MEGAphone Module Building Progress: Cellular modem and DC:DC modules

Goran has been working on refining the MEGAphone modules I designed earlier in the year. So let's check in on the status of those.

Goran has improved the structure of the castellated pads, so that they can be fabricated more cheaply, without needing to pay the premium for castellation.  A part of this is adding "backup holes" behind the castellations

 

2.4 Cellular Modem, including SIM card Module 

2.4.5: Assembly 

Let's start with the cellular modem module. After fabrication, Goran has assembled it, and checked the fit of the cellular modem:

2.4.7: Revision 

You can kind of see in the photo below, that we had the wrong clip on the left, making it squish too low.

 

This was fixed by revising the part used for that, to one that stands taller:

 Now it all fits together nicely, with clearance:

There was also a complication with the UART voltage selection, as some cellular modules use 1.8V and others 3.3V for this, so Goran had to also do some remediation on that to be able to test it. 

So let's hook it up, and test it:

Without the 1.8V/3.3V fix, we see output from the modem, but we can't talk to it.

After fixing it to bypass the level converter, to allow 3.3V directly into it, we can get two-way communications, i.e., we can send a command like ATI, and get a response:

As mentioned, this is because some cellular modules require 3.3V for the UART:

We thought about just making the level converter voltage selectable, but that doesn't work, because the PCM digital audio interface is 1.8V on all modules we found so far:

And adding a pull-up to 3.3V didn't work, because our level-converter has a Xener or some similar to clamp the output to 1.8V regardless.

The interim solution was to cut a track, and route it elsewhere:

2.4.5 Testing

To test this module more thoroughly, we actually need the SD card/SIM card module and a DC:DC power module. Like this:

Gee, this whole idea of making the system modular is paying dividends in so many ways.  They modules are designed to work together, and so we just bread-board them up, and can happily test away. It's even running from a battery, like it will need to.  This is effectively letting us burn down integration risk early.

Here's a bit closer view of the SIM card adapter -- with a SIM card inserted -- connected to it: 

 

 

So if it powers on, then we know the integration with the DC:DC converter is good. If it can see the cellular network and it responds READY to a  +CPIN (pin unlock on SIM card status) with READY, then we know it's talking to the SIM card. Likewise, it has to be talking to the SIM card if it can read the CCIM from the card. 

And yay! It does.  So this confirms that both the cellular modem board (following its revisions) and the SIM/SD card modules are working, and the DC:DC module.

But let's be explicit in ticking off the DC:DC converter board...

2.1 High-Efficiency DC:DC Converter

2.1.4: Assembly 

Here is the assembled 2A DC:DC converter board:

2.1.5: Testing

As described in the discussion for the cellular modem module, we have verified that this module works, by using it to power the cellular modem and SIM card modules.

We also checked that the 3.3V output is close enough (we built it using a 300K instead of 290K resistor, because that's what we had on hand, so the open-circuit voltage is a little higher (3.3V x 300/290 = 3.41V, plus a little for resistor tolerance and open-circuit voltage being a bit higher. 

2.1.6: Revision

As Goran was working through this module, he found that I hadn't included as many GND pins as he thinks is sensible.  He's probably right, so we have incorporated this in the updated version of the module.

 

So the summary is that we now have the cellular modem, SIM card holder and DC:DC regulator -- all the key ingredients we need to be able to start using these with a desktop MEGA65 to work on the software and integration of further modules.