FT232R Self Powered Config

TomS

Has anybody used the FT232R in self powered configuration? It's described in Fig 12, p20 of the FT232R datasheet, v1.04. Any Gotcha's or quirks I need to be aware of?

Thanks
Tom

Forrest

Bean uses the FT232RL in a USB self powered configuration on his Prop Dongle http://forums.parallax.com/showthread.php?p=653637

TomS

No, that's a bus powered design. The FT232R Vcc pin is connected to +5V from usb and VCCIO is connected to 3V3OUT. I want to power the FT232R from the same supply as the prop uses, not from the usb bus, ie, 3V3OUT isn't used, VCC, VCCIO and the prop are connected to an external source of 3.3Volts not derived from usb +5V.

Phil Pilgrim (PhiPi)

The FT232R self-powered configuration will likely be much less troublesome than the various bus-powered configurations, so long as you follow the FTDI guidelines. One possible gotcha is that when you unplug from the USB port, DTR# will be pulled high by the FT232R's internal 200K pullup. This may cause the Propeller to reset, assuming a 16uA pulse through the base capacitor is enough to cause the reset transistor to switch.

I've worked with this chip enough to know not to take anything for granted (or to trust FTDI's datasheet and appnotes either, for that matter). Any data gleaned from experimentation should be given the most weight.

-Phil

TomS

Thanks Phil,

The 200k pullup forms a divider with the 10k base resistor so false reset shouldn't be a problem. Even so, I don't mind if a reset occurs. Right now every time I unplug power from my custom board and plug power back in with the usb cable connected, I get a latchup in an i2c bus chip that I think is caused by power flowing through a substrate diode on the propeller. I measure almost 1 volt on Vdd. I suspect the demo board would do the same if I tried with the same ic but it's surface mount so I can't try it easily (my circuit is essentially the same as used on the Demo board).

Although I have plenty of excess capacity in my power supply I'm curious if it goes into USB Suspend mode when reset# is pulled low, thereby lowering current draw. Do you happen to know?

Tom

Phil Pilgrim (PhiPi)

You can look at the SLEEP# output when RESET# is pulled low. If it goes low, too, the device is likely in suspend mode.

-Phil

TomS

I just finished testing the FT232R in self-powered mode and with the USB cable connected and the FT232R not reset, the chip draws 8mA. When I remove the USB cable and the FT232R is reset, the current draw drops to 1.5mA. For some reason it doesn't drop to what the datasheet states (70uA Typ) but a delta of 6.5mA is better than nothing.

Tom