Nuts and Volts - August 2010

■ 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 logically low.

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.