Sara-R4 LTE CAT M1/NB IoT Shield not accessible to linux as a breakout board

Wireless & IoT Cellular
1

I’m using Debian on a Pocket Beagle and trying to connect that to the sparkfun sara-r4 shield as a breakout board. I am simply trying to use screen or minicom to send a basic AT command so that I can get an OK response from the modem but I can’t connect to it.

Using the shield as a shield on top of an Arduino UNO does work and I can run the example scripts with Hologram. I have since used the Reset button on the shield to reset it to its defaults

Wiring:

  • Power: So my shield is not connected to the Arduino. It is standalone as a breakout. I have connected USB to it from my mac – but not with the intent of mounting it as a USB device, just for power. Note, I have also tried powering it via the 5V and GND pins with jumper wires. Both power options turn the sheild’s power light on.

  • I have GND on the breakout connected to GND (P2 pin 15) on the pocket beagle.

  • I have TX on the breakout connected to RX (P2 pin 05) on the pocket beagle.

  • I have RX on the breakout connected to TX (P2 pin 07) on the pocket beagle.

  • the breakout’s PWR SEL switch is set to Arduino

  • the breakout’s SERIAL switch is set to HARD

  • I don’t have any added solder bridges on the back of the shield.

I am connecting with minicom using sudo minicom -D /dev/ttyS4 -b 115200 (though I have also tried 9600 baud). I don’t see characters when I type, AT and I don’t get a response from the board either.

Any other ideas?

2

You might try moving the switch to SW serial (use pins 8 & 9) https://learn.sparkfun.com/tutorials/lt … e-overview …also, it might be ‘browning out’ if powered over an iffy usb port/cable…if it still works as a standalone shield that’d be my first suspect

3

Thank you for your thoughts.

I have verified that my USB power cable is fine.

I have done a physical loopback serial test on the pocket beagle, connecting tx/rx with a wire and using minicom and tio (serial device tools) to prove that the UART device on the microcontroller is functioning properly. I am also now mapping LF character to CRLF which sara-r4 appears to require based on the docs.

I can’t understand how software serial would be the right setting if there is no arduino attached.

I suspect the issue is the logic levels. https://learn.sparkfun.com/tutorials/lt … logiclevel Although the page says it can auto-switch, I suspect that with no arduino attached it needs to be set to 3.3V via the jumper in the back which I have unfortunately solder-sucked the pads right off of now so I can’t use it any more.

Another thing about the logic level jumper that I’m not sure about is if it shifts both TX and RX (you’d hope so)

Might have to buy a new one but it sure would be nice to know if anyone has gotten this board to work as a breakout before I drop more money on it.

4

Maybe I should get a couple of these instead of buying a whole other board. https://www.sparkfun.com/products/12009 … 1653238088

5

sporkfun23:
Maybe I should get a couple of these instead of buying a whole other board. https://www.sparkfun.com/products/12009 … 1653238088

I did order one. Will report back.

6

Follow-up

TL;DR I don’t think this shield actually works as a breakout

Before using the external logic level module, I powered the shield with a USB cable and probed the GND & hardware TX of the shield with a multimeter (MM) which indicated 3V (prob 3.3V). I should have done that before ordering the logic level module as there should be no need for it after all.

I did a couple more tests with the FTDI cable plugged into the FTDI pins on the shield, USB end of the FTDI cable attached to my mac. I experimented with tio again and confirmed a few things from trial and error. This command on my mac worked:

tio --baud 9600 /dev/cu.usbserial-A601EELY
``` (your dev device might be different).

* needed to hold the Power button for ~3s on the shield after plugging it in to USB power source to ensure it started up. Within 30s, I was getting the blue flashing LED indicating successful connection to the carrier (at least at some basic level). This is prior to sending any commands – it's doing whatever the Sara chip does by default.

* 9600 baud worked, 115200 did not

* mapping the line endings was NOT necessary (ex. LF to CRLF)

* When I sent the AT command by typing ```
AT
```, the characters I was typing did not display but when I pressed the Return key, my letters appeared and the ```
OK
``` response appeared on the next line. the command, ```
ATI
``` also returned a valid response.

Having confirmed the correct connection settings, I removed the FTDI cable and tried once more to use the 3-pin [[USB to TTL serial cable](https://www.adafruit.com/product/954), connected to GND, hardware TX and RX. Using the same settings, I was unable to connect. I also tried attaching the cable to the GND, TX and RX of the FTDI header with the same results. Note, tio defaults to "flow control: None" so it shouldn't be an issue. Ultimately, I would want my 3.3V microprocessor to be able to connect to GND, TX, RX but this test seems to prove that this will not be possible with this shield.

(Yes, I was connecting RX to TX and TX to RX as one should expect to and out of desperation even tried swapping them).

@Sparkfun? Has anyone else gotten this shield to work as a breakout? 

Reluctantly, I have abandoned this shield and purchased the Sara-R5 breakout.](https://www.adafruit.com/product/954)