Designing for XP Variants of the Blues Notecard

Introduction

The numerous variants of the Notecard collectively referred to as "XP" variants have the following high-level differences from their non-XP counterparts bearing the same SKU designators:

1. They are designed to have an "eXternal Power supply" design for powering the on-board modem, as compared with the standard Notecard whose design incorporates an internal power supply focused on extreme robustness.

2. They are designed to offer flexibility between three SIM options: one MFF2 or USON-6 SIM, another MFF2 SIM, and a plastic nano-SIM. This contrasts with the standard Notecard, which features a fixed SIM design with one onboard SIM and the option to use a plastic nano-SIM via a Notecarrier.

3. Working in conjunction with the Notecard firmware, the XP variants enable certain "a la carte" manufacturing options to support refinement of customer SKUs.

A subset of Notecard SKUs are available in XP variants. XP-designed Notecards are not compatible with non-XP Notecarriers, nor vice-versa. Although it will not work to plug an XP Notecard variant into a non-XP standard Notecarrier, and although it will not work to plug a standard Notecard into a Notecarrier designed for XP, doing so will not be electrically harmful to either the Notecard or the Notecarrier. That is, there should be no worry about "magic smoke" or harm.

Hardware Design Considerations

Physical Design

The serigraphy on the customer's Notecarrier SHOULD have a clear designation that it is for "Notecard (XP ONLY)" in the area underneath where the M.2 Notecard is inserted.

SIM Design

The hardware designer must decide whether they are designing for an XP variant which has a "Notecard-provided SIM" or a "customer-provided SIM", because the XP variants do not support dynamic SIM switching. In the Notecard documentation, instructions are provided for attaching an external SIM to the Notecard.

If using a Notecard-provided SIM on an XP variant, the designer MUST ignore all the M.2 pinouts related to SIM. If using a customer-provided SIM on an XP variant, the designer MUST comply with all of the M.2 pinouts related to the SIM.

Outboard DFU Design

The standard Notecard supports the Notecard Outboard Firmware Update (ODFU) function on its AUX pins, where their function is multiplexed with numerous other AUX functions. More recent models of the Notecard also support five dedicated M.2 pins devoted to ODFU, referred to as the Alternate DFU (ALT_DFU) pins. Because certain XP-specific AUX functions may be provided in the future, all XP variants utilizing ODFU MUST use the new dedicated ALT_DFU pins for the ODFU function, and certainly SHOULD implement ODFU if they are designed to utilize an MCU type that is supported by ODFU.

Power-Fail Resilience

All Notecarrier designs which are powered by a potentially-interruptable "line power" source (rather than batteries) SHOULD incorporate power-fail resilience into their designs for two reasons:

1. Although no modem manufacturer will highlight this fact, all commercially-available modems are accompanied by warnings related to the fact that there is a small risk their internal file systems may become corrupted if power is unexpectedly removed without a clean soft power-down procedure. These modems all have software-based mechanisms by which they are supposed to recover, but for all there is a small chance that they will become "bricked". For example, the manufacturer of the modems included on some Notecard SKUs warns in their hardware datasheet: "NOTE: To avoid damaging internal flash, do not switch off the power supply when the module works normally. Only after the module its shut down by PWRKEY or AT command, then the power supply can be cut off." [SIC]

2. In implementing its extreme "bandwidth savings" mechanisms, the Notecard works closely in conjunction with the Notehub such that each one intimately understands the state of the other. That is, in the process of "database and queue state synchronization", the Notehub knows precisely what data is present on the Notecard, and the Notecard knows precisely what data is on the Notehub, so that neither needs to redundantly send "changes" to the other. In order to ensure this process is correct and robust, every time the Notecard is restarted from a power-off state, the Notecard will do what is referred to as a "full state synchronization" upon its first connection to the Notehub. This generally takes in the realm of 25KB-50KB of bandwidth, but it could be more depending upon how much customer data is stored and queued on the Notecard. If there is a sudden power failure, the first connection between the Notecard and the Notehub will always perform this resynchronization procedure (which takes more bandwidth than usual) in order to ensure robust behavior on an ongoing basis.

Because of these, when making tradeoffs of cost vs risk behavior, it is strongly recommended that even line-powered hardware designs SHOULD incorporate some sort of power-fail resilience into their hardware designs. On the hardware side this generally means incorporating a Lithium Ion Capacitor like the Blues Scoop (an inexpensive component that exhibits properties of super capacitors and also of LiPo batteries, but which is extremely safe, long lasting, and can be shipped globally without declarations).

All of this said, for line-powered applications where power fails are quite infrequent in practice, and where cost considerations prohibit adding a power-fail resistance mechanism which could add $2-$3 dollars to the BOM, it is understandable that the customer may wish to bear these risks in their overall designs.

Modem Power Supply Design

The Notecard's datasheet specifies that the Notecard's modem (the VMODEM net as labelled on the M.2 pins) requires a power supply capable of providing 2A bursts of energy, because of the requirements of modems when they attempt to power-on and do their initial transmissions to distant cell towers over GSM or even LTE.

Notwithstanding this requirement, it is the rule, not even the exception, that Notecards are designed-into equipment where power supplies undergo periods in which they are unable supply the quick bursts of current. Most commonly, this is because:

• A battery is low and, because of its nature, is able to provide an "in-spec" voltage, but is unable to deliver the full burst required.
• The temperature conditions are low enough such that they are impacting a battery's ability to deliver the full burst required.
• Another component is sharing the core power supply and is impacting the power supply's ability to provide the current burst.
• A developer is powering a Notecard through a Notecarrier using a USB cable that, without realizing it, is incapable of delivering more than 500mA.

In order to provide an unprecedented level of resilience, the Notecard has a robust on-board power supply design which consists of:

• The ability to switch the modem off when it is not in use, with a shutdown current budget of less than 1uA so that energy is not wasted when the modem is off.
• A 2A buck-boost regulator with an input range of 2.5V-5.5V and an output voltage that is within spec of the on-board modem.
• A large pool of capacitance - specifically 1320uF implemented by using six tiny capacitors each of 220uF - to assist in satisfying the burst requirement of the modem.

On the XP variants of the Notecard, this modem power supply is externalized and becomes the sole responsibility of the Notecarrier. The interface to this power supply is manifested by using three new pins which are unused, and which must be "not connected" on standard Notecarrier designs, as well as incorporate new restrictions on the VMODEM line:

1. M.2 Pin 1: XP_VREF is a new and essential INPUT pin to the Notecard. This is an analog input pin that must be in the range of 2.5V-5V. The function of this pin is to tell the Notecard what voltage will be reported by the card.voltage API request, and will be used for all voltage-variable operations within the Notecard.
2. M.2 Pin 75: XP_PRESENT is a new OUTPUT pin from the Notecard that can be used by Notecarriers to change their behavior in the presence of an XP Notecard. If this is an XP variant of a Notecard, this pin will be output as HIGH at the VIO level supplied to the Notecard on M.2 Pin 2. If this is not an XP variant of the Notecard, this pin will be floating (not connected). In order to minimize wasted energy, we recommend use of a very weak pulldown (10M) in order to detect the level of this line.
3. M.2 Pin 23: EN_MODEM is a new OUTPUT pin from the Notecard. This is a digital output pin at the VIO level supplied to the Notecard on M.2 Pin 2 that will be either HIGH or LOW at any given time. It will be HIGH when the Notecard requires power to be immediately supplied on the VMODEM pin, and it will be LOW when it does not care whether or not power is supplied to the VMODEM pin as the Notecard will not be using it.
4. M.2 Pins 70,72,74: VMODEM are pins defined by the standard Notecard as needing to be between 2.5V-5.5V with 1A continuous and 2A burst capability, and that VMODEM needs to be continuously supplied to the Notecard. On the standard Notecard, the card.voltage API request reports the voltage of this line, which is most commonly connected directly to a battery. This input is redefined in XP variants:
   1. First, although it MAY be continuously supplied and the Notecard will draw less than 1uA when it is not being used, the requirements are reduced in that it MUST be supplied only when the EN_MODEM output line is HIGH.
   2. Second, when it is enabled, the VMODEM voltage MUST be a regulated supply that is within the range and current requirements of the specific modem present on the customer-selected XP variant of the Notecard. Although you should consult the specific datasheet for your XP modem variant, as of this writing all modems supplied with Notecards will operate with a regulated 3V8 power supply capable of 1A continuous and 2A burst.

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