■ SCREENSHOT 1. The FT_Prog Utility can be used to
inspect and/or change the FT232R’s innards. We’re
about to alter the default CBUSO configuration
TXLED# to TX & RXLED# in this shot.
I/O pin. The PWREN# signal is used to activate external
current loads once the FT232R has been successfully
configured, and deactivate those current loads when the
device enters USB suspend mode. We’ve emulated the
functionality of this pin with a microcontroller I/O pin in
past Design Cycle USB projects.
Looking at Schematic 1 again, the PWREN# signal
drives the logic-level gate of a P-channel MOSFET. Resistor
R6 insures that the DMP2123L- 7’s gate is pulled to VBUS
when the PWREN# signal is inactive (logically high). When
the PWREN# pin transitions to its active-low state, resistor
R7 and capacitor C9 act as a soft start circuit. The soft
start configuration prevents the FT232R or any attached
USB host from resetting due to the transient surge current
that occurs when the MOSFET switches on.
CBUS4 is output only and defaults as an active-low
SLEEP# output. In Schematic 1, CBUS4 is labeled as the
active-high TXEN pin. Recall that the TXEN signal is used
to control the flow of debugger interface data through the
three-state buffer pair. The FT_Prog Utility calls our TXEN
signal TXDEN. TXDEN is normally used in half duplex RS-
485 links to switch the direction of transmission. In that
we’re also switching transmit and receive circuitry, our
application of TXDEN is very similar to its originally
intended purpose. When TXEN is logically high, data is
allowed to flow from the FT232R’s TXD pin, to the 2A
input of the 74LVC2G241 three-state buffer, and out of
the 2Y buffer output to the Vinculum-II’s debugger
interface. Data flowing from the Vinculum-II through the
debugger interface is allowed to flow into the FT232R via
the alternate gate of the three-state buffer when TXEN is
If you look back at some of the previous Design Cycle
USB designs, you’ll see that we used a TPS2041BDBVT
instead of a MOSFET to switch the VBUS voltage to an
external load. The TPS2041BDBVT is equipped with an
OC (Over Current) output pin which we monitored with
the I/O pin of a microcontroller. In our Vinculum-II
debugger/programmer design, the TPS2041BDBVT and
associated microcontroller are replaced by a DMP2123L- 7
and an SMT PTC fuse. Resettable fuse F1 holds at 200 mA
and trips at 500 mA.
If you wish to consider the TPS2041BDBVT as an
alternative power control design, it is rated for 500 mA
continuous, and includes circuitry that will limit the
current to a safe level in the event of a continuous or
severe overload condition. However, if your Vinculum-II
design will draw current that is always hovering near the
500 mA USB portal maximum, you will need to provide
external power to your circuitry regardless of the power
control design you choose to use.
The debug interface circuitry we’ve just designed can
stand alone or be included on the Vinculum-II application
circuit board. Low-power Vinculum-II applications will
want to see the this circuitry out board. Vinculum-II
application development platforms will benefit by
including the debug interface circuitry on board.
As it turns out, we can ride our donkey all day long
since the Vinculum-II IDE takes care of configuring the
FT232R’s CBUS I/O pins for the debug interface. With
that, let’s talk a little about the Vinculum-II.
August 2010 57
■ SCREENSHOT 2. Not only does this utility spill the
beans on the Vinculum-II’s I/O, it writes the code too.
To keep things simple, I’ve asked the utility to only
write code for the 64-pin part.