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I have a GNS FM9 dongle, which is a RDS receiver that came bundled with a sat-nav, intended to be used as a receiver for TMC traffic reports (which are transmitted as part of the RDS data stream).

These devices have a UART interface but come in various flavors—connectors differ depending on the device the dongle came with, and logic level can be either 3.3V or 5V. The chip seems to operate at 3.3V; the correct logic level for the device can be identified by the presence or absence of extra circuitry on the board. The one I have lacks the level-shifting circuitry, suggesting it operates at 3.3V. It has a USB cable, which is plugged in between a USB power supply and the device, although the device-side data pins on the USB cable carry UART signals, not USB signals (which is why the device-side plug does not carry a USB logo). It dies mean, however, that it operates on a 5V power supply.

In order to control it from a PC, I have bought a FTDI232 breakout board. The board has a jumper to select 3.3 or 5V. Looking at the PCB, the middle jumper pin seems to be connected to the Vcc pin on the pin header, though I don’t know for sure what else is governed by the jumper setting. The chip on the board is an FTDI232RL.

What is the correct jumper setting for this device (3.3V logic level, 5V power supply)?

In particular, what are the TX voltage levels at either jumper settings? Do I risk frying the FM9 by setting the jumper to 5V, or is TX voltage guaranteed to not exceed 3.3V with either jumper configuration?

Or do I need to go for a hacky solution, setting the jumper to 3.3V (for the logic level) but taking Vcc from a different source, not the Vcc pin on the 6-pin header?

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  • \$\begingroup\$ This jumper is controlling the the voltage levels by driving VCCIO pin of the chip. It is connecting it to either VCC or to 3V3OUT pin. \$\endgroup\$ – Eugene Sh. Jun 29 '17 at 21:33
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The FTDI232 IC has a pin called VCCIO that controls the levels of its IOs.

If it is connected to the 5V supply from ths USB port, then the TX signals will have a level of 5V and the RX signals will be 5V compatible.

If it is connected to the 3V3OUT pin of the IC, which is a 3.3V output from the integrated LDO, then the level of the TX signals will be 3.3V.

So, in your case, you have to select the 3.3V levels with the jumper on the board.

Note that if you choose the 5V levels, you will most probably fry the FM9, if this is indeed operating from 3.3V! So don't try it!

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Based on nickagian’s answer, I’ve tried the following:

Setting the jumper to 3.3V works, but the FM9 gets fed a voltage of just 3.3V. I’ve had some tuning issues (stations are identified as transmitting RDS but no RDS data ever arrives), which I suspect may have to do with the voltage.

Measuring the voltages on the jumper pins against GND revealed that the outer pins of the jumper carry 3.3V and 5V, respectively.

So now I have the jumper in the 3.3V position and take Vcc for the FM9 from the free jumper pin rather them from the pin header, which worked at least in a brief test. Adding the missing pin headers on the side and using the 5V pin on those would probably work as well. This should now cause the FTDI232 to work at 3.3V while still supplying 5V to the FM9.

That being said, the tuning issues I observed before are the same with 5V supply voltage. So apparently the FM9 is OK to run at 3.3V.

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