Using UART on a brand new Nova Global Cellular Modem


#1

Hi There!

I just received my brand new and fresh Nova Global where the main modem is an Ublox SARA-R410 and I am planning to use the UART interface instead of USB to issue AT commands.

Reading a bit more on the https://hologram.io/docs/reference/nova/datasheet/ _

The USB_D+ / USB_D- lines carry the USB serial data and signaling. The USB interface is automatically enabled by an external valid USB VBUS supply voltage (5.0 V typical) applied on the VUSB_DET pin.

_

and also Ublox SARA-R410 datasheet I figured out USB VBUS probably is connected after the TPS63020DSJ regulator in some part of the 3.8V path. I could no enable the UART interface even removing the regulator from the board.

So my question are:

Is there available a schematic for this module? to just cut the USB VBUS track (only if it is possible)
Is there any information available regarding how to disable USB VBUS on this board?.
why have leave exposed the UART pads if they are not going to work? ( only if the 2 question above are negative)

Regards
Arepa :smiley:

PD: I soldered back the regulator to the board and it works like a charm :smile:


#2

This is the old Nova but I think hte USB wiring is the same. Note the VBUS is at 5V and direct from the USB plug (through a 0-ohm resistor): https://github.com/hologram-io/nova-hardware/blob/master/Gerber/Fab_Assembly_Schematic.PDF

checkout L1 and pin 17 on the modem. Best bet is to try to cut this trace somewhere that doesnt cut power to the regulator.


#3

Hey @Arepa,

To communicate to the Nova through the UART pads you’d need to use a 1.8V UART convertor or a level translator. You don’t need to cut any power lines because VBUS powers the regulator which produces the 3.8V needed by the u-blox Modem. Otherwise you’d need to hack in a clean 3.8v supply.

We are working on getting the schematics for the R410 Nova up to our github page. In the mean time I’d recommend you check out the schematics for the U201. Both boards use the same PCB, but have a few BOM changes.

Also, please keep in mind that the UART pads are directly connected to the u-blox modem so using these I/O or improperly handling the board runs the risk of damaging the u-blox modem. Additionally, we do not officially provide support this interface.

Best,
Maiky


#4

I’m not sure, per the SARA-R System Integration manual under 1.9.2.1:

The USB interface is available as AT command / data communication interface only if an external valid USB
VBUS supply voltage (5.0 V typical) is applied at the VUSB_DET input of the module since the switch-on
of the module, and then held during normal operations. In this case, the UART will be not available.

Per this paragraph and my experience with the SARA-R410M, if VBUS is applied before and during module boot, the UART port will not work. I just tested this now, and if I boot with 5V attached to pin17, UART AT commands don’t go through.

Maiky can you double check the configuration where VBUS power can be applied and the UART still works? FOr UART I am specifically using pins 12 and 13 on the module.


#5

Hey @AndrewGifft,

Thanks for the follow up. Let me double check this and get back to this post.

Best,
Maiky


#6

I just got my Nova with a Sara-R404 working using the uart :-). But I’d love to kill someone at U-Blox! It took umpteen readings of the manual for me to finally realize what these sentences mean: “The module data output line (RXD) […]” and “The module data input line (TXD) […]” Yes, you read that correctly, the module transmits on RXD and receives on TXD. Crazy! I just couldn’t get it to respond for hours, no wonder.

What I did was to cut the 3.8v output of the regulator and the “3.8v discharge” circuit. Using the R404 Nova schematic and bottom layer mask as reference, I cut the (fairly wide) trace between the feedback resistor R13 and the first cap C36. And I desoldered R9 and resoldered it turned 90 degrees so it doesn’t get lost but doesn’t do anything. This makes it relatively easy to put things back in place. I’m feeding power right onto one of the big caps.

In an earlier thread of mine about all this (Using a Nova without USB) the suggestion was made to use a step-up regulator, but that won’t work because as soon as 5v is applied the R4 modem will disable the uart pins and only talk USB. For test purposes I’m powering the thing using a 3.3v regulator, but once I have it working I’m going to hook it up straight to a LiPo since that’s pretty clearly what it’s designed for (see sect 2.2.1.6 of the system integration manual) and where the 3.8v come from.


#7

@AndrewGifft @tve guys Thanks a lot now I am able to use the UART interface to send/retrieve AT commands :smiley: I have removed previously the whole regulator IC with not luck and put it back, but just removing L1 and R9 works just fine for me I didn’t cut any track :smiley:

I added an external 3.3V to supply the SARA using the VCC hole close to the antenna connector and use and voltage translator to the UART pads and it is alive!!

Maiky what is the purpose for the +3.8 Discharge circuit? I could not get it


#8

Cool!

Yea I think Hologram messed up a bit on the UART implementation. You shouldn’t have to remove parts from the board to make it work. And although you could leave the discharge circuit active (it draws ~7-8mA) that would about double the power drain of the device when idling. This mode also requires you to provide your own regulated power which adds complexity / cost.

I think Hologram intended the device to always be USB powered even if using UART (reading between the lines on why 3.8V is a test point vs a pad like the UART lines) but this wont work without providing a jumper or switch in line with USB_5V going into the modem.


#9

Yup they indeed messed that part up and should have included a solder bridge or similar that could be cut.
My interpretation of the datasheet & sys guide is that non-regulated Li-Ion power is fine for the “3.8v” rail, though.


#10

Hey guys, some great discussion in this thread and I’m glad @tve was able to get that working.

I think part of this might be a difference between the U201 and R410 and the Nova using the same PCB for both.

One thing I want to shed some light on is that our thinking behind those pads was just to expose the GPIO pins for the users who want to hack around on the modem or mess with GNSS or UART and isn’t necessarily a supported method for actually controlling the modem. All of our software and documentation is designed with using the USB interface as the primary means of communication and the physical hardware design is optimized for that use case.

This thread has been interesting though and we’ll definitely think about some of these suggestions for a future design revision.


#11

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