What is the preferred USB interface circuit design for the Prop?

Don M

No doubt there have been numerous designs and with Tracy's latest post concerning some issues- http://forums.parallax.com/showthread.php/146831-Fuzzy-tail-on-RESET-pulse-from-FT232R it got me to looking further into what is being used successfully.

On some of my projects I have used the FT232RL along with a logic gate 74LVC2G07 (as suggested by Phil).

I just looked at the latest Prop Project development board and see they are now using the FT231XS. First of all the FT231 is half the cost ($2.12 vs $4.50 DigiKey singles) of the FT232 so that gets my vote if it is a stable design. I'm trying to finalize a design so I'm looking for some ideas here to finish up.

Here's what has been suggested from some posts and what I see in some of the diagrams:

1. Using a 1uf instead of a .1uf on VCCio of FTDI chip
2. The cap from DTR change to .01uf instead of .1uf
3. Use a combination of 10K / 270 ohm resistor in base circuit of reset transistor

Not sure if the logic gate is needed? I had never had any problems as reported by some with the random resetting issue. Not sure if the newer FT231 may solve that?

Also I see now a mosfet switch being used to power a board and being controlled by the FTDI chip once enumeration and communication has been established. What are the advantages / disadvantages of using the mosfet other than isolation between power sources which a diode could do. The mosfet would have a lower voltage insertion loss than a diode but what else?

Don M

Don M wrote: »

Also I see now a mosfet switch being used to power a board and being controlled by the FTDI chip once enumeration and communication has been established. What are the advantages / disadvantages of using the mosfet other than isolation between power sources which a diode could do. The mosfet would have a lower voltage insertion loss than a diode but what else?

After reading a bit further into the FT231 spec sheet I see why they suggest this method. It has to do with the USB suspend mode so that the current draw is minimized.

Don M

Concerning the mosfet circuit- here is the diagram for the Parallax version of the Propeller Project board-

And here is the one from FTDI's data sheet-

Looks like one of them has the drain and source reversed. Which one is correct?

Looking at the diagram of the suggested mosfet from the data sheet- IRLML6402-

It looks to me like the internal diode is going to allow current to flow in the reverse direction without the mosfet being on.

tonyp12

As I see no charge pump circuitry, Parallax must be using P-Channel and is using wrong symbol for it.

Don M

So looking further at the Parallax circuit here is the USB power input along with the Schottky diodes for the main power input-

It looks to me as though the 5V from the USB port will just flow right on through whether the mosfet is turned off or on. The only thing keeping the external power from flowing back into the USB 5V is the schottky diode.

Edit: here is a more complete drawing-

Duane C. Johnson

Hi Don.

Don M wrote: »

It looks to me like the internal diode is going to allow current to flow in the reverse direction without the MOSFET being on.

Yes, MOSFETs ALWAYS have a reverse body diode as part of the intrinsic structure. Its always there even if you don't want it. In the symbol its the little arrow attached between the conduction zone and the source.

In some MOSFETs, as in the IRLML6402, they add a extra zener protection diode used to limit or snub the maximum Drain to Source voltage. In this case this diode also enhances the reverse current flow capability of the intrinsic body diode.

Duane J

tonyp12

>internal diode is going to allow current to flow in the reverse direction without the mosfet being on.
Parallax do have the D2 Schottky Diode to prevent that.
Ftdi uses a softstart curcuit.
I would use a single Power-Switch to do it all , soft-start, reverse current blocking and 1.5A current limiter.
http://www.mouser.com/ds/2/115/AP2161-6258.pdf

Don M

@tonyp12- if you were to use that IC would you still put a diode in series with the output if it is tying into the 5V bus that may be powered by other means? I know you wouldn't need one for the reverse current block to the USB source but what about back feeding from the other 5V sources but into the IC?

Don M

Here's something else that seems confusing. Either Parallax is re-progamming the FT231 or they have the component labeled wrong.

Here's their pinout for the TX & RX led's and the PWREN-

And here's the pin description from the data sheet-

I would think that they are using them as stock and not re-programming them.

Parallax?