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Designing with ST87M01: technical FAQs

The questions below are from real customer interactions and cover the recurring design questions that engineers ask once they have moved past evaluation and are actively integrating ST87M01 into a product.

 

The page is organised by theme so you can jump straight to your area of interest. For broader background on NB-IoT, or for help choosing between LPWAN technologies, see NB-IoT essentials and NB-IoT vs LPWAN.

1. Antenna and RF design

These questions come most often from teams designing compact multi-radio devices such as asset trackers, smart labels and small sensor nodes, where antenna performance and PCB size are in direct tension.

Is ground-plane efficiency affected by digital traces that break the plane into narrow strips? Should I re-evaluate the recommended ground sizes when the PCB has heavy digital routing?

Yes, it can be affected if the digital routing fragments the ground plane into narrow strips or introduces slots under or near the RF paths. The key point is not only the total ground area, but its electrical continuity and the quality of the return path. If the digital traces significantly break the plane, the effective RF ground is reduced, and antenna performance may degrade. So yes, we recommend re-evaluating the effective ground size and continuity in that case, especially around the antenna and the 50 ohm RF routes.

Can I achieve the same efficiency with a GNSS active ceramic antenna?

Yes, an active GNSS ceramic patch antenna can deliver good GNSS performance, and in many cases better raw receive gain than a passive solution. That said, it also comes with clear trade-offs compared with the passive antenna approach used here: it is typically larger and more expensive, it requires bias power so it increases current consumption, and its radiation pattern is more directional rather than wide-angle. In addition, the need to power and integrate the active LNA adds design complexity. So while it can improve GNSS performance, it is not always the best choice when cost, power budget and compact integration are key constraints.

If I use an external NB-IoT antenna, how small can my PCB be?

Using an external NB-IoT antenna can allow a smaller PCB, because the radiating element is no longer integrated on the board. If the external antenna already includes its own ground plane and provides good RF performance, the PCB can be very small and mainly host the components. If the antenna does not include its own ground plane, then the PCB must also act as the ground plane, and its size should be as close as possible to lambda/4 for good RF performance.

What is the difference between a PIFA and an Ignion chip antenna?

The main difference is that a PIFA is a PCB-implemented antenna structure with a more custom geometry, while an Ignion chip antenna is a compact, predefined antenna component that is easier to integrate. Both still need the PCB ground, clearance and matching network to be well designed. In terms of performance, neither is automatically better: a well-designed PIFA can give good performance in a product-specific form factor, while an Ignion chip antenna can deliver very good and repeatable performance in a much smaller footprint. The final result mainly depends on the available ground plane, placement, keep-out area and matching, rather than on the antenna type alone.

Is it feasible to integrate the NB-IoT antenna on a PCB as small as 30 x 20 mm?

Yes, it can be feasible, but with important RF trade-offs. At 30 x 20 mm, the PCB is much smaller than the ideal lambda/4 ground reference at NB-IoT low bands, so efficiency will drop, especially in the lower bands. If the design uses a well-optimised antenna, matching network and careful keep-out, it may still be workable, but you should expect lower performance than on a larger board, and validate it with measurements on the final product.

2. Power management and low-power operation

These questions matter most for battery-powered designs aiming at ten-year-plus operation in the field.

Why should I not switch off the ST87M01 completely when it is in sleep mode?

When you cut the ST87M01 power supply, instead of staying in Power Saving Mode (PSM), the context of the last connection is lost. At the next connection, the modem will have to restart a full RF scan to find the right band and operator. This wasted energy has to be balanced with the 1.2 uA consumption when you are in sleep mode. Cutting power completely should be considered only if the sleep period is very long.

Is there a way to supply the ST87M01 power amplifier only during Tx emission, to save power?

Yes, by using the TX_ON indicator pin. This signal, enabled by an AT command, will toggle approximately 1 ms before and after every RF transmission.

For power saving, can I limit the output power of my modem?

No, this is not allowed for devices certified to Class 3 (such as the ST87M01). Note also that a reduction in power output will lead to a reduction in coverage, which may mean the device enters EC1 or EC2 levels sooner than a Class 3 device, requiring more repetitions to deliver a message.

Can I wake up my application host from the modem?

Yes, by using the RING indicator pin. This signal can be used to wake up your host depending on modem activity, with a programmable delay and level.

3. Host integration and hardware interfaces

These questions come up at the schematic stage, when engineers are deciding how the ST87M01 sits in the wider device design.

Can I use the ST87M01 as the host for my application?

No, you need a microcontroller to run your application and to control the ST87M01.

Is the ST87M01 module pre-programmed initially, or does it need to be flashed manually?

Yes, the ST87M01 module is pre-programmed by STMicroelectronics during the manufacturing process with a default firmware version. However, you may need to update this firmware to a newer version to incorporate functional improvements, additional features or security enhancements.

Does the ST87M01 provide GPIOs I can use for my application?

Yes, the ST87 offers 24 GPIOs that can be programmed for LED indication, ADC, traces and similar uses. All can be configured using AT commands.

4. Connectivity and protocols

These questions concern what runs on top of the NB-IoT radio link, and what to choose for different application profiles.

What is the most efficient transport protocol in NB-IoT?

UDP (User Datagram Protocol) is the lightest protocol with low overhead, so we recommend this protocol for applications which can tolerate data losses.

What are the main differences between MQTT and CoAP?

Both are IP-based protocols, but MQTT is TCP-based whereas CoAP is UDP-based. MQTT is most suitable for long-standing connections and large data payloads. CoAP (and LwM2M) are better suited for resource-constrained IoT applications and devices, where the payloads are very small and data losses can be overcome.

What are the differences between TLS and DTLS?

TLS stands for Transport Layer Security. DTLS is an evolution of TLS, targeted at UDP connections. DTLS is more efficient and quicker, while TLS is more reliable as it is based on a stable connection (thanks to TCP).

What is the maximum size of a received frame?

The reception is limited to one IP frame at a time, so 1,600 bytes. If there is more data to receive, you will get a message from the modem.

5. Software-defined radio and multi-radio capability

These two questions sit together because they describe both what the ST87M01 software-defined radio enables and the practical boundary of its Wi-Fi mode.

What are the benefits and drawbacks of a software-defined radio?

The drawback can be the consumption in specific use cases. The benefit is its full versatility: it can be updated to be compliant with the latest NB-IoT evolutions, and it can be programmed to support different protocols like GNSS, W M-Bus, ADS-B, Wi-Fi positioning and others.

Is the ST87M01 capable of receiving Wi-Fi frames?

No. The ST87M01 can only retrieve the Wi-Fi hotspot identifiers provided in the header. The frame payload cannot be decoded.

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