Olimexino Nano GSM not reacting to UART

[derchris]

Dear All,

I know the Nano GSM isn't meant as a standalone product, but we're trying to use it as a daughter board for a bigger PCB and so I still hope what somebody is willing to help.

Connection during test is via the 2x4P headers on the card edge, RX/TX is properly connected, GND too, of course, and VBat2 is fed with 3.8V (we could give it 5.5V but I guess that's too high).
When the PWR btn is pressed, I can see it turn out, the LED is blinking, alright. But it doesn't respond via UART, neither with 9600, 115200 or 57600 baud. I know chances are that it's my fault, but still, any help?
Plus, extra question: as the system is powered by a LiPo, does VBat2 really require > 3.7V?

Edit: Something to add is, that it works nicely with the Olimexino MCU. Our TTL logic is 3.3V, might this be the problem?

[LubOlimex]

> Connection during test is via the 2x4P headers on the card edge,

One of these is USB. One of the 4p connectors is UART, the other is USB. Is there a chance that you used the USB connector instead of the serial one? I've done it before so it should be possible to mix those.

> Plus, extra question: as the system is powered by a LiPo, does VBat2 really require > 3.7V?

Battery is sufficient as long as it is between 4.2V and 3.8V.

> Edit: Something to add is, that it works nicely with the Olimexino MCU. Our TTL logic is 3.3V, might this be the problem?

Which Olimexino? Olimexino-NANO?

[derchris]

Thanks for the quick reply 

One of these is USB. One of the 4p connectors is UART, the other is USB. Is there a chance that you used the USB connector instead of the serial one? I've done it before so it should be possible to mix those.

Nope, I've used the right ones... it works with Olimexino Nano using the same ports.
But I noticed that the Olimexino Nano has 3.3V TTL voltage and maybe that's why there are resistors populated for RX/TX. Since our board has a logic high of 2.8V I could imagine that - because of the resistors - it's too low to be recognized as logical high. I've tried to remove them but broke my board (already ordered a new one). Never did that, so please excuse the question: would it be possible to add a pull-up resistor to our RX/TX to shift it back up to 3.3V so we can use the Nano GSM without tinkering?

> Edit: Something to add is, that it works nicely with the Olimexino MCU. Our TTL logic is 3.3V, might this be the problem?

Which Olimexino? Olimexino-NANO?

yes, correct

[LubOlimex]

You can use either resistors in series or a dedicated level-shifter chip.

Shows us the part of your schematic with the main chip and the UART, maybe it is something obvious.