part. So, we’ll also use the output of the FT232R’s LDO
voltage regulator to drive the FT232R’s I/O pins at 3. 3 volt
logic levels via the VCCIO power input pin.
In reality, the FT232R’s active-low RESET pin is
internally pulled up and can be left disconnected. The
datasheet gives us the option to tie the RESET pin logically
high or control it with an external device such as a
microcontroller. There are no microcontroller I/O pins
available to us in this situation. So, to play it safe and
avoid any possibility of a spurious reset, we’ll physically
connect the FT232R’s RESET pin to VBUS.
The Vinculum-II’s debug interface is a single-wire
bidirectional data pipe. To adapt the FT232R’s two-wire
TXD/RXD serial interface to the Vinculum-II’s single-wire
debug interface, we’ll multiplex the TXD and RXD signals
using a 74LVC2G241 dual three-state buffer. The data flow
direction on the Vinculum-II interface is controlled by one
of the FT232R’s programmable CBUS I/O pins (CBUS4).
The CBUS I/O configuration is retained within the bit
structure of the FT232R’s internal 1024-bit EEPROM.
However, we can view and change the CBUS I/O
configuration using the FTDI FT_Prog Utility which (like
most other FTDI tools) is a free download. With the help
of this Utility, let’s see if we can make some educated
guesses as to how the CBUS I/O is configured for the
Vinculum-II debug interface application.
CBUS0 is a configurable I/O pin that defaults to an
active-low TXLED# output signal. The "#" character
denotes an active-low FT232R or Vinculum-II signal. In its
default mode, this signal pulses logically low for
transmission events. According to our debug interface
layout, CBUS0 is configured as TXRXLED#. As you can see
in Screenshot 1, TXRXLED# is termed TX & RXLED# in the
FT_Prog Utility and causes CBUS0 to pulse logically low
and blink the TX/RX LED at every TXD or RXD bit that
■ FIGURE 1. Once again, a picture is worth a thousand
words. This is a block diagram of the FT232R’s system cells.