Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
61
5.6 Internal clock accuracy
5.6.1
Clock of MTi’s without GPS 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 secon
ds (= 3600 s ∙ 10 ppm) or 15 sample counts in 1,440,000 at 400 Hz
sample rate (± 25 ns/data packet @ 400 Hz).
5.6.2
Clock of MTi-G-700 GPS/INS
In the case that the MTi-G has a GPS fix the bias of the clock will be estimated and on the long term
there will on average be no deviation from GPS time. On the short time scale, the clock jitter is the
determining factor. The MTi-G is capable of generating a quite accurate hardware-synchronised time
pulse when GPS is available. The time pulse is synchronised to UTC time. This accurate Time Pulse
can be used to correct the sampling clock of the MTi-G. This clock bias estimation will improve the
accuracy of the crystal used in the MTi-G, under normal operating conditions to below 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. The cable delay is 5.5ns/m for PFTE, resulting in 16.5ns delay with the
development kit antenna (PCTel 3910D).
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.
The first point is compensated in the MT GPS 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. f there is no GPS fix available. Using a
typical MT (with an accuracy of 80 ppm), this means that the worst case deviation after a 1 hour log is
± 0.288 seconds (=
3600 s ∙ 80 ppm) or 29 sample counts in 360,000 at 100 Hz sample rate (± 0.8
μs/sample @ 100 Hz).
5.7 Default Serial Connection Settings
Setting
Default Value
Bits/second (bps):
115200
Data bits:
8
Parity:
none
Stop bits:
2
Flow control:
none
These settings are the same for the RS-232 as the RS-422/RS485 versions. The baud rate (bps)
setting can be changed by the user. The maximum is 921600 bps and the minimum 4800 bps. Please
refer to the [LLCP]
for details. Should the communication fail, it can be helpful to change the number of
stop bits to 1.
Note that there is no terminating resistor in the MTi with RS422/RS485 communication. Should
communication problems arise, terminate the connection either at the application side or alternatively
close to the MTi.
