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Document MT0605P.2018.E
© Xsens Technologies B.V.
MTi User Manual
6 Communication
Communication with the MTi is done via the binary Xbus communication protocol, except for data
coming from the MTi device in ASCII format (NMEA). The communication protocol is extensively
documented in [LLCP].
6.1 Communication Timing
For many applications it is crucial to know exactly the various delays and latencies in a system. This
has been extensively discussed in
https://base.xsens.com/hc/en-us/articles/209306289
6.2 Triggering and synchronization
In case multiple systems are used during a measurement it is important to have the measurement data
synchronized between the systems. Processing synchronised data is much easier because there is no
need to resample the data to compensate for timing inaccuracies like clock drift and clock deviations.
Synchronization using multiple systems involves 2 important issues: starting the measurement at the
same time and having a fixed time relationship of the sampling instances. Refer to
https://BASE.xsens.com for more information on triggering:
6.3 Internal clock accuracy
6.3.1 Clock of MTi’s without GNSS receiver
The internal clock jitter of the MTi is less than 25ns.
The internal clock of the MTi which generates the sample timing based on the set sample period is
accurate to ±10 ppm with a maximum of ±15 ppm (this differs per MTi) over the temperature operating
range. Using a typical MT (with an accuracy of 10 ppm), this means that the worst case deviation after
a 1 hour log is ± 0.036 seconds (= 3600 s ∙ 10 ppm) or 15 sample counts in 1,440,000 at 400 Hz sample
rate (± 25 ns/data packet @ 400 Hz).
6.3.2 Clock of MTi-G-710
In the event that the MTi-G-710 has a GNSS fix the bias of the clock will be estimated and on the long
term there will be no deviation from GPS time. On the short time scale, the clock jitter is the determining
factor. Note that only GPS time pulses (not other GNSS reference times) are used to determine the time
reference. This clock bias estimation will improve the accuracy of the crystal used in the MTi-G-710,
under normal operating conditions to <1ppm.
The time pulse used to correct the clock of the MTi-G has minor inaccuracies, caused by the following:
Delay caused by distance between antenna phase centre to input pin of the GPS receiver
module in the MTi-G-710. The cable delay is 5.5ns/m for PTFE, resulting in 16.5ns delay with
the development kit antenna.
Quantisation loss, clock of 23.104MHz, results in a resolution of 43ns.
Rise time of timepulse 7-25ns, best results when loaded with a high impedance.
Software delay, for handling the time pulse interrupt clock_ticks/300Mhz.
Delay caused by the antenna cable length is compensated for in the GNSS receiver, but will vary with
cable length.
The internal clock jitter of the MTi is less than 25ns. The internal clock of the MTi which generates the
sample timing based on the set sample period is accurate to ±10 ppm with a maximum of ±15 ppm (this
differs per MTi) over the temperature operating range, if there is no availability of GPS. Using a typical
MT (with an accuracy of 10 ppm), this means that the worst case deviation after a 1 hour log is ± 0.036
seconds (= 3600 s ∙ 10 ppm) or 4 sample counts in 360,000 at 100 Hz sample rate (± 0.1 μs/sample @
100 Hz).
