Circuit assistance: DTR high on cellular module

Setting low power mode on the sim900 modem requires a command and then setting dtr high. Taking it out of sleep requires setting dtr low.

Any experience in this stunt?

Yep, that was my plan. But there has been talk about exposing other pins to monitor status etc.

Maybe there was some mistakes I could avoid.

The cellular module from GHI does not have the DTR line connected so you may need to modify the board to use this.

How are you finding it with regards to SMS? With the driver and a SIM5320E I was not getting any SMS reception. I’ve had to modify the commands to get the driver to work with the SIM5320E and tell it to use ME as the destination for all messages.

The driver is a little strange though as there is no correlation with sending and receiving of commands. For example, I had an error with sending SMS but didn’t know there was one because the sending of the AT command was not waiting for the reply.

I am working my way through the driver but it needs a lot of changes to work with pure NETMF as there is no CheckAndInvoke for instance. I have it working now and I can send and receive SMS.

My next big challenge is to get the MUX protocol working so that we can do both SMS and DATA at the same time. Right now, if you use GPRS you can’t receive any SMS or calls. That’s going to need a lot more work and may need to be integrated with what GHI has or some kind of hook into the PPP driver to allow this to work.

I was aware of that, and therefore my circuit assistance heading, I think the pins on the SIM900 module are a little too small for my soldering skills, and was looking for at shortcut to get access to it. I cant seem to find other places on the PCB where the DTR pin actually is exposed…?

Last time I tried SMS it was working ok, but have not tried SMS in the latest and greatest version of the driver. The stuff @ John had been doing looked promising.

As usual you are ambitious :whistle:

Maybe you should @ Byrons old code, I did a few rewrites of his code to make it work, without luck. But the codebase gave some good inspiration.

I have still not been able to pull of a real-world system using the cellular features, partly because of the power requirements, and partly because it is unstable. I am on my third year with the cellular module waiting for coolness.

If the pin is not connected on the schematic to anything then there is unlikely to be able track from the pin. They are not too bad to solder to if you have small bit and a steady hand. :slight_smile:

Same here and waiting patiently for the PPP library later this month. I had a very good meeting yesterday with a client and he wants 50 of my new G400 based designs with some customisation but I desperately need the PPP to work with this modem.

@ John - do you have any information indication of an expected work effort here?

@ njbuch - Expected work effort?

@ John - Yeah, did you plan to do some work on this driver in the nearest future, or are we left with our own incompetencies :wink:

@ njbuch - I do plan to take another look at it before the next SDK. No promises on anything specific yet.

1 Like


@ njbuch I have that working.

Actually we’ve been working internally on a GPRS module that fits our requirements for IoT projects. (I guess those shouldn’t be that much different from anybody else’s…)

It’s based on the SIM800H. We’ve focused on size, low power and exposing only the absolute minimum pins.
We have implemented SMS, TPC with/without SSL, HTTP/S, SNTP and location services. Voice support was left out as IoT modules are usually not very talkative :wink:
We’ve been successfully connecting to general Web APIs, Azure Service Bus and IoT Hub. For SB and IoT with both HTTPS and AMQP.

The board measures 39x48[mm]. I’m attaching a photo.
If this of interest to anyone out there we can release this as a product and publish the driver as a Nuget package.


@ JSimoes - I am very interested. Please provide more details.

Is the driver NETMF? What is the power requirements? Expected price? When can I have one? Does it come with a prototyping version with pins/headers?

Since this is based on the SIM800H, you could use the open-source driver I wrote for the Adafruit SIM800L, though you would need to add a couple commands for SSL (which the SIM800L does not support). If someone sends me one of these, I will update the driver to support it and other 800H variants. I am currently revising the driver to support the SIM808, which includes GPS functionality.

1 Like

Sounds awesome, did not know that. Which ones of these do you recommend for starter package? And can I have a look at the driver?

There is another thread here with valuable info: [url][/url]

In Europe, I would recommend the [url][/url] for 2G (there are uFL and SMA versions).
If you want 3G coverage, then [url][/url] (this is the Euro one. there is another for the US, and uFL and SMA versions)
If you want 2G+GPS, then [url][/url] (again, there are uFL and SMA versions)

All of these require a LiPo battery, and uFL or SMA antenna, and the 808 requires a GPS passive antenna (which are linked to from the individual product pages).

Note that the 800, which is based on SIM800L, does not do SSL. The 808 does do SSL, as does the 3G one, which is based on SIM5320E.

My driver code (tested with the 800) is here : [url][/url] It does not currently include SSL support.
I’ll happily update the code in exchange for a device, but I have so far not had a reason to lay out the money myself to cover the newer variants of the device.

@ mcalsyn - Awesome, how low can it go on power consumption?

There’s a current draw profile on pg 60 of this doc : [url][/url]

50uA in ‘off’ mode, 1mA in sleep and up to 2A in bursts during transmit at the highest power. The profile is a bit complicated based on what the chip is doing. 300-500mA during GPRS sessions it not unusual, but again, there are a lot of variables, not least of which includes how reachable the nearest tower is.

I should also mention that the adafruit devices (all but the unpopulated breakout) include LiPo charging circuits and require a dedicated battery. That can be problematic in a lot of designs. If that’s a problem, better to just get a SIM800H/L/808/5320E module or search around for another brand that doesn’t include the power block. For example: [url][/url]

@ njbuch -

  1. Absolutely: the driver is NETMF
  2. Power requirements: 3.4 to 4.3V
    the power source has to be able to deliver the 2A peaks (these are really short burst, specially when the GSM signal is week)
    as @ mcalsyn mentioned a LiPo (or LiOn) is highly recommended
  3. We’ve validated SIMCOM claims about the power consumption: 60uA for power off; 0.8mA with minimum functionality; 1.02mA fully funcional idling
  4. to clarify SIM800H is Quad band850/900/1800/1900MHz; GPRS multi-slot class 12/10; GPRS mobile station class B
  5. Despite SIM800H doesn’t have GPS it’s able to call a SIMCOM service that (this is me guessing) maps cell towers across the world and is able to provide a pretty decent location coordinates.
  6. Still fine tuning production but expect around 45 EUR/un for one unit
  7. Available in 3 to 4 weeks
    8 ) No prototyping board available. I guess we could work on something for that although the castellation pins give you plenty of room for soldering

@ JSimoes - love what I am seeing. Perfect size for most applications.

Are you going to share any azure examples? It just happened that I am trying you do the same thing!

@ mcalsyn - what about adding azure iot hub to your driver? :wink: