[Ovmsdev] OVMS v3 First Board Layout

Tue May 9 21:16:07 HKT 2017

We’ve come across a small power problem with the modem board. It seems that 3.3V is not enough for some of these modem modules, but ESP32 and SPI flash chips are sensitive to power so if we go too high we may run into problems with those. 3.6v is a compromise, but I’m concerned that if the power draw goes too high, voltage may drop too low for the modem. OVMS v2 used 4v (measured around 3.8v on a selection of OVMS v2 modules I have).

Simplest solution seemed to be to route EXT_PWR (5v USB or 12v vehicle, whichever is bigger), USB_5V and EXT_12V through to the expansion connectors. Then, if the modem works on 3.3V fine we can just use that. Otherwise, we use EXT_PWR and a power supply just for the modem (tuned to whatever the modem requires). An alternative would be a DC booster, but that seems messy. Anyway, best to just route the power lines up to the expansion connectors and let us decide that later once we’ve had the ability to test it. If we want to tune the main AOZ1280 power supply output we can (it is just a resistor that sets the desired level). The revised expansion connectors shown on the schematic attached shows this new arrangement.

One nice thing about this overall design is debugging the modem is simple. It is on it’s own expansion board with two single rows of pins. Easy to breadboard. We’re including a USB connector for direct manipulation from a PC (for debugging) - something like the serial port we had on OVMS v2.

For the modem, it seems that one design of PCB will work with a number of different SIM5xxx (3G) and SIM7xxx (4G) modules. The attached schematic shows how that is wired. Nothing finalised yet - this first board with SIM5360 is just for initial testing and development. That said, the SIM5360 is a pretty nice 3G modem with three variants available for different frequency ranges. All have GPS/GNSS. It is a more modern version of the SIM5320 I played around with a year ago.

Switch Mode power supply to 4V (from EXT_PWR so input maybe from USB 5V or 12V car).
FXLA108 (or FXLA104 as we only need 3 bits) level shifter (1.8v <-> 3.3v).
SIM socket (I suggest put this on top of board, and modem on bottom, to make easier to put in/out sim - or both on top depending on space available).
Single internal blue LED for NETLIGHT status. Like OVMS v2 no need for this to be visible outside the case.
I haven’t drawn it on the board, but perhaps we need a simple (internal only) push switch to toggle power (connected to MDM_EN circuit). Same as OVMS v2.
A USB socket for debugging / firmware upgrade. No need for this to be visible outside the case.
SIMCOM SIM5360(J)E (frequencies suitable for Hong Kong).
GSM and GPS antenna.

Regards, Mark.

> On 8 May 2017, at 8:07 PM, Mark Webb-Johnson <mark at webb-johnson.net> wrote:
> 
> Interestingly, SIMCOM seem to be addressing compatibility. Here is an example:
> 
> http://www.mt-system.ru/sites/default/files/documents/sim7100_sim5360_sim800_compatible_design_v1.01.pdf <http://www.mt-system.ru/sites/default/files/documents/sim7100_sim5360_sim800_compatible_design_v1.01.pdf>
> 
> That covers making one board for SIM7100 (4G), SIM5360 (3G), and SIM800 (2G).
> 
>> On 8 May 2017, at 7:36 PM, Mark Webb-Johnson <mark at webb-johnson.net <mailto:mark at webb-johnson.net>> wrote:
>> 
>> Both SIM7000C (LTE-M) and SIM5320 (3G) are available include GPS/GNSS functionality. Price of the SIM7000C is about 50% more expensive than SIM5320. They are very compatible at the pin level so not hard to support both.
>> 
>> Longer-term LTE-M sounds good, but I think maybe too early for this today (even if networks available, how easy is it to get a data plan?).
>> 
>> Regards, Mark.
>> 
>>> On 8 May 2017, at 3:18 PM, Dominik Westner <me at nikwest.de <mailto:me at nikwest.de>> wrote:
>>> 
>>> It is supposed to be rolled out across Europe over the next months. In US it already seems to be available. 
>>> As this is a software only feature of the existing infrastructure it shouldn't take too long. But that information is only second hand. 
>>> I'm mostly following what these guys are doing 
>>> https://www.pycom.io/pycom-and-canonical-to-enable-lpwan-communications-via-snaps/ <https://www.pycom.io/pycom-and-canonical-to-enable-lpwan-communications-via-snaps/>
>>> 
>>> 
>>> 
>>> 
>>> Sent from my iPad
>>> 
>>> On 8. May 2017, at 08:42, Mark Webb-Johnson <mark at webb-johnson.net <mailto:mark at webb-johnson.net>> wrote:
>>> 
>>>> LTE-M
>>>> 
>>>> Is that ‘prime time’ already? I thought that wasn’t gonna be ready for another year or so (on a large scale, globally).
>>>> 
>>>> There is a SIMCOM module for LZTE-M (SIM7000C), with GNSS (GPS) and 2G but no 3G, but that is really new.
>>>> 
>>>> My concern with LTE is that there are still lots of areas with no 4G coverage, so fallback to 3G seems necessary.
>>>> 
>>>> Mark.
>>>> 
>>>>> On 8 May 2017, at 2:18 PM, Dominik Westner <me at nikwest.de <mailto:me at nikwest.de>> wrote:
>>>>> 
>>>>> Isn't LTE-M the way to go? 
>>>>> Something like the sequans monarch?
>>>>> 
>>>>> http://www.sequans.com/products-solutions/streamlitelte/monarch-lte-platform/ <http://www.sequans.com/products-solutions/streamlitelte/monarch-lte-platform/>
>>>>> 
>>>>> 
>>>>> On 8. May 2017, at 07:41, Mark Webb-Johnson <mark at webb-johnson.net <mailto:mark at webb-johnson.net>> wrote:
>>>>>> 
>>>>>> The side project of looking at 3G/4G modems has also started. This is going to be on one of those optional expansion slots to allow us to use different modems for different functions. I’m used to SIMCOM, but it seems their selection is getting complicated (particularly for 4G). I see the SIM5320A has 3G and GPS, which seems good but is quite dated now.
>>>>>> SIM5320A:Dual-Band UMTS/HSDPA 850/1900MHz, Quad-Band GSM/GPRS/EDGE 850/900/1800/1900MHz
>>>>>> SIM5320(J)E:Dual-Band UMTS/HSDPA 900/2100MHz, Quad-Band GSM/GPRS/EDGE 850/900/1800/1900MHz
>>>>>> SIM5320J:Dual-Band UMTS/HSDPA 850(800)/2100MHz Quad-Band GSM/GPRS/EDGE 850/900/1800/1900MHz
>>>>>> It does have the CMUX, which is very nice (gives us four virtual ports over the single async port, so easier to deal with AT command control).
>>>>>> 
>>>>>> If anyone has any suggestions for the modem, it would be helpful. Something with GPS would be ideal.
>>>>>> 
>>>>>> Regards, Mark.
>>>>>> 
>>>>>> 
>>>>>> <OVMS_V3_Schematic_CN_20170503.pdf>
>>>>>> <OVMS_V3_Layout_CN_20170508.pdf>
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