homechevron_rightDocschevron_rightDatasheetschevron_rightApplication Noteschevron_rightStarnote Carrier Board Design Guide

Starnote Carrier Board Design Guide

Starnote is the Blues satellite companion device. Like Notecard, it uses a standard M.2 Key E connector and can be embedded into a custom carrier board. Starnote is offered as Starnote for Skylo and Starnote for Iridium, which use the same M.2 Key E form factor.

This guide covers the Starnote-specific considerations involved in designing a custom carrier board: mounting, power, and the satellite antenna. Because every Starnote design also includes a Notecard, it should be read alongside the Notecard Carrier Board Design Guide.

note

Starnote is always paired with a Notecard

Starnote is not a standalone device. It works alongside a Notecard (e.g. Notecard Cellular, Notecard Cell+WiFi, or Notecard WiFi), adding satellite as an available radio access technology. The host communicates with Notecard, and Notecard in turn communicates with Starnote over UART.

As a result, a Starnote-capable carrier must accommodate both devices. For the Notecard side of your design (e.g. host serial interfaces, power, SIM, GPS, AUX pins, DFU, and more) follow the Notecard Carrier Board Design Guide. Note that Notecard for Skylo is a Notecard for the purposes of these guides, and is covered in that guide.

note

Blues always recommends starting from an existing Notecarrier design!

All Notecarrier design files are open source and available in the note-hardware GitHub repository. These Notecarriers all adhere to the concepts described below, and starting from one is the fastest, most reliable path to a working custom carrier board. We suggest Notecarrier XS for Starnote for Skylo and Notecarrier XI for Starnote for Iridium.

Table of Contents

Required Items

The following items are required for any host product that embeds a Blues Starnote.

M.2 Connector and Mounting

Starnote plugs into a standard M.2 Key E connector such as Amphenol MDT420E01001 (the same connector used by Notecard), and is secured along the opposite edge to a grounded standoff by a mounting screw. Blues reference designs use a Würth Elektronik 9774025151R paired with an M2.5x4 metric machine screw.

Because Starnote is larger than Notecard and its mounting-hole position differs (both from Notecard and between Starnote for Skylo and Starnote for Iridium), the standoff must be placed to match the specific device the carrier is designed for. See Footprint and Mounting for more information.

Notecard-to-Starnote Link

Because a Starnote-capable carrier must accommodate both devices, plan for two M.2 Key E connectors: one for Notecard and one for Starnote, linked over UART.

  • Starnote for Skylo can alternatively connect to a Notecard through its 6-pin JST connector and a connector cable instead of a second M.2 connector.
  • Starnote for Iridium connects to a Notecard over its M.2 connector exclusively.

Because the Notecard-to-Starnote link uses UART, the I2C interface can be left available as the host's interface to Notecard. For that reason we recommend routing both the UART and I2C interfaces on the Notecard side of your carrier; see Serial Notecard Request Interfaces in the Notecard Carrier Board Design Guide.

Notecarrier XS Block Diagram

The block diagram below shows how the Notecarrier XS reference design (used for Starnote for Skylo only) puts this together. Power arrives over USB-C or the battery JST and is combined by the charger into the main system rail (VMAIN), which supplies VMODEM directly and derives the 3.3V logic rail (VIO) through a buck regulator; both rails reach Notecard and Starnote. UART carries the dedicated Notecard-to-Starnote link, while the pin header exposes I2C, AUX UART, AUX GPIO, DFU, NRST, and ATTN from Notecard to the host:

Notecarrier XS block diagram

This is a simplified view intended to show the overall topology. For the complete circuit detail behind it, including an interactive schematic viewer, the schematic PDF, and KiCad project, see the Notecarrier XS design files.

Power

Starnote is designed to be powered continuously and should be left powered at all times. Provide constant power to VIO_P and VMODEM_P, and avoid power-cycling or resetting the device after boot. See the Low-Power Hardware Design application note for more on power architecture.

warning

For Starnote, a power-loss event (i.e. losing power on VMODEM_P or VIO_P, or pulling the RST pin low) risks rendering the satellite modem permanently unusable. It is the designer's responsibility to ensure that Starnote-based designs provide constant power to VMODEM_P and VIO_P.

Replicating the Reference Power Architecture

The power subsystem is the part of a Starnote carrier that is best implemented by copying the matching Notecarrier verbatim, especially for Starnote for Iridium, whose transmit-burst current demands are the single most important thing to get right. Both reference designs share the same front end and differ only in how they generate VMODEM_P.

The shared front end (Notecarrier XI and Notecarrier XS) is:

  1. Source OR-ing. OR-ing is a small circuit (built from diodes or an ideal-diode controller) that combines several power inputs onto one rail: the board runs from whichever source is present, and the sources are blocked from back-feeding one another. Notecarrier XS OR-s its USB-C and ESLOV inputs with an LM66200 ideal-diode OR; Notecarrier XI OR-s its two USB-C inputs and its solar input with Schottky diodes. The LiPo battery is not part of this OR.
  2. Battery charger with power path (BQ25628). The OR-ed input feeds the charger, and the LiPo battery connects to the charger's dedicated battery (BAT) terminal. Through its integrated power path and buck stage, the charger produces the main system rail (VMAIN) that feeds everything downstream, drawing from the external input when present and from the battery otherwise.
  3. Buck-boost regulator (ISL9122) converts VMAIN to the 3.3V logic rail (VIO_P) shared by both Notecard and Starnote. The reference budgets ~150mA for Starnote on this rail.

The difference is the VMODEM_P supply: Starnote for Skylo takes it straight from VMAIN, while Starnote for Iridium inserts a supercapacitor charger/balancer between VMAIN and VMODEM_P.

Starnote for Iridium

Notecarrier XI reference power tree for Starnote for Iridium

Starnote for Iridium's satellite modem (an Iridium 9704) draws large current bursts during transmit. Internally, Starnote for Iridium re-regulates VMODEM_P to a 4.1V rail whose power traces and load switch are dimensioned for up to 8A and configured to deliver on the order of 5A peak to the modem's power amplifier. A battery or upstream supply generally cannot source those bursts cleanly on its own without the VMODEM_P rail sagging.

The Notecarrier XI reference solves this by feeding VMODEM_P from a buck-boost supercapacitor charger/balancer (LTC3128) that charges two 10F, 3V supercapacitors (Eaton/Maxwell BCAP0010) and holds the output steady. The supercapacitors act as a local energy reservoir: the LTC3128 draws a modest, current-limited average from VMAIN to keep them topped up, and the caps themselves deliver the high-current transmit bursts. Because Starnote for Iridium buck-boosts VMODEM_P internally, the priority is a constant supply that can source the burst current, which the supercapacitor reservoir provides.

warning

Do not feed a custom Starnote for Iridium carrier's VMODEM_P directly from a battery or a general-purpose regulator sized only for average current. Without a supercapacitor reservoir (or an equivalent supply proven to source the modem's transmit bursts), VMODEM_P can collapse during transmit. As with any loss of power on VMODEM_P, this risks rendering the satellite modem permanently unusable.

tip

Replicate the LTC3128 supercapacitor charger/balancer stage (the two supercapacitors, the balancing network, and the output/current-limit settings) from the Notecarrier XI power schematic to reproduce its burst-reservoir behavior and its nominal 4.83V VMODEM_P output. The exact schematic is in the note-hardware GitHub repository.

Starnote for Skylo

Notecarrier XS reference power tree for Starnote for Skylo

Starnote for Skylo is simpler: no supercapacitors are required. The Notecarrier XS reference connects VMODEM_P directly to VMAIN (through a 0Ω link), so Starnote for Skylo's VMODEM_P input is driven straight from the carrier's charger/power-path rail. On the Starnote itself, VMODEM_P is then re-regulated by an on-board buck-boost and load switch before it reaches the satellite modem (a CC660D-LS). Because the modem's current draw increases significantly when active, size the VMODEM_P and GND traces for the sustained current and connect VMODEM_P to a supply capable of 350mA or more, as described below.

VMODEM

VMODEM_P powers the device's radio and must be applied to pins 70, 72, and 74 of the M.2 connector. The supply requirements depend on the product:

  • Starnote for Skylo: 2.5VDC to 5.5VDC, capable of sustaining 350mA or more. Connect it directly to the battery/power-path rail and size the VMODEM_P and GND traces to handle this sustained current.
  • Starnote for Iridium: a nominal 4.83VDC rail supplied from a supercapacitor charger/balancer (as in the Notecarrier XI reference) so the rail stays constant and can source the modem's transmit bursts. See Replicating the Reference Power Architecture above.

VIO

VIO_P powers the device's MCU and peripherals and must be applied to pins 2 and 4 of the M.2 connector. Supply either 1.8V or 3.3V. In both reference designs VIO_P is generated by an ISL9122 buck-boost regulator and shared between Notecard and Starnote; budget ~150mA for the Starnote on this rail.

Ground

The 12 M.2 connector pins labeled as GND must be connected to the ground of the host system (pins 3, 5, 6, 11, 18, 33, 39, 45, 51, 57, 71, and 73).

Satellite Antenna(s)

Starnote connects to the satellite network through a U.FL antenna connection, and the host system must provide an appropriate antenna. Requirements differ by product:

  • Starnote for Skylo (u.FL variant) provides two U.FL connectors, one for the satellite radio (labeled SAT) and one for GPS/GNSS, and operates on S-band and L-band (bands B23, B255, and B256).
  • Starnote for Skylo (Ignion variant) integrates both antennas on-board, so no external antenna connection is required.
  • Starnote for Iridium provides a single U.FL connector for an Iridium-certified L-band antenna that also serves GPS/GNSS.

Satellite antennas require a clear, unobstructed view of the sky and are intended for outdoor placement. Be sure to observe a 5mm keep-out area around all antennas in all directions, avoiding ground planes, metal screws, metal components, and metal enclosures to avoid RF interference. Please see the Antenna Guide application note and the Starnote datasheet antenna requirements for Skylo and Iridium for additional information.

Recommended Items

Footprint and Mounting

Starnote follows the same swappable philosophy as Notecard but is physically larger, and its mounting-hole position differs by product. Plan for the largest footprint and the correct standoff location for the device(s) you intend to support:

  • Starnote for Skylo: 30mm x 42mm (u.FL variant), or 30mm x 60mm for the Ignion variant, which integrates its antennas.
  • Starnote for Iridium: 42mm x 42mm (wider than any Notecard) and 16g, which also warrants attention to mechanical mounting.

Because the standoff and mounting-hole positions differ between Notecards, Starnote for Skylo, and Starnote for Iridium, each Starnote is validated only against its matching Notecarrier (e.g. Notecarrier XS for Skylo and Notecarrier XI for Iridium).

Designing in the Notecard

Every Starnote design also includes a Notecard, and the host communicates with Starnote through that Notecard. For the rest of your carrier design (the host serial request interfaces, Notecard power, external SIM, cellular antenna(s), outboard DFU, GPS antenna, and the AUX, reset, and enable pins), follow the Notecard Carrier Board Design Guide.