Powering SMT via battery

Started by BMcFrizzle, March 15, 2014, 05:22:22 pm

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I am attempting to power the SMT board via battery; however, it seems that unless the data pins are connected to something, it turns off. Is there any way around this?
Essentially, I want to power the SMT board via a USB battery but grab the data from the Tx/Rx pins on another device.

In order to power the SMT board, I am using two stripped USB cables: one plugs into the SMT board and the other plugs into a USB battery. I am hooking the power pins up between each stripped cable, while leaving the data pins open/unconnected (the data pins will later be connected to another device). The SMT will not turn on in this configuration. Even when the data pins are connected to the other device, it still won't turn on.

Everything works if I hook the same setup into a laptop's USB with the data pins connected.

So I know it is related to the data pins, but I'm not sure how to solve this issue. Any help would be much appreciated.


Not sure if this is directly related to your issue, but I've found that these USB batteries switch off their output if the power drawn is too small as normally they are used to charge phones.


That's what I originally thought, so I hooked it up to a DC power supply and adjusted the voltage and it still never came on. Whenever I mess with the data pins it seems to work -- like the device MUST be able to send data or it'll just turn off... idk?


March 17, 2014, 05:07:20 pm #3 Last Edit: March 17, 2014, 05:14:27 pm by dave-at-axon
OK, I've had a look at the schematic and you are right. It requires the bus to be active to make it work but there might be a way around this. The FT232RL is made by FTDI and the IO Pin 14 is the output that is used to enable the FET (IRLM6402) to allow power to be output to external devices. It enables this pin after negotiating with the USB host.

I can't see any configuration that will allow you to do this with the FT232RL config programme so the only way would be a hardware change.

The gate connection to the FET is driven from the CBUS3 output. If you remove R14 and then connect the FET side to VCC it should enable the FET and give you power out to the rest of the circuit.

Hope this helps?


March 18, 2014, 03:33:47 am #4 Last Edit: March 18, 2014, 03:50:50 am by BMcFrizzle
Great info. I really appreciate it. I might try to figure this out -- I'll be sure to check back with my results if I can pull this off. I am curious, any idea how this bypass may affect the rest of the system (especially if the FT232RL hasn't resolved the USB connection)?

It seems like it may still be ok, but the data will be pushed across the USB at some time after the data is being "pulled" from the electrodes (as long as the USB host is connected to something to receive the data).


If you plan to grab the TX and RX lines, you're going to need to isolate then from the FT232RL, more so the TX data line as you can't have 2 drivers active at the same time. RX will be OK but better to isolate it.

You'll need to make sure you take them from the isolated side, ie the FT232RL side.


One thing I have found in the datasheet for the FT232RL is for the CBUS3 (PIO14) is that it is brought LOW after the USB is resolved (look at #PWREN in the "CBUS signal Options" of the datasheet). So I should hook up Pin14 to GRND, not VCC -- is this correct?

Also, I am not sure what you mean when you say I should isolate the RX and TX? Do you mean isolate them from the power (VCC/GRND) lines? As I described earlier, I have stripped USB cables where I am connecting the VCC and GRND to the battery and the RX and TX wires are hooked to another device that reads serial data. Is this what you are referring to?

Ref: http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

Thanks again!


Oops. Yes, connect to GND. I forgot that it was driving a FET and not a transistor.

Good to see you did your homework though and found it out. :)

By isolation, I mean that the RX and TX from the isolators to the FT232RL are not connected to the FT232RL otherwise you will get 2 buffers trying to drive at the same time.


Ah yes, of course. By isolator, you are referring to the ISO7221xD in the schematic -- the little TI chip.

Needless to say, this all works out nicely. I have now tested this out and I can indeed power the SMT board via a battery and grab the data from the ISO7221xD (U3 tag on the SMT board) using its Pin2 (OUTA in the datasheet). Moreover, analyzing the output in HEX seems to follow the P2 packet format, as expected.

So in conclusion, this works! Connect Pin14 from FT232RL to GRND, grab the output serial data from the ISO7221xD via Pin2 (OUTA) and voila! Battery powered SMT with data to another device.

Thank you so much for your help.


Thumbs up.

Nice work. It's always good to here back when someone gets something working. :)