SIM7080G not responding to serial due my design flaw?

Using Arduino Networking, Protocols, and Devices
1

Hello all,

I designed custom PCB to communicate STM32F030C8T6 with SIM7080G (schema below). JLCPCB made board + assembly for me. Right now I'm facing issue that SIM7080G is simply not responding to any AT command over serial. Via USB AT commands works. I feel this is similiar issue to SIM7000 responds to AT commands over USB but not UART connection

List of known parameters / tips I tried already:

  • SIM7080G UART levels are 1.6V-1.8V
  • SIM7080G voltage is 4.18V
  • There is not SIM card inserted yet.
  • USB responds to AT commands.
  • Baud rate 9600 / 115200 is not answering.
  • STM flash is burned via serial connected to SIM7080G.

List of what I havent tried yet:

  • My design missing on logic level shifter EN pin 100nF capacitor.
  • Desolder logic level shifter -> use transistors / other shifter
  • Factory reset / firmware reflash (however usb works.. so why)

Scheme:

image
image1931×1201 201 KB

PCB:
I have RX/TX connected to PC via header (picture below green zone). Purple zone - logic level shifter

image
image1920×1323 254 KB

Any ideas what might solve my problem?

Thanks for tips!

2

I started the thread you mentioned. Unfortunately I'm a software guy, so my ability to comment on wiring diagrams is nearly non-existent. My probably dumb question is prior to getting the PCB printed, did you ever get your 7080G and STM communicating over UART? Is it something that basic or is it definitely something more subtle like a potential level shift issue as you've noted?

3

This may also be unhelpful. Have you tried buying a 7080 breakout board and looking at how they've done things? ebay: SIMCOM SIM7080G LTE CAT-M NB IoT Module NB-IOT Global SIM7080G Breakout board | eBay

I can likely help with software questions about SIM modules. I've worked quite a bit with sim7000 sim7500, sim7600, and each has hardware bugs.

oof, formatting.

4

Hi,

7080G and STM should work like between nano and 7080G. I studied a lot of different PCBs. They all do provide level shifters.

I ordered some transistors similiar to design guide reference. I will desolder my level shifter and try via breadboard make hopefully PC-7080G connection. I never used before 7080G but I do have experience with SIM800 which should have same pins / etc, however different UART levels.

I managed sucessfully PC-STM communication, then I uploaded to STM program which should boot up 7080G, wait 10s and send AT command to shut it down. 7080G never shuts down which means there is an issue on 7080G side.

5

Hi,

@jwallis So today I removed logic level shifter and replaced it with transistors + voltage divider. Properly measured all connections. UART levels are about 1.7V.

However no luck.. no response for AT command sended via PC. So it might be burnt. Last thing I am willing to do is firmware update but I cant find any tool nor firmware binary file. :confused:

6

Oh, ok. So you are saying that on the breakout board I use:

there is a logic level shifter between the pins and the SIM module itself? That makes sense, but I never realized that. I have only used the breakout board, never directly connected to the module. That is much more difficult, and beyond my ability.

You should talk with Tim at botletics.com He is a very nice guy, very helpful, and you could probably pay him for a couple hours of time. I think he can fix your problem, because he has developed his own Arduino Shield based on the SIM7000:
Botletics™ SIM7000 LTE CAT-M1/NB-IoT + GPS Shield Kit — Botletics

Sorry I cannot be of more help. Good luck!

7

Yes there is actually on-board logic level shifter can be seen on picture in your link U103.

Thanks for tip about Tim. I will try to contact him.

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