#46, H2: Cat-M1/NB-IoT Modem

Function: Cat-M1/NB-IoT modem

Form factor: H2

Category: Input/output module

Special needs: [SER]

Power requirements: 5V/300mA average

Temperature range: –40°C to +85°C

Mates with: ---

See also: #45-1~3, #47

Details

This Tibbit comes in the H2 hybrid form factor and is based on the SIM7000G NB-IoT (Cat-M1/Cat-NB/LTE/GSM) modem manufactured by SIMCom. It features an SMA connector on the front for use with an external antenna, which is not included with the Tibbit but is required for it to operate. This Tibbit is compatible with LTE Cat-M1 and NB-IoT services.

The modem has standard TX, RX, RTS, and CTS signals, as well as several additional control lines.

Control Lines

This Tibbit has four control lines:

• –SDWN (shutdown): When LOW, turns off the Tibbit's power regulators. When the line is switched to HIGH, the power regulators are enabled, but the modem remains off.
• PWRKEY (power key): A LOW-to-HIGH transition on this line toggles the power on and off; it turns the modem on if it was off or off if it was on. Toggling the module on requires keeping a HIGH state of signal (after a transition from LOW) for about one second (see Fig. 1). Turning the module off requires keeping a HIGH state for about 1,200ms (see Fig. 2).

• STATUS: This line is LOW when the modem is off and HIGH when it is on. This line can be used to check the current power state of the modem. It takes about five seconds for the STATUS line to reach HIGH after the modem is powered on, after which the module can begin operating. Note that the line's state is undetermined when the modem is shut down (i.e., the –SDWN line is LOW).
• RESET: Bringing this line HIGH for about one second resets the modem. There is no need to reset the modem after turning on its power.

Before establishing a connection, the modem should be initialized. First, pull the –SDWN line HIGH to enable the power regulators. Next, toggle the PWRKEY line HIGH for about one second to turn on the modem. After the STATUS line goes HIGH, the modem is ready to communicate using AT commands via a standard UART interface with RTS/CTS flow control. At this point, you can decide whether to allow the system to establish a PPP connection automatically or to remain in AT command mode (see the Library section below).

SIM Card Slot

The SIM card holder in Tibbit #46 accepts a micro-SIM card and features a "close-and-slide" locking mechanism. The card holder is located on the bottom of the Tibbit, meaning that insertion or removal of the SIM card requires opening the Tibbo Project Box (TPB) enclosure and removing the Tibbit from the Tibbo Project PCB (TPP). We chose this relatively inaccessible location for the SIM card to reduce the chances of theft — accessing the card holder requires quite a bit of time and work, making it impossible to remove the card surreptitiously.

LEDs

There is one red and one green LED. The red LED is connected to the TX line, while the green LED is linked to the RX line. The remaining lines are not equipped with LEDs due to the internal board's space constraints.

Power Considerations

At full bandwidth, Tibbit #46 consumes an average current of 300mA, with peak current consumption of about 2A. While these current "spikes" are so narrow that they can't be seen on a regular multimeter, they occur regularly when the modem is operating. We advise using an adequate power source to prevent unexpected reboots or unstable operation. Consumption can fall to as little as 100mA in low-bandwidth usage scenarios.

The Tibbo Project System (TPS) platform offers several power supply Tibbits capable of providing the necessary current.

The TPS Online Configurator can help you decide which parts to use in conjunction with Tibbit #46. The configurator lists the Tibbit as consuming 300mA of current, which we found to be a good approximation of the current burden it places on a power source, but does not take into account the spikes or that it often operates with lower power utilization.

While Tibbit #46 supports power-saving mode (PSM), Tibbo does not currently supply a Tibbo BASIC library for its management. PSM places the Tibbit into a state similar to the power-off mode, but the connection does not need to be reestablished after the module is powered back on. In other words, the link is "remembered" even while the Tibbit is powered off. This provides massive power savings for low-power applications. If you need to use the PSM, please refer to the SIM7000G manual for instructions on its implementation. Tibbo supplies the CELL library (see below), which has provisions for the AT command mode that can be used to implement the PSM manually.

Operating Temperature

In testing, this Tibbit operated normally in the –40°C to +85°C range. However, SIMCom recommends the –30°C to +80°C operating temperature range for its modem IC and warns of reduced performance when operating outside of this range.

In addition to the temperature constraints on the module, there are also limitations for off-the-shelf SIM cards. While regular consumer SIM cards are certified to operate in a temperature range of –25°C to +85°C, industrial/automotive SIM cards can operate from –40°C to +85°C, with some reaching even up to +105°C. In internal testing, some consumer SIM cards deformed at high and low temperatures. Therefore, we highly recommend that you assess your application's projected environmental conditions and choose an appropriate SIM card.

Library Support

The CELL Library unlocks the capabilities of this Tibbit and can be easily included in your project through CODY, Tibbo's hardware configuration and project code generator.

The library allows you to automate the process of initializing the modem and automatically establish and maintain a PPP connection. It also offers an AT command mode for manual control of the SIMCom modem. When using the library, make sure to select which service — Cat-M1 or NB-IoT — is provided by your network operator.

Additional Considerations

As Cat-M1 and NB-IoT services provided by telecoms are typically cheaper than LTE (4G), their appeal for mass deployment is obvious. However, before subscribing to any telecommunications service, you must properly assess your project's needs.

The most important criterion is whether your application will be mobile or stationary. While Cat-M1 is suitable for either, NB-IoT is specifically intended for stationary applications only. For example, if your application is being deployed in a vehicle, Cat-M1 should be chosen because it supports tower handovers. By contrast, NB-IoT does not support such handovers, but is ideal for stationary applications with low data requirements, such as vending machines, street lighting, as well as heating, ventilation, and air-conditioning (HVAC) systems.

You must also factor in your project's expected bandwidth usage. If you need to transfer large amounts of data relatively quickly, Cat-M1 will be more suitable. If your system will only be transmitting small amounts of data that are not time-critical, NB-IoT will be sufficient for your needs and likely more cost-effective. If you require significant bandwidth for longer periods, Tibbo recommends that you consider using LTE(4G) and our Tibbit #45-1~3.

Finally, consider the cellular signal strength at your target deployment locations. NB-IoT has better signal penetration than LTE(4G) and Cat-M1. This means that if NB-IoT service is available at all, it may have a better signal reception. At the same time, NB-IoT usually has a much less stable "spot" bandwidth, meaning that the actual data throughput fluctuates wildly from one moment to the next.