Synchronization options

• Trigger signal
• SyncIn
  • TriggerIndication function
  • SendLatest function
  • StartSampling function
  • Clock Bias Estimation function
  • 1PPS Time-pulse function
• SyncOut
  • Interval Transition Measurement function
• Combining multiple Sync functions

This chapter describes the synchronization functionalities of the MTi-600 series. In the remainder of this chapter, synchronization will be abbreviated as sync, synchronization input as SyncIn and synchronization output as SyncOut. A set of one or more synchronization options and their corresponding parameters are stored in a structure referred to as SyncSettings.

The sync functionalities are in line with the other Xsens motion trackers. However, some minor differences exist because of the different architecture of the MTi-600.

The MTi-600 series has two input lines available for SyncIn, and one for SyncOut (see Functional description). It is not possible to configure a SyncIn line as SyncOut or vice versa.

On devices which employ an external GNSS receiver, a 1PPS signal can be gathered directly from the GNSS receiver. However, it is also possible to configure a 1 Hz output reference signal using the SyncOut functionality. The output reference signal is synchronized with the 1PPS signal of the external GNSS receiver when using an MTi-670 or MTi-680, or the internal GNSS receiver when using an MTi-670G or MTi-680G.

Trigger signal

A trigger signal is expected to be a pulse wave. When using a SyncIn function, the trigger is an input signal. When using a SyncOut function, it is an output signal generated by the MTi. When configuring a sync function, various parameters can be set by the user to interpret/generate the trigger signal. These parameters are reported in the table below.

SyncIn

This chapter describes the functionality and behaviour of the SyncIn line. The SyncIn lines can be used to synchronize the sampling of data or data output of the MTi 600-series to an external device or clock by inputting a trigger signal through the SyncIn lines. See System and electrical specifications for the trigger signal requirements. 

TriggerIndication function

With this function, the user can input a trigger signal to mark the output data (e.g. orientation) with the trigger event. Upon receiving a trigger, this function will set the trigger indication status bit (SyncIn Marker) of the Status Word to 1. Furthermore, the user can configure the MTi to also output a TriggerIndication message through the MtData2 stream. The advantage is that this message is timestamped with the trigger moment, so it has better accuracy than just the status flag.

SendLatest function

Enabling this function, the last measured/computed desired data available at the trigger instant is outputted by the MTi. The internal signal pipeline of the MTi works as usual, but the desired data is outputted only when a trigger is received.

The trigger can be sent either on a SyncIn line or by means of a software command (Line must be set to ReqData).

StartSampling function

When this function is enabled, the device will start outputting data after a SyncIn trigger has been received. It does not trigger every consequent sample, but it accurately starts outputting the first sample, after which the next samples will follow the selected output data rate. A skipFirst value can be set to ignore the first n pulses before considering a pulse as a trigger.

Similarly, a delay parameter can be set to tune when to start the sampling after the trigger.

Clock Bias Estimation function

This function enables the user to synchronize the output stream of the MTi with an external reference signal. The internal clock of the MTi-600 has an accuracy of about 10 ppm. When a reference clock of better accuracy is available, it is possible to use this clock to improve the timing of the output stream of the MTi-600. If the Interval Transition Measurement function is enabled, then the SyncOut signal will also be synchronized with the ClockSync reference.

If the user does not have a reference clock that is better than 10 ppm, but it is still preferred to run multiple devices at the same clock speed, it is better to use the SyncOut line of the MTi-600 as a clock source for other devices.

On the MTi-670G/680G the internal clock is always referenced to the internal GNSS receiver. Therefore, on the MTi-670G/680G the clock bias estimation is not user-configurable. On the MTi-670 and MTi-680, the internal clock is by default referenced to the external GNSS receiver, but it is user-configurable (although not recommended).

Please note that not all reference clock frequencies are supported by the ClockSync functionality. This is because the reference clock period needs to be configured in integer milliseconds. For example, a reference clock frequency of 100 Hz (period = 10 ms) is supported, but 60 Hz (period ~ 16.67 ms) is not supported. The ClockSync functionality can cope with small deviations (-0.5%) between the configured and actual reference clock periods, but a 60 Hz reference clock signal will exceed this when a 17 ms clock period (f ~ 58.8 Hz) is configured.

Please also note that although the output data stream is synchronized with the external ClockSync reference, the timestamps provided by the MTi-600 are defined based on the (unadjusted) internal sampling clock of the MTi.

1PPS Time-pulse function

When an MTi-670/680 is connected with an external GNSS receiver, the 1PPS signal of the receiver can be used as input to synchronize the MTi-670/680 with the external receiver (using the Clock Bias Estimation function). This function is enabled by default, and will configure a connected (u-blox) GNSS receiver to output a 1PPS signal. On the MTi-670G/680G, this function is always enabled (and not user configurable) with the time pulse from the internal GNSS receiver.

This function is not available for the MTi-610/MTi-620/MTi-630(R).

SyncOut

The MTi 600-series has one output synchronization function.

Interval Transition Measurement function

The Interval Transition Measurement function provides a sampling time indication, allowing the user to synchronize external applications with the same clock employed by the MTi-600 to output the data. It generates a SyncOut trigger based on the internal 400 Hz SDI sampling clock.

Combining multiple Sync functions

Multiple Sync functions can be combined, under the restrictions described in the table below.