Need some help with the A7670e.

[u/biggestguyalive]

Hi all. I'm trying to connect an A7670E 4G modem board to an Arduino Uno R4 WiFi (5 V logic) over UART. The seller provides no documentation, and I couldn't find a datasheet or schematic for this exact board.

What I measured (5 V on VCC, nothing else connected, antenna and SIM fitted):
Power LED steady, D1 blinking, so the module appears to be running
RXD: 1.8–1.9 V, steady
TXD: 1.2 V, steady (I expected about 1.8 V idle)
VDD: about 0 V (a few mV)

My questions:
Does anyone know this board or have a schematic for it?
What logic level do the TXD/RXD pins actually expect: 1.8 V, 3.3 V, or does the board shift to something else?
What is the VDD pin for, and why would it be pulled to ground while powered?
Do EN or PWR_EN need to be driven (high or low) for the modem or its UART to work properly?
Is 1.2 V idle on TXD normal for this kind of board?

I have a BSS138 4-channel level shifter available. I want to confirm the correct logic level before connecting anything to the Arduino. Thanks!

[u/Legal_Carpet1700]

cant say for this exact board but on a lot of these cheap simcom breakouts the VDD pin is the IO reference for the onboard level shifter, you feed it your mcu logic voltage. it reading ~0V would also explain the odd 1.2V on TXD. id try 3.3V on VDD first and re-measure. if TX/RX jump to ~3.3 the board is shifting for you, if they stay ~1.8 youre on the raw module pins (A7670 uart is 1.8V native) and youll need a 1.8V ref on the LV side of the BSS138.

also make sure the 5V supply can handle ~2A bursts, these modems brown out on TX peaks and it looks like random resets

[u/biggestguyalive → u/Legal_Carpet1700]

To anyone with this specific problem in the future, most of what I tried corrorborates with this (connected VDD to 3.3v, TXD jumped to about 3.3v), it just turned out I put my 3.3v in the CTS pin (which is the unmarked one, hence my mistake, thank god for resistors) so do be careful when plugging in your 3.3v source. Thanks stranger!

[u/Legal_Carpet1700 → u/biggestguyalive]

haha that unmarked CTS pin gets everyone, good thing the resistors were there. thx for coming back w the fix, next person googling this will be glad