what are the possible ways to use GPRS with the propeller?
Bobb Fwed
Posts: 1,119
What are the possible ways to use GPRS with the propeller? It doesn't need to be GPRS specifically, but some type of cellular communication. What hardware is used to do this?
How is this setup with the wireless carriers? We want to setup a data-only (low quantity of data at that) system that conveys about 60 bytes of data per hour (maximum of 500KB per month). Is there some way to work with the carriers to get low cost accounts? We would likely add a few hundred accounts per year.
I can figure out the carrier stuff, that's just going to be a lot of talking with the companies, the hardware is the big thing I care about.
How is this setup with the wireless carriers? We want to setup a data-only (low quantity of data at that) system that conveys about 60 bytes of data per hour (maximum of 500KB per month). Is there some way to work with the carriers to get low cost accounts? We would likely add a few hundred accounts per year.
I can figure out the carrier stuff, that's just going to be a lot of talking with the companies, the hardware is the big thing I care about.
Comments
GPRS is a good way to go. A couple of years ago i picked up a Telit GPRS modem, played with it for a few hours, and was surprised at what it could do. I wouldn't do it any other way.
Telit easy-gprs scripts are built in for email, ftp, web page get and put, socket connections for sending data like for your application, and more. There are other brands such as Sagem, Siemens, Wavecom that have equivalent gprs modules.
You're right about getting the SIM cards and plans being the hardest part. Its easier to get started now there are so many smartphones needing data plans. We pay something like $3 a month for a SIM that would do 500kB.
regards
tubular
You have to be careful with the power supplies. The Telits are designed to run from Lithium batteries, about 3.4 to 4.2 volts. But the modems have sharp current spikes (about 2 amps) for short periods when transmitting, so you need big caps to try and smooth it out. The prop is only rated to 3.6v, so there are a couple of approaches:-
1) Try and run everything at 3.6v. Your regulator will need big caps so the Telit voltage doesn't sag below 3.4 volts when transmitting (and drawing its 2A spike).
2) Run the Telit from a LiPo battery (3.7..4.2v), Prop from a separate 3v3 LDO reg off that battery, and have small protective resistors in series on each of the interfacing lines (RXD TXD ON/OFF etc), to protect the Prop.
Last week I rushed out a small Telit breakout board that would fit the prop proto board. I'm only half expecting it to work, it will definitely require a further revision or two. I'll let you know what happens with it
You say "big caps" that's a broad range. How short of a spike is the 2A draw? I was hoping a 1A linear regulator (don't want to mess with switching regs on an RF device) with a couple thousand micro-farads of caps would be enough. The problem will likely be the ESR required to pump out during the spikes. What kind of setup did you go with? I think at some point it would be cheaper to just get a 2A/3A regulator instead of loading the board up with caps which could produce questionable for reliability especially over the long haul (5/10+ years down the line is very important for us).
Where did you order your Telit from? Directly from them? The only supplyer we use that has any is Newark, but they only have 1. I figured I could get the 1 and mess with it, then find a normal supplier.
If you have any extra breakout boards, or want to share the design files, I would love to have them so I can start messing with one. Are they functional for standard breadboards? I don't have a Parallax proto board.
We are just now having a second crack at a suitable switcher from Linear Tech. We'll see how it goes this time round.
Regarding suppliers, we get ours from Glyn (Germany company with local offices here). Not sure if they are in the US. Sparkfun have a range of Telit modules and breakouts. Arrow electronics have some in stock according to FindChips.com. But if they aren't "readily" available to you, I would look at another brand. I know Newark/Farnell carry other brands too. I found the Easy-GPRS feature of the Telits very easy to use, but the others probably have equivalent scripts that are equally good. Its quite a competitive little area, GRPRS modems.
Average current - I seem to remember a few hundred mA for a 2G module, and up to about 800mA for a 3G module. 1 amp with lots of smoothing should be fine at least for 2G.
I'm curious about those Telit modules. They look very small. At first glance, it appears that they have to be mounted on a PCB using a ball grid scheme. Is that right?
GE864 is the small essential modem, cheapest form factor
GC864 adds a pcb w/sim pads, and 80 pin molex connector (so its thicker and longer)
GT864 is the terminal in plastic case with RS232, power supply etc.
I also plan on trying to keep output bursts as low powered as possible (so 1X TX?), so hopefully that will bring the duty down to 14%, which will make it all better as well (@19V it goes down to 3.75V). I think it's worth a try with this setup....
With both duty draws, at 14V it only drops 20mV or so. This all assumes I can dissipate the heat that will be produced. I will have to do the math on that.
Tubular, does your new PCB use the ball grid devices?
I wonder why the manufacturers insist on unfriendly connection methods. I don't like the 80-pin Molex sockets either. So many connections, all for tx, rx, power and ground and maybe a few handshaking lines. The MultiTech socket modems have pins on 2mm spacing, but only a few of the 64 possible positions are implemented, and that is done in a way that leaves it mechanically unstable as it sits in a socket. Go figure.
I didn't have time on that board to lay out the power supply, so its just brought out to a corner terminal. For test purposes I will just use a Max155x and 2000 mAH lipo battery. There is an Attiny85 onboard for initialisation if required.
I need to preserve the reset button somehow on the next rev, since this board covers the central core of the proto board.
I forgot to say, yes, I am using Linear Technology's SPICE.
Current limit @ input voltage (for 4V output)
1.05A @ 20V
1.6A @ 18V
2.15A @ 16V
2.7A @ 14V
3A @ <13V
This is interesting, because they say 1.6V minimum current limit if input is only 1V above output. At 18V, that's a 12V difference. And I don't see in the datasheet anything about the 3A max.
So, above 16.1V, the capacitors will be powering the device for a bit. My target input is 14V, but it could periodically be close to 16V.
What I will likely do on my production design is switch the voltage down to 5V maybe. That way I could power my 5V devices, and use the linear regs for 3.3V and 4V, with very little heat. At 5V input, there is only a 10mV fluctuation.
So if I keep it cool (below 50 or estimated 70C), at 5V (1V differential) the current limiter won't kick in. But now that makes be want to bump the switching pre-reg up to 8V maybe, so I get more into that hump above 2A, but still fairly low to keep the heat dissipation to a minimum. Ug. Maybe I should just get a 2A reg.
Seeing as this device is supposed to last at least a decade (hoping for decades), I shouldn't skimp too much here, and it will put my mind more at ease for removing the electrolytic caps. 2A reg would allow me to keep the 5V pre-reg which would save me the money I lost by not needing another, separate, 5V reg.
Any suggestions on a fast transient response linear technology 2A regulator? I'll look around.
For a linear regulator solution, the LT1528 works (and is used on several designs including Sparkfun)
tubular
The NCP5663 looks like a good find, attractive price for the BoM. It appears to be an NPN output regulator, with 1.5V dropout, as opposed to the LT1963 which has a PNP output, 300mV dropout. Same pinout, higher maximum current, and fast transient response. I'm tempted to use it, except there is the 30µA current in shutdown compared with <1µA for the LT1963. 30µA isn't bad, but my apps are solar powered and I obsessively count microAmps.
There is a Linear Tech app note, an104, written by LT tech guru Jim Williams about how they do load transient testing. The example is the LT1963. His cartoon...
Since the D2PAK pinout is the same for the OnSemi and LT parts, makes sense to design the PCB around those, and load with the right part for the application.
1.3V dropout is the theoretical maximum dropout at 3.0A. According to the charts and graphs, nominal dropout is closer to 1.0V @ 3A or (more importantly) 0.92V at 2A. My Li-Ion battery shuts off at 5.5V, so 5.5 - 1.3 is still above 4V, so I am good, no matter what.
The shutdown amperage is important to me as well. My project has to go into a standby mode and run on battery for weeks at a time. I will continue looking, but I think I can manage with a few extra µA that this will use.
The telit goes face to face with the prop chip, exposing the SIM card uppermost. All the useful pins are broken out around the rim, and the essential ones are in a single bank of 8.
The pads are for an AtTiny45v if required for initialization or standalone operation
On the back on the board is the SIM holder, bulk caps, status Leds, antenna connector, etc. No reg on it at this stage (doing that externally)
That one hasn't been connected just yet, but I have used '864s successfully in several projects over the past 3 years, and some Sony GR64s before that.
When you purchase a data sim you need to find out what the APN is for the provider you're purchasing from. You will need that so the Telit knows which of the many networks to connect to.
See the screenshot in this message for the commands to get started. Change the vfinternet.au to the APN given to you by your provider.
http://forums.parallax.com/showpost.php?p=877355&postcount=26
If you have any questions, I'll be happy to help where I can...
Just getting up to speed with a couple of threads. Do you still need the Diptrace footprint? I can probably organize this (extract from existing design) for the 80 pin molex (GC864 and/or UC864). I don't have anything in DipTrace for the GE865 yet, again could solve this if you still need it.