Propeller Activity Board USB hardware bug?

Peter Jakacki

I noticed on the Propeller Activity Board schematic that the USB circuit has a tristate gate between the FT231 Txd and the Prop's "Rxd" pin. Is that a mistake?.
It looks like it was meant to be from the Prop's "Txd" to the FT231's Rxd so that the Prop doesn't end up phantom powering the FT231 when the USB cable is unplugged.

Phil Pilgrim (PhiPi)

That was my first reaction, too, when I saw the schematic. But the Activity Board does not use the FT232R, which exhibits the parasitic power-up issue. Apparently, the newer USB chip that it does use does not have this problem.

-Phil

Peter Jakacki

Phil Pilgrim (PhiPi) wrote: »

That was my first reaction, too, when I saw the schematic. But the Activity Board does not use the FT232R, which exhibits the parasitic power-up issue. Apparently, the newer USB chip that it does use does not have this problem.

-Phil

Still, it looks a bit strange, just like a mistake was made in transposing rx and tx lines. Floating the Prop's Rxd line does not make any sense, but floating the FTDI chip's Rxd does if that is where it was meant to go. Still, a series resistor is all that is really required, any kind of gate is really overkill.

jazzed

I'm just happy that the FTDI RX doesn't have an accidental oscillator attached to it like on the Rev A Quickstart board when Propeller P30 is floating.

Maybe there was still some concern over parasitic power. If it's a non-issue, then it's a cost reduction opportunity.

Personally I'd like to see a set of jumpers to choose between the USB IO and the XBee IO (default to USB of course). Alternatively one might replace the USB entirely with another XBee socket to allow choosing which type of interface to use in the event that another interface like a Wifi or Bluetooth module becomes a viable and popular option. There are already USB XBee modules.

David Carrier

The buffer (U4) prevents the FT231X from driving the Propeller microcontroller RX pin when the Propeller isn't powered. The FT231X TX rests high, so if it were directly connected to the Propeller RX pin, it would power the Propeller through one of the pins ESD protection diodes, making it impossible to turn the Propeller off when the FT231X is powered on. The reason it is a 74AHCT1G126 tri-state buffer is that it costs 25% less than the cheapest non-tri-state buffer. The buffer's OE pin could can be connected to either the 3.3 V or 5 V USB supplies, it doesn't really matter, but with the 5 V USB connection we used it does save about three fourths of a micro amp of current when the USB port is unpowered, so the batteries should last about 0.00003% longer

— David Carrier
Parallax Inc.

Peter Jakacki

David Carrier wrote: »

The buffer (U4) prevents the FT231X from driving the Propeller microcontroller RX pin when the Propeller isn't powered. The FT231X TX rests high, so if it were directly connected to the Propeller RX pin, it would power the Propeller through one of the pins ESD protection diodes, making it impossible to turn the Propeller off when the FT231X is powered on. The reason it is a 74AHCT1G126 tri-state buffer is that it costs 25% less than the cheapest non-tri-state buffer. The buffer's OE pin could can be connected to either the 3.3 V or 5 V USB supplies, it doesn't really matter, but with the 5 V USB connection we used it does save about three fourths of a micro amp of current when the USB port is unpowered, so the batteries should last about 0.00003% longer

— David Carrier
Parallax Inc.

Thanks for explaining that David, I wasn't sure what the real reason was
BTW, I would have just powered the whole darn board when USB was plugged in anyway!
Oh, and I love the 0.00003% - can't underestimate any savings you know.