Seeking advice for a cellular modem in an IoT project

Hi All...

I have a device based upon an ESP32 and I would like to add a cellular modem to it. Ideally, it would either work globally or have various versions available for different countries. For now I'm primarily concerned with the US and Canada. My board currently has I2C and SPI available, but not a UART or any other GPIOs for bit banging. Ideally this would be a card I can plug into an expansion header on the I2C bus. I control the board, so I can add headers in whatever physical configuration is needed.

I'm confused about what I need to move data and SMS (although I would be very happy with just data if I can not have both). I'm not at all concerned about voice, as this is an IoT application. I see there are modems available in 2G, 3G, LTE and other things I'm not familiar with. I see Particle makes a few but it appears that their offerings are locked to their cloud. Mostly I want to move data without being locked to a particular vendor cloud. Can that be done? Are protocols, like RPC / HTTP / WS implemented in the modem, or by my application above it?

I'm sure I have other questions once I understand enough to ask them. Thank you!

I’ve been using SIM 5320 (3G) and 7600 (4F) chips for a few years now. They’re well supported and documented, and should do what you want.

The only exception is support for VoLTE voice, which will require one of their newer modules.

The data sheets and application notes are available from SIMCIM (China) There are certainly other manufacturers in the space - read the data and pick one!

You may want to be aware of the cost is around $70 per device to add a cellular modem with LTE-M capability and a bit more if you want 2G/3G fallback features in case LTE-M isn't available.

I've used digi international's xbee cellular modem. They have carrier approvals in multiple countries so you can deploy more easily. You need a serial port. Do you have to have all 3 serial ports for your project? If you reserve one for program upload and console prints, you still have two you can allocate. Can you deallocate one of them when you do your cellular modem thing and reallocate back to whatever is using it?

Are you making your own PCB ?
The chips themselves are a lot cheaper, and only need about $5 worth if support chips…

Hi everyone and thank you for the responses. I have been doing a lot of reading since I posted this, and have a better understanding. Still not great but getting there. I'll checkout the SIM7600. I did run across the xbee cellular modem and it looks like a good option. I also found the Skywire modems. These are global and have GPS built in (as many do) and already meet compliance requirements. I also found some very cheap ones, but while the modem is cheap, the cost of FCC/DOC/CE certification won't be.

It does look like I need a serial port. While many of these have I2C and SPI, it seems those are only for interfacing with an external codec. All three of the UARTs are in use, one for USB and two for RS422. I'm looking at building a daughter card with an I2C to UART bridge, but more likely I'll remove one or both of the RS422 channels.

@lastchancename yes I'm making my own board, and yes I see I can do it with just a small design but as I just mentioned the cost of compliance is killer. I may do it though because I also want to be able to use a satellite modem. Those are available (at great cost) but they will have a different footprint than the cell modem. If I make my own it solves that, but puts me back into the compliance nightmare.

Thanks everyone! If anyone has any thoughts please let me know, and I'll report back when I end up doing.

lastchancename:
Are you making your own PCB ?
The chips themselves are a lot cheaper, and only need about $5 worth if support chips…

What? Which chip is $5, at what qty? u-blox?

The actual modem chips are around $10$-$15, plus the support chips - allow an extra $5. Add them onto your PCB with connectors, and you’re good to go. Depending on what antennas you want, add $2 and upward. Extra fir the GPS antenna!

OK. I'd say that if you do your own layout and get FCC approval, your parts cost is about half of a modular modem, so $35 instead of $70 if going with LTE-M for instance, as long as you have enough qty, say 5K units. It's a more difficult way and not necessarily the goal to achieve before the first couple of prototypes or small productions.

liuzengqiang: OK. I'd say that if you do your own layout and get FCC approval, your parts cost is about half of a modular modem, so $35 instead of $70 if going with LTE-M for instance, as long as you have enough qty, say 5K units. It's a more difficult way and not necessarily the goal to achieve before the first couple of prototypes or small productions.

The plan is to find a product that plugs into headers. That way I can start with a certified product. If demand is sufficient I can always later focus on building my own that is pin compatible, but to start, that's too much time and money. This is going to be a low volume product anyhow, if it even makes it to market. And FCC is relatively easy... CE though...

Then you’re paying someone else for the work.

Nothing wrong with that, that’s what I do, but you probably still need to test and certify the whole assembled product for RF/EMI emissions.

lastchancename:
Then you’re paying someone else for the work.

Nothing wrong with that, that’s what I do, but you probably still need to test and certify the whole assembled product for RF/EMI emissions.

I do. What I don’t know is, if I get the base device certified, and I add a certified device, do I need to recertify the combination? I don’t know, but for now I’m happy to pay someone else to do the work, as you said. Time is money too!