FT232RL powered by Rx/Tx pins - Page 1
Forum section: electronics / beginners

author: Dasypus
author_rank: Contributor
timestamp: June 21, 2025, 01:14:28 pm
content: Hi, I have an FT232RL breakout board that I have already gone to the trouble of fitting inside an Omron M2 basic Heart Rate Monitor. "Great, now I can automate BP readings on my laptop", so I thought, but there's an unexpected catch, doh! The FT232 is powered through/by the Rx/Tx pins, see:https://electronics.stackexchange.com/questions/145073/ftdi-ft232rl-powers-itself-through-rx-tx-linesThe breakout I have is a bulk standard jobby and has LED's for power and Rx/Tx. The green LED's regularly switch on and off for some reason.I want to maintain battery power and maximise battery life, for simplicity and minimum of faff.Any ideas please?

author: Peabody
author_rank: Super Contributor
timestamp: June 21, 2025, 02:11:23 pm
content: Can you clarify how this is set up? Are Rx and Tx connected to the Omron's processor, or have you added an AVR?It would seem you need to bring Rx and Tx low until you need to transmit, or until USB is connected. Can you do that?

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 21, 2025, 02:52:30 pm
content: Depends on what sort of flexibility you have with modifying each board, or adding one in between.For the data line going to the FT232RL, you could use a buffer from a logic family (like AHC) which does not have clamping diodes from input to Vcc. This buffer would be powered from the same supply as the FT232RL.You could do the same for the other data line (powering the buffer from the processor side). But might not be necessary (unless there is a strong pullup resistor).I'm betting there are chips specifically designed for this purpose as well....

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: June 21, 2025, 04:30:54 pm
content: You can use 1kΩ–10kΩ resistors in series with both RX and TX lines. This limits the current and reduces phantom powering, though it doesn’t fully eliminate it.

author: Dasypus
author_rank: Contributor
timestamp: June 23, 2025, 02:35:50 pm
content: Thanks to all who replied, it's been a long while for me to do any real component work, it just so happens I have some 74HC32N's from a previous repair job. I'm very old skool, but I can knock up a Vero board to fit over the FT232 breakout. A circuit what I think may do follows, but I have very little idea about any pull up/down resistors, and decoupling caps (I suspect not at this speed, 9600Bd). I do remember, way back, to tie unused inputs to ground, especially CMOS

author: Gyro
author_rank: Super Contributor
timestamp: June 23, 2025, 05:43:40 pm
content: The 74HC32, in common with pretty much all CMOS ICs contains protection diodes to the supply VCC and GND pins. It the diagram you show, the Omron TX pin will be trying (and probably failing) to power the VCC output of the FT232 board via the HC32 protection diodes. This will load down the Omron TX pin with unknown results.As ahsrabrifat says, series resistors are the way to go to reduce loading.

author: langwadt
author_rank: Super Contributor
timestamp: June 23, 2025, 07:06:01 pm
content: series diodes and pull ups

author: Dasypus
author_rank: Contributor
timestamp: June 24, 2025, 07:00:01 pm
content: Thank you all.The resistors seem to me like a good solution, but because the quiescent state of the OMRON Tx pin being high (RS232, 0=high), means that the three LED's on the breakout (red-power, green tx/rx) are all on, thus taking current, as well as the FT232 (which seems to process input state and other stuff, hence the green LED's regularly switching off and on) so battery life is extremely compromised. Won't the value of resistor chosen either allow the LED's and FT232 to be on, or prevent the breakout board from even operating at all? I will try this, but...I also don't understand "Diodes and pull-ups", I don't understand enough about discrete logic and what drives the Tx up or down (source or sinking current)? Which way to put the diode? does the pull-up (value?) go before of after the diode?I can kinda see a transistor may work here, but again this is at the limit of my understanding. Here is a stab at a solution, note the emitter being used as I would need two transistors because of the inversion:Again, many thanks for your input.

author: Dasypus
author_rank: Contributor
timestamp: June 24, 2025, 07:30:30 pm
content: Yeah, in hindsight, that circuit gains nothing, the transistor will be on, drawing current, and the FT232 breakout will also be on.

author: langwadt
author_rank: Super Contributor
timestamp: June 24, 2025, 08:29:09 pm
content: Quote from: Dasypus on June 24, 2025, 07:30:30 pm

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 24, 2025, 08:56:56 pm
content: Quote from: Dasypus on June 24, 2025, 07:00:01 pmYour description of your circuit and its problems isn't coherent...Normally when using a FT232 in a battery powered application, the FT232 itself gets power from the host via the USB cable. Thus the FT232 (and the LEDs on the board) are only powered on when a host is present and connected, and this does not take any current from your battery.When the host is not connected, the FT232 might act as a parasitic load on the battery through its RXD pin (and possibly its TXD pin) via its clamping diodes. Is that your problem?

author: langwadt
author_rank: Super Contributor
timestamp: June 24, 2025, 09:47:50 pm
content: Quote from: mtwieg on June 24, 2025, 08:56:56 pmthat is how I understand it, and it is a common problem when some parts of a circuit it not powered

author: Buriedcode
author_rank: Super Contributor
timestamp: June 25, 2025, 12:02:12 am
content: It's a common problem I've had when hooking up USB-UART devices to microcontrollers. As was recommended by others in a thread I created, the 74LVC1T45 worked well. It worked for my application because one side could power down, whilst the other is still active, but that could be either side, but they could both share a common ground connection.If the signals are >2V (not 1.8V logic) you could use AHC/VHC logic, as that doesn't have an input clamping diode going to VCC, so if the logic chip/FT232 isn't powered, there is no path for a high input to power it.If you justhave the Tx line going to your FT232R's Rx line, then as suggested above the reverse diode + pullup on the FT232R's side works well. This can however, send that pull-up's current to the device in question, which may or may not be a problem (it could be that it has enough current to keep the MCU on, preventing any auto-turn-off circuits from working).For true isolation where multiple pieces of equipment are powered from the sources with different grounds, or the same ground via multiple wires, that has the potential for ground loops, the brute force approach is to use an opto isolator like the ancient 6N137 (there are newer, faster and cheaper options available these days, but it works).

author: Dasypus
author_rank: Contributor
timestamp: June 25, 2025, 04:25:03 pm
content: I've just tried a 1N4148 & 10K resistor and it seems to do the job, thank you all.

author: MrAl
author_rank: Super Contributor
timestamp: June 25, 2025, 04:34:00 pm
content: Quote from: Dasypus on June 21, 2025, 01:14:28 pmIf the LEDs draw power too, you could try to cut them out.