Microchip Technology Vectron MD-261 User manual

Vectron GNSS Timing Modules
MD-261, MD-171, MD-174
Reference Manual
Rev D, 10-05-18

Vectron GNSS Timing Module Reference Manual, Rev. D, 10-05-18
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Vectron • 100 Watts Street • Mount Holly Springs, PA 17065, USA
Revision History
Revision
Date
Description
A
04/02/2015
•First Release
B
05/21/2015
•Update for Firmware 4.0
C
12/13/2016
•Update for Firmware 4.3
D
10/05/2018
•Update for Firmware 4.4
•Updated section 2.4.3 Reference
Selection
•Updated section 2.3 Operating Modes

Vectron GNSS Timing Module Reference Manual, Rev D, 10-05-18
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Table of Contents
CHAPTER 1 INTRODUCTION ...................................................................................................................................... - 4 -
1.1 INTRODUCTION.................................................................................................................................................. - 4-
1.2 SCOPE............................................................................................................................................................... - 4-
1.3 TERMINOLOGY................................................................................................................................................... - 4-
CHAPTER 2 DEVICE OPERATION.............................................................................................................................. - 5 -
2.1 STARTUP ................................................................................................................................................................. - 5-
2.2 COMMUNICATIONS................................................................................................................................................... - 5-
2.3 OPERATION MODES................................................................................................................................................. - 5-
2.4 MODULE CONFIGURATION ....................................................................................................................................... - 8-
2.4.1 Output Messages Configuration................................................................................................................... - 8 -
2.4.2 Module Output Configuration........................................................................................................................ - 8 -
2.4.3 Reference Selection...................................................................................................................................... - 9 -
2.4.4 Survey Mode & Position Hold..................................................................................................................... - 11 -
2.4.5 Cable Delay.................................................................................................................................................. - 11 -
2.4.6 GNSS Fixmask ............................................................................................................................................ - 11 -
2.4.7 GNSS System Select.................................................................................................................................. - 12 -
2.4.8 GPS Week Rollover .................................................................................................................................... - 12 -
2.4.9 Manual Holdover Mode............................................................................................................................... - 13 -
2.4.10 Manual Frequency Setting........................................................................................................................ - 13 -
2.4.11 Device Reset / Restart.............................................................................................................................. - 13 -
2.5 FIRMWARE UPDATES............................................................................................................................................. - 14 -
2.6 OPERATION MODES &CONDITION REGISTERS...................................................................................................... - 15 -
2.6.1 Operation Modes......................................................................................................................................... - 15 -
2.6.2 Condition Register....................................................................................................................................... - 15 -
CHAPTER 3 DEVICE INSTALLATION ...................................................................................................................... - 16 -
3.1 SERIAL RX AND TXCONNECTIONS........................................................................................................................ - 16 -
3.2 RECOMMENDED MINIMAL CONNECTIONS............................................................................................................... - 16 -

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Chapter 1
Introduction
1.1 Introduction
The Vectron GNSS Disciplined Oscillator Module is an OEM Micro-Disciplined Oscillator
Module designed to provide a time and frequency reference to systems requiring
synchronization. These modules can be configured to use either GNSS constellations or an
external reference signal, which can be 1PPS (standard) or a reference frequency (contact
factory). This document outlines the available VSIP commands and queries supported by
the module and explains how the commands are used. This document should be used
along with the datasheet and VSIP Reference Manual to properly configure the device.
1.2 Scope
This document currently applies to the MD-171, MD-174, and MD-261 series modules with
firmware versions 4.3 and 4.4.
1.3 Terminology
VSIP
Vectron Serial Interface Protocol
VDOM
Vectron Disciplined Oscillator Module
Device or Target
The VDOM device sending reports and responding to commands
Host
The device, typically a PC, sending commands to the VDOM device; a host may also be
another embedded system, typically communicating with the VDOM device via a UART.
Incoming
From Host to Target
Outgoing
From Target to Host
Permanent Setting
A permanent setting is written to nonvolatile memory and will persist between power
cycles and resets. Permanent settings can be modified by the user any number of times.
Temporary Setting
A temporary setting will only maintain its state during the current runtime, and will revert
to the last permanent setting or factory default upon device power-up or reboot.

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Chapter 2
Device Operation
2.1 Startup
At startup or after reset the module may take from five to ten seconds to boot up and
begin serial communications. Standard communications startup begins with a
$PVEC,03,VECTRON,init_ok… message indicating that the device successfully loaded and
initialized all settings. If the init_ok field is not given, the device was unable to start due
to a hardware failure or invalid configuration. After the $PVEC,03 message, the first
$PVEC,00 status message will be transmitted in approximately 1.5 seconds. After
transmitting the first status message, the unit will begin processing normal serial
communications. Do not attempt to send any data to the unit before the first status
message is received.
2.2 Communications
The module communicates using an idle high CMOS-level serial UART, transmitting both
VSIP and NMEA talker sentences. The standard port configuration is 115,200 baud, 8 data
bits, no parity, and one stop bit (115200 8N1). Configuration of the device is done using
VSIP commands and queries which are outlined in the section below. See the VSIP
Reference Manual for more information on VSIP commands, queries, and reports. Serial
communication is not required for normal operation; once the device is configured it can
be used as a standalone GPSDO without a host controller.
In standard operating mode, the module will output a status message (VEC,00) every
second. This message indicates the operation mode, DAC value, temperature, uptime, and
other information. Additionally, all supported NMEA messages (see below) can be
configured to be output up to once per second as well. Note: the time fields in GNSS
messages are up to one second ahead of UTC and correspond to the next PPS pulse.
The Vectron VDOM Monitor 3 software application can be used to monitor and configure
the module via the serial interface.
A 3.3 volt USB-to-serial converter, such as the FTDI TTL-232R-3v3, may be used to
connect to the module for configuration and firmware update purposes. Alternatively, the
module can be interfaced with another microcontroller or logic on the customer’s board;
however, it is advisable to allow direct access to the module’s RX and TX pins from a PC
interface (such as the TTL-232R-3v3) to allow for firmware updates using the VDOM
Monitor 3 software. WARNING: DO NOT CONNECT THE CMOS-LEVEL SERIAL PORT TO AN
RS-232 LEVEL SERIAL INTERFACE. THIS WILL DAMAGE THE UNIT.
2.3 Operation Modes
Upon power-up, the module will perform a self-diagnostic test and initialize its internal
registers. This takes approximately ten seconds. After this is completed, VSIP Status
Reports and NMEA 0183 data are transmitted at regular time intervals via the serial TX

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pin. The default interval is once per second, but the intervals for all NMEA messages are
configurable. The status information is transmitted using the VEC,00 format. See the VSIP
Protocol document for details of the commands and message format. Errors that occur
during self-diagnostics or any other stages are reported real-time through error messages
and/or the error flag bytes in the status packets.
The disciplined oscillator module has 8 operation modes (opmodes). They are shown in
the flow diagram below. The current mode of operation is included in the serial Status
Report. See the VSIP Protocol document for details.
Figure 2.3a: Operation Modes (opmodes)
Opmode 0 –System Warmup
After the completion of the self-diagnostic test, the module enters the System Warm-up
mode. The duration of this mode after reset or power-on is fixed, but may be from 0 to
1200 seconds. The module may return to this mode momentarily while exiting from a
holdover mode (manual and auto) before re-starting the locking process.
Opmode 1 –Reference Evaluation
In this mode the reference(s) and local oscillator are evaluated for stability. Additionally,
the local 1PPS is coarsely aligned to the reference, if available. The module will remain in
this mode until a suitable reference is detected. If using GNSS constellation(s) as a
reference, the GNSS receiver parameters are evaluated to determine the fitness for use of
the reference.
Opmode 2 –Frequency Locking
Once a suitable reference is detected, a frequency locking routine will begin.
Assuming the reference is determined to be usable, moderate adjustments to the local
oscillator’s frequency are then made to gradually approach nominal. The time spent in this
mode is mostly related to the initial frequency error. Once the local oscillator’s frequency
is within approximately 2ppb or less of nominal, the module will enter Phase Alignment
mode. If the routine is unable to achieve and hold these criteria, the module will return to
reference evaluation.
Opmode 3 –Phase Alignment
Phase alignment is a digital phase adjustment which is intended to bring the local
oscillator within approximately 100ns or better of the reference signal. If the frequency of
both the local oscillator and reference is stable, this mode will exit to opmode 4.

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Opmode 4 –Phase Locking
In order to maintain synchronization to the reference source, small frequency corrections
to the local oscillator are made. The magnitude of these changes is configurable by the
factory depending on application requirements. These corrections happen at variable
intervals that are related to the stability of the oscillator and/or reference signal. In most
cases, this is the desired steady-state operating mode of the module. Upon entering this
mode after initial reference evaluation, the first (approximately) 5 minutes of this mode
are considered “Coarse Phase Locking” and PPS is not valid. After the stability of the local
oscillator and reference has been established, the PPS is considered valid and the lock bit
(see pinout and VSIP Status Report) will go high, indicating the module is locked to within
its configured threshold. At this point, the module will also enable the output 1PPS and/or
RF if configured to do so (See VEC,12 –Output Configuration).
Opmode 5 –Manual Holdover
The module can be placed into holdover mode at any time using the Holdover Mode Set
command. See the VSIP protocol document for details. This can be used in applications
where the user demands control of when holdover mode is entered, or for testing,
evaluation, or control of module performance. The module will remain in this mode until
the Holdover Mode Set command is issued, with hmode set to 0. The module will then
momentarily return to warmup mode before restarting the normal evaluation and locking
process.
Opmode 6 –Auto Holdover
If the signal is determined to be unusable during phase align or phase locking, the module
will enter auto holdover mode. The more time the module spends in phase locking mode,
the longer it can maintain accuracy during holdover. In Auto Holdover mode, the module
will continually check for the presence and of any reference. If a reference is detected, the
module will transition to either opmode 1, 3, or 4, depending on the phase error of the
local reference compared to the reference at that time.

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2.4 Module Configuration
2.4.1 Output Messages Configuration
The module can be configured at runtime to output various GNSS NMEA talker sentences
at user-defined intervals. To configure these messages, use VSIP Command VEC,19
(Configure Messages) at any point during normal device operation. The maximum interval
is 255 seconds, while an interval of 0 disables the message. An ACK indicates that the
given message interval was set correctly.
The module supports the following NMEA outputs, per NMEA standard 0183 (June 2012):
Message ID
Description
GGA
Global Position System Fix Data
GLL
Geographic Position –Latitude/Longitude
GNS
GNSS Fix Data
GSA
GNSS DOP and Active Satellites
GSV
GNSS Satellites in View
RMC
Recommended Minimum Navigation Information
VTG
Course of Ground and Ground Speed
ZDA
Time & Date
The module’s periodic status report message (VEC,00) can also be controlled using
VEC,19. In this case, a message ID of “STATUS” should be used to set the interval of the
VEC,00 report.
By default (after power-up or restart) STATUS, GNS, GSA, and ZDA will be output every
second.
2.4.2 Module Output Configuration
The device can be configured to output both PPS and 10MHz RF in various modes. By
default, the module will output PPS only while locked to a reference (GNSS PPS or an
external PPS) and will always output a 10MHz RF signal. This behavior can be modified
using VSIP Command VEC,12 (Output Select) and can be set either temporarily or
permanently.
VEC,12 controls the PPS Output Source, PPS Output Enable, and RF Output Enable. The
PPS Output can be configured to output the locked PPS (default), the GNSS PPS, or to
pass the external reference PPS (note: there is approximately a 13ns delay through the
module in this case). PPS and RF outputs can be configured to output always, never, or
only when the device is locked to a reference.

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2.4.3 Reference Selection
The module can lock to either an external reference input or to the internal GNSS
receiver’s PPS. If both references are enabled on the module, the device will use
whichever is available. If both are enabled and available, the device can be configured to
prefer one over the other (GNSS or PPS preferred) or to continuing using the last
reference as long as it is available (No Preference). These settings can be modified using
VSIP Command VEC,10 (Reference Select) and can be set either temporarily or
permanently.
The module is capable of switching from GNSS to and external references in a hitless
fashion; that is, a phase offset will be applied to the external reference so that the
internal timing controls will not see a significant timing shift –allowing the device to stay
locked during the process of switching out of GNSS reference to an external reference,
and back again (assuming the external reference frequency is accurate). Figures 2.2.3a
and 2.2.3b show the reference switching process and holdover exit process, respectively.
Figure 2.4.3a: Reference Switching Process (From Locked Condition)

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Figure 2.4.3b: Exit from auto holdover process
For example, if both GNSS and external PPS references are enabled, and GNSS is
preferred, the device will use GNSS whenever it is available. In the event that GNSS
reference is lost (i.e. the antenna is disconnected) the device will attempt to switch to the
external reference. If hitless is disabled and the external reference phase is not closely
aligned with the GNSS PPS, the device must switch to mode 1 (evaluating reference) and
start the locking process from the beginning. If hitless is enabled, the device will apply a
phase offset to the external reference and the lock will not be interrupted. When the
GNSS reference is restored, the device will attempt to switch back to GNSS reference (as
it is preferred in this case). If the phase of the on-board reference (previously locked to
the external reference) is aligned with the GNSS PPS, the device will return to the locking
state. If not aligned, the device will re-start the locking process from mode 1.
Bit 12 of the condition register indicates if a hitless offset is actively being applied to the
external reference input. A ‘1’ in this field indicates there is an offset active. Note that the
hitless offset is only applied to the external reference after switching out of GNSS; it
cannot be used to align the GNSS to the external reference.
Note: It is not shown on the flowchart diagrams, but when switching between aligned
references (with or without an active hitless offset), the module will briefly enter “Auto
Holdover” mode before switching to a new reference. PPS Output will not be affected.
Note: Refsel mode 4 is not supported prior to firmware version 4.5 (selecting this mode
will result in mode 7 operation).

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2.4.4 Survey Mode & Position Hold
The module has a Position Hold feature that can be used to provide better timing and
positioning solutions if provided with accurate location information. Additionally, a mobile
/ no survey mode can be enabled as well. By default, the device will use a built-in survey
solution that continuously evaluates the devices’ position which is used in the timing and
positioning solutions.
To configure the survey mode, use VSIP Command VEC,14 (Survey Mode Select). To
switch to mobile mode or self-survey mode, set the surveymode to 1 or 0, respectively,
and fill the location fields (latitude, longitude, and altitude) with zeros. To enable fixed
position mode, use surveymode 2 along with the latitude, longitude, and altitude of the
antenna. These fields are formatted as +/- decimal degrees (lat/long) and meters
(altitude). Note that accurate location data must be provided for the device to operate
correctly and to see an improvement over self-survey.
The survey mode determines what information is reported as location data in the NMEA
output strings. If using self-survey, the location determined by the self-survey process will
be reported in the NMEA strings; if using mobile mode the current calculated location is
reported, and if using position-hold the given position-hold location is reported.
2.4.5 Cable Delay
A delay can be applied to the PPS output of the device to slew the PPS timing. VSIP
Command VEC,1A (Configure Cable Delay) is used to set the cable delay from +10,000ns
to -10,000ns and can be configured either temporarily or permanently. A negative cable
delay will shift the output PPS ahead of normal PPS while a positive cable delay results in
a delayed PPS. Changes to cable delay will take some time for the control process to apply
to the standard output (the device will unlock if the change is great enough); however, if
the device is configured to output the GNSS PPS directly (see VEC,12) the change will be
immediate. Cable delay does not affect the output if VEC,12 is configured to use the EXT
reference PPS.
2.4.6 GNSS Fixmask
VSIP Command VEC,18 can be used to configure the GNSS Fix Mask. This command
requires 2 masks to be defined; as follows:
Elevation Mask
0 to 90
degrees
Only SVs with elevation above the given
elevation are used
Signal Level Mask
0 to 99 dBHz
Only SVs with signal level above the given
signal level are used
Note that the ranges given in the table above for Elevation and Signal Level mask are
minimum / maximum ranges accepted by the device; settings these values at or near the
maximum ranges will result in the unit not be able to achieve lock. For example, an
elevation mask above 75 degrees or a signal level mask above 50 dBHz may prevent the
unit from locking. The command will not accept any value above 50 dBHz signal level or

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80 degrees elevation. Satellites below the given masks will be reported in GSV messages
(satellites with an SNR below the SNR mask will be reported as zero SNR) but not used for
position and time calculation.
The default elevation mask and signal level mask are 15 degrees and 25 dBHz.
2.4.7 GNSS System Select
The module can be configured to use select GNSS Systems for timing and location
solutions. VSIP Command VEC,1B (GNSS System Select) is used to configure this feature.
By default; GPS, GLONASS, and QZSS are used (SBAS is not enabled and Galileo is not
yet implemented). This field can be set temporarily or permanently.
If the device is switched to or from GLONASS only, the receiver will perform a cold reset;
clearing the time information and restarting the locking process. In any other case, a hot
reset is performed.
2.4.8 GPS Week Rollover
The week number field, as reported by GPS, will roll over to zero on April 7th, 2019. After
this rollover, the reported date will begin at August 22nd, 1999. However, GPS receivers in
Vectron GNSS Timing Modules apply an offset to this field to extend the operational
lifetime of the module. As of the date of this revision, all modules currently have an offset
rollover date of August 15th, 2032. Note that future updates to the module’s GNSS
receiver firmware may extend this time.
If the module is kept on constant power, the receiver will not be affected by the rollover
(it will automatically change the internal offset and continue reporting correct dates after
the rollover). Any time the module is power cycled after a rollover; however, the reported
date will be affected.
On firmware version 4.4 and later, the VEC,1F command allows the user to manually set
the internal offset to correct the reported date after this rollover. This command allows
the user to specify a “valid year” to set the rollover offset. For example, after August 15th,
2032 the user may specify a valid year of 2035 to move the offset up one 1024-week
step. Since 2035 is within the 2032 to 2052 range (2052 being the approximate time of
the next rollover), the receiver will properly offset the week field for this duration of time.
Using this command, the correct date can be reported up to December 31st, 2099.
GPS week rollover does not affect the ability of the module to perform proper timing and
synchronization. Only the reported time will be incorrect. Phase lock and holdover are not
affected.
Note: If GLONASS is enabled and available, the module is not affected by the week
rollover, even in the event of a power cycle after the rollover. This is because the week
field reported by GLONASS has a larger range. The module will be able to correctly output
the date and time until December 31st, 2099. If GLONASS is used, do not use or enable
the VEC,1F feature as this may significantly affect TTFF.

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2.4.9 Manual Holdover Mode
The module can be placed into (or taken out of) manual holdover mode at any time by the
user by using VSIP Command VEC,11 (Holdover Mode Set). The device will stay in manual
holdover until a power cycle, reset, or another VEC,11 command is sent instructing the
device to return to normal operation.
Upon exiting manual holdover, the device will restart the lock process from the
warmup/evaluating reference stage.
2.4.10 Manual Frequency Setting
While in manual holdover, the frequency of the 10MHz RF output can be manually set
using VSIP Command VEC,23 (Frequency Set). The frequency offset can be from 0 to
65535, with center at 32768. If the VEC,23 command is sent while the device is not in
manual holdover, the device will first switch to manual holdover and will continue in this
mode until VEC,11 (Holdover Mode Set) is used to take it out of manual holdover.
2.4.11 Device Reset / Restart
A software reset can be performed by sending VSIP Command VEC,78 (Software Reset)
during normal operation. Asserting a software reset will cause all temporary settings to be
reverted to last known permanent values or factory defaults, the GNSS receiver is reset,
and the locking process is cleared and restarted.

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2.5 Firmware Updates
All firmware updates are performed through VDOM Monitor 3. The PC running VDOM 3
should have direct and dedicated serial access to the module’s RX and TX pins for the
duration of this process. The reset line should be pulled high or left floating for the entire
duration of any firmware update process (unless otherwise specified). Do not reset,
disconnect, or power cycle the module during any firmware update process.
In-field updates to various components and configurations of the device are supported
after hardware revision 2.0 and forward. Updating the controller can be done through
VDOM Monitor 3 using firmware package files provided by Vectron. If the firmware update
is successful, the module should reboot itself upon completion and resume normal
operation. In the case that the update fails for any reason, the module will not boot and
VDOM Monitor 3 will be unable to recognize and automatically connect to it. In this case,
with VDOM Monitor 3 disconnected from any module, follow the normal sequence for
updating firmware. When the firmware update is started, you will be prompted to select a
COM port on which the device should be connected. Select the appropriate port, manually
reset or power cycle the unit, then click continue (within 5 seconds of the reset or power
cycle). The process should then continue as normal. See the VDOM 3 Users’ Guide for
more information on firmware update procedures.

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2.6 Operation Modes & Condition Registers
2.6.1 Operation Modes
Opmode
Description
1
Evaluating Reference
2
Frequency Locking
3
Phase Alignment
4
Phase Locking
5
Manual Holdover
6
Auto Holdover
8
Test Mode
153
FAULT
2.6.2 Condition Register
VEC,00 Condition Register
Group
Bit
Field
Description
Error
0
Tune Limit
OSC EFC approaching limits
1
Antenna Current High
GNSS Antenna short / high current
2
Out of Temp Range
Device out of temp specification range
3
AHO Expired
Auto-holdover timer expired
Status
4
Lock
Unit is locked to reference
5
Status
Unit OK
6
TRAIM OK
TRAIM Solution OK
7
Holdover Ready
Unit is ready to enter holdover
Reference
8
Track GNSS
GNSS Reference Enable / Selected
9
Track External
External PPS Reference Enabled / Selected
10
Have GNSS
GNSS PPS Detected
11
Have External
External PPS Detected
Alerts
12
Offset Active
Hitless External PPS Offset is Active
13
RESERVED
<reserved>
14
BIT Fail
Build-In-Test Failure
15
Holdover Active
Unit is in holdover

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Chapter 3
Device Installation
3.1 Serial RX and TX connections
While the device will run without any additional control via the serial interface, it is
recommended to provide access to the serial RX and TX pins on the customer PCB. A PC
running VDOM Monitor 3 with a direct connection (through a USB-to-TTL-Serial converter,
for example) to the device is required to perform any firmware updates. A header
providing access to RX, TX, and GND is recommended, along with a jumper or switch to
disconnect the RX line from another microcontroller or logic on the board, if applicable in
the customer design.
The module’s RX line must have an external 10k Ohm pull-up resistor to VCC attached. If
not used, the TX line should be left floating.
3.2 Recommended Minimal Connections
In addition to the serial connections outlined above, the customer should provide 3.3 volts
to the VCC connections and an antenna supply to the VCC ANT pad (typical GPS/GNSS
antenna voltage is 5 volts. Do not exceed 5 volts on this pad). Both VCC and VCC ANT
connections should have a 100uF tantalum bypass capacitors near the module.
The reset_n connection should, if not driven by external circuitry, be connected to a 10k
Ohm pull-up resistor.
The GPS/GNSS antenna pin (ANT) should be connected to a 50-ohm impedance line
connecting to the antenna or antenna connector. Keep this line as short as possible by
placing the antenna or RF connector as close to the ANT pin as possible. The area around
the ANT pad as well as the customer’s antenna trace should be kept clear of any other
high-speed lines and should be provided with a proper ground plane.
It is recommended that the PPS and RF outputs are connected to external circuitry
through appropriate 3.3 volt CMOS buffers to protect the module from external short
circuits or other interference.