USB Power USB to Serial Adapter

Macgruber

Hi Everyone,

Had a question regarding the USB to Serial Adapter... I wanted to use this hardware to provide communication to an IR circuit which has some LEDs (total consumption ~200mA tops) and I was wondering if the USB BUS power from my laptop will be available through this adapter (couldn't find a datasheet). I read online that a USB device can request that the host categorize the device as high power and supply it with 500mA but I don't even know if this is possible via the adapter.

Any feedback would be appreciated!

http://www.edn.com/article/CA220400.html

Erik Friesen

I'd just give it a try. I think the FTDI chip enumerates with a 100ma request. It may work depending on your computer.

Macgruber

I'll give it a try... I'd just like to avoid hurting the BUS on my laptop in the event that it is enumerated incorrectly

SRLM

You could use a fuse or a zener diode rated for the current that you want.

Erik Friesen

I think the port will be shut off if you draw too much current but again that depends on your machine.

Macgruber

Thanks for the suggestions guys... I think I'm just going to give it a shot and see how it goes. I looked at the datasheet for the FTDI chip and it definitely supports different type of enumerations but I think that Parallax must set these up in factory so I have no idea which configuration will be used. It also seems possible to configure the chip to pass through the usb power and also "elegantly" shutdown the rest of the circuit when the laptop / usb host goes into suspend (I'm kind of wondering if I could achieve the same effect with the brownout detection of the uC... I'm using the SX).

If anyone from parallax is listening... A datasheet on the usb to serial adapter would be really really useful!

Next time I might just order the raw chip and go from there...

Chris Savage

I’m not sure why we would need a datasheet. The datasheet for the FTDI chip covers everything and we’ve made no changes so it would be a duplicated effort.

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Chris Savage
Parallax Engineering

Macgruber

Chris Savage (Parallax) said...
I’m not sure why we would need a datasheet. The datasheet for the FTDI chip covers everything and we’ve made no changes so it would be a duplicated effort.

Chris, I have no idea if the chip is configured for triggering the enumeration of the USB port as a high powered device??? Isn't this a parameter that you program on the chip? I take it that the Pin Out of the DB9 connector is like a standard computer serial port? Vcc is not passed through one of the pins? The FTDI chip also allows for multiple configurations to make the device support the powering / signaling of the slave device for different USB conditions... Are these utilized?

Maybe I'm clueless but I just don't seem to have a good handle as to how I need to incorporate this device. I noticed multiple test points on the board... Can I solder a lead to one of these to get the USB power?

Chris Savage

Our USB to 232 device is like any other unit you would buy at a store to convert your USB to work with a device that requires a serial port. The pinout for the DB9 in this manner does not include such a connection, which would potentially make the adapter incompatible with many devices. As for high-power mode, yes, you do have to program the chip for that, but since USB to Serial Adapters don’t supply power to a target circuit, there is no reason to do this. The chip is unprogrammed and comes with its factory default settings.

I'm wondering if we're even talking about the same adapter, since our USB to 232 doesn't have any test points on it.

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Chris Savage
Parallax Engineering

Macgruber

I think that we are talking about the same thing... By test points I meant the little open holes under the shrink wrap of the device (maybe I'm just to new to know what those are actually called ). Anyway, thanks for the info! I used this adapter along with the PDB and the SX chip to design a pretty cool circuit and it worked great to provide com with my PC.



I was considering making this a new thread but figured that for continuity I would just ask this question about making a circuit that uses the USB bus power...

I'm thinking that I am going to order the DS_MM232R (link below) from FTDI to make the final circuit. As far as I can tell, this device is like the USB to serial adapter but it just gives you a USB port that connects to the FTDI chip and then provides header pins to access the IO of the FTDI chip along with the RS232 com lines. Now, on the Parallax USB to serial adapter there are a couple of extra ICs that as far as I can tell are used to bring the IO of the FTDI chip to RS232 level??? If that is the case, do I need to do anything special when connecting the DS_MM232R to my SX chip? I imagine I can just use the TTL signals from the header pins and run them right to the IO pins of my SX chip without any need for an RS232 level converter? Should I place a resistor between the header pins and the SX IO pins?

Ultimately, the objective of using the DS_MM232R is that I can get the virtual com port and then wire IO of the FTDI chip to control the power of my circuit through a P-channel logic level MOSFET. One of the IO pins on the DS_MM232R can be programmed to switch the circuit on and off when the PC goes into suspend and when the device is being enumerated so I don't draw too much current off the USB bus. I'm thinking that I can use this technique along with the brownout detection of the SX chip to switch the circuit on and off and run everything off USB!

Do you gurus think that this is possible or am I oversimplifying my design?

www.ftdichip.com/Products/EvaluationKits/MM232R.htm

Chris Savage

The holes you’re referring to are called vias and allow traces to route from one side of the PCB to the other. They most often don’t work as test points because they’re typical solder-masked. That is, the copper is covered so you cannot solder to it.

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Chris Savage
Parallax Engineering