PTPH1 Precision Time Protocol & Holdover

J5 carries the 5 V supply and the CAN bus. The 5 V on pin 2 is an input: in the 5V CAN position the board draws its supply from it (about 0.76 A typical, see the specifications table), so the supply on the bus must be able to deliver that. The mating connector is the Phoenix Contact 1027605. Slide switch S1 chooses the supply: in the 5V CAN position the board runs from J5 pin 2, in the 5V JACK position it runs from the DC jack. The CAN transceiver and the termination are not affected by the switch.

Fitting a jumper on JP1 connects a 120 Ω termination resistor across the CAN bus. Fit it only on the two nodes at the ends of the bus.

J4 is designed for use with the ST-LINK-V3MINIE and can be used to debug the board or load firmware onto the microcontroller. The debugger must work at the 3.3 V logic level of the board (pin 3). The SWO and virtual COM port pins are not connected. Mating part: 14-pin STDC14 Cortex ribbon cable.

J3 is a USB Type-C connector wired to the USB device port of the mosaic-T, with ESD protection (USBLC6-2P6). It is a data connection only: the board is not powered from it. Connect it to a host computer to reach the mosaic-T web interface, which the receiver serves over USB at http://192.168.3.1 (the default address given by Septentrio for its USB Ethernet interface).

T1 is an RJ45 connector with integrated magnetics for a 10/100 Base-T network. A KSZ8041NLI Ethernet PHY connects it to the Ethernet (RMII) interface of the mosaic-T, which provides the web interface, the NTP server and the PTP engine. The PTPH1 firmware turns the receiver’s Ethernet interface on; the network address, NTP and PTP settings are those of the mosaic-T and are set through its web interface (references [2] and [3]).

ANT1 (J1) is the GNSS antenna input of the mosaic-T. The receiver supplies the antenna with 3.3 V on the same SMA connector (the supply pin of the module is tied to the 3.3 V rail), limited to about 150 mA, and switches the supply off on an overcurrent [3]. Use an active antenna rated for a 3.3 V supply, or one that does not need a supply with a DC block fitted. ANT2 (J2) is wired to the second antenna pad of the module footprint, which only the dual-antenna modules (mosaic-H) use [3]; the mosaic-T is a single-antenna module, so ANT2 is not used.

Six SMA connectors (10MHz 1 to 10MHz 6) carry the 10 MHz from the SiT5811 oscillator through the LMK00101 clock buffer. A seventh output of the buffer goes to the reference input of the mosaic-T. The frequency is steered to GNSS by the microcontroller, so the outputs are traceable to GNSS time while the receiver is locked, and keep the last frequency when GNSS is lost.

PPS1 to PPS5 are 1 Hz pulse-per-second outputs. They are generated by the AD9545 from the receiver’s PPS, so each pulse starts on the GPS second and is aligned to the 10 MHz outputs by the static skew trim. The pulse width is 100 ms. PPS6 (connector J17, OUT0CN of the AD9545) is not a PPS output: it carries a 5 kHz signal, which is the AD9545’s own zero-delay feedback tap and must not be used as a PPS reference.

The DC jack is an alternative to powering the board from J5. Move S1 to the 5V JACK position. It takes a 5 V power adapter and supplies the board in place of J5. The polarity and plug size are given in the Wurth Elektronik datasheet for 694103107102; the board’s designer notes a 4 mm plug.

A coin cell holder on the bottom side backs up the receiver’s supply (VDD_BAT) when the board loses power, so the mosaic-T keeps its almanac and settings and restarts warm (typically under 20 s) instead of cold (under 45 s) [2]. A BQ25306 charger charges the cell from the 5 V rail. Use only a rechargeable lithium coin cell of a type that suits the charger setting; do not fit a non-rechargeable cell.

Two LEDs (D2 and D3) show the receiver’s status outputs (GPLED); see the mosaic-T hardware manual [3] for their meaning.

The Septentrio mosaic-T has built-in interference monitoring and mitigation, which Septentrio calls AIM+. According to Septentrio, AIM+ monitors for interference and mitigates narrow band, wide band and chirp jammers, and the receiver is described as highly secure against jamming and spoofing [2]. The receiver also supports Galileo OSNMA (navigation message authentication) and RAIM+ (receiver autonomous integrity monitoring) [2]. Septentrio’s product datasheet gives no suppression figures, such as the jammer-to-signal ratio the receiver tolerates, so none are given here.

These functions run inside the mosaic-T. How well a PTPH1 performs near interference also depends on the antenna, where it is placed, and any external filtering or limiting, which are not part of this board.

When interference degrades the receiver’s clock data, the PTPH1 treats it like any other loss of GNSS: after 5 s of unhealthy data the oscillator control word is frozen, and the board keeps its 10 MHz and PPS outputs running for up to one hour. The state is reported in the DroneCAN node status. The holdover was verified by disconnecting the antenna, not by applying jamming or spoofing signals; the PTPH1 has not been tested against either. Whether OSNMA authentication is enabled on a unit has not been verified on this board, and the DroneCAN messages do not report interference or spoofing status.

Every node on a DroneCAN bus has a unique node ID from 1 to 127, which is the source address of the messages it sends. The PTPH1 has no node ID jumpers and no parameter service: its node ID is fixed in the firmware at 42. Give each node on the same bus its own ID and keep the flight controller’s own node ID different from 42.

The board publishes three messages and answers one service. It does not subscribe to any message.

The NodeStatus message reports the board’s health in its health field and the disciplining state of the 10 MHz in the vendor-specific status field. The mode field is INITIALIZATION while the board waits for the oscillator, the AD9545 and the receiver, or for a healthy GNSS clock (states 0 and 1 below), and OPERATIONAL afterwards.

NodeStatus health

Disciplining states (vendor-specific status, bits 2 to 0)

NodeStatus vendor-specific status field (16 bits)

Holdover works as follows. When the receiver’s clock data are unhealthy for 5 s (no fix, no external frequency, stale data, or no PPS), the microcontroller freezes the oscillator’s frequency control word. The receiver then keeps emitting its PPS from the held 10 MHz for up to 3600 s, and the AD9545 keeps following that PPS, so all outputs stay on frequency and time. Only if the serial link to the receiver itself is lost for more than 30 s does the microcontroller also put the AD9545 into its own holdover, so that it coasts on its tuning-word history. When GNSS returns and stays healthy for 30 s, the board steers back to lock; after a one-hour antenna disconnection on the bench the recovery needed only a fine correction.

The board publishes uavcan.equipment.gnss.Fix2 (type ID 1063) when the receiver sends a new position (once per second), and at least every 2 s otherwise. The table lists the fields as defined in the DroneCAN DSDL [7] and the value this board writes.

uavcan.equipment.gnss.Auxiliary (type ID 1061) carries the number of satellites in view and used; all dilution-of-precision fields are NaN.

The firmware configures the mosaic-T at start-up and again if its data stop. It asks for the PVTGeodetic, ReceiverStatus and xPPSOffset blocks and NMEA GGA and GSV messages at 1 Hz on COM1 (115200 baud, 8N1, hardware flow control), turns the receiver’s Ethernet interface on, and sets the PPS output to 1 Hz, rising edge, no delay, GPS time scale, 100 ms wide, with a maximum synchronisation age of 3600 s. The receiver’s clock runs free on the 10 MHz reference: the microcontroller reads its clock bias and drift from the PVTGeodetic block and does not use the receiver’s own clock steering.

PX4 can use the PTPH1 as a GNSS source through its uavcan driver. Set the following parameters (a reboot is needed after changing them). The parameter names are from the PX4 parameter reference [8]. Operation with a flight controller has not been tested.

Connect an ST-LINK-V3MINIE to J4 (STDC14) to load firmware or debug over SWD. The board must be powered; the debugger takes its logic level from pin 3 (3.3 V). The microcontroller is an STM32G0B1KBT6, which STM32CubeProgrammer supports. Do not erase the whole device unless the replacement image is available.