the GPS spits out as UTC.
> 
> So, the FPGA catches some of the GPS messages, but also allow 
> for a straight path through to a PC. Everything that comes in 
> is passed on the the PC - this way existing software could 
> still be used to interface with the GPS etc.
> 
> Stephan.
> 
> 2008/11/20 Ulrich Bangert <df6jb at ulrich-bangert.de>
> 
> > Stephan,
> >
> > can you give us a clue what it is all about? What happens with the 
> > data after being decoded by the FPGA?
> >
> > Best regards
> > Ulrich Bangert
> >
> > > -----Ursprungliche Nachricht-----
> > > Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] 
> > > Im Auftrag von Stephan Sandenbergh
> > > Gesendet: Mittwoch, 19. November 2008 15:30
> > > An: Discussion of precise time and frequency measurement
> > > Betreff: [time-nuts] M12+T ASCII interface - I'm confused?
> > >
> > >
> > > Hi All,
> > >
> > > Up until now we've been interfacing my Motorola M12+T's using the 
> > > Oncore software. However, at this point we are trying to have it 
> > > interfaced directly to a FPGA. To my mind this should be simple - 
> > > the commands are discriminated (framed) by looking at the 
> start and 
> > > terminating bytes sequences when they enter the FIFO, 
> check summed, 
> > > decoded etc.
> > >
> > > However, I noted something very peculiar about the motorola ASCII 
> > > protocol: The start bytes @@ and he terminating byte 
> <CR><LF> aren't 
> > > unique with respect to the data bytes. For instance one could 
> > > receive a time of 13hrs and 10mins which would look 
> identical to the 
> > > terminating characters. Initially I thought it made sense 
> since the 
> > > data is also sent in ascii format. It appears not to be the case.
> > >
> > > It seems to me that the only way in which a command could be 
> > > robustly identified and check summed is when the 
> interface knows the 
> > > length of the expected return.  Obviously, the data lengths are 
> > > dependent on both the actual command and the specific 
> request. This 
> > > type of intelligence is cumbersome to implement in FPGAs.
> > >
> > > It would be of great help if you could point me in the right 
> > > direction here. I feel rather stupid in asking such a simple 
> > > question, but at this point I can't seem to see the light. I'm 
> > > flabbergasted...
> > >
> > > Best regards,
> > >
> > > Stephan
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