5V Uart setup

[Andries Kruger]

Hi, 

I'm trying to integrate a GSM modem to my project (EFCOM PRO from Elec Freaks). I can connect the module to my pc fine using a usb to uart converter, send AT commands and receive the responses.

When trying to connect it to the IOIO, I cannot receive the reply. I am using pins 39 and 40 (tx and rx). Now I know that the modem replies correctly as I have sent a command from the IOIO and received it using the USB-UART converter and TeraTerm.

I use the following code to set up my UART:

//Setup Comms UART
DigitalInput.Spec rx = new DigitalInput.Spec(40, DigitalInput.Spec.Mode.PULL_UP);
DigitalOutput.Spec tx = new DigitalOutput.Spec(39,DigitalOutput.Spec.Mode.OPEN_DRAIN);
commsUart = ioio_.openUart(rx, tx, 9600, Uart.Parity.NONE, Uart.StopBits.ONE);
commsRx = commsUart.getInputStream();
commsTx = commsUart.getOutputStream();

Am I doing anything wrong here? Any advice or tips would be greatly appreciated.

[Ytai Ben-Tsvi]

Pins 39, 40 are not 5V tolerant. You really shouldn't connect a 5V signal to them. They may already be damaged at this point.

See this wiki page for information on how to interface the IOIO with 5V logic:

https://github.com/ytai/ioio/wiki/Digital-IO

Make sure to use 5V tolerant pins for both. You definitely need a pull-up to 5V on the output and might as well have one on the input.

I totally don't get the design of the RX/TX circuit on the EFCOM schematic. Either it's totally weird or I'm missing some very clever trick. Definitely not a standard way of shifting voltage levels...

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[Andries Kruger]

Really embarrassed about using the wrong pins, but switching to 5v tolerant pins now did not solve the issue (new IOIO board just to make sure). I have a 10k pull up on both the input and output. Think it will be easier to just use another GSM module at this stage.

[Ytai Ben-Tsvi]

Don't see any reason why this one wouldn't work if it works with a PC.