First, a study of the FTDI chip shows a pin, VCCIO, which control the interface signal levels for the chip. This pin should be tied to 5V, if driving a 5V devices such as the PIC 18F2520 and the ATmega328, or to 3.3V for 3.3Vdevices. If left unconnected, it defaults to 3.3V. On the Sparkfun it is left floating and a work-around is not easy; therefore, the DEV09716 produces 3.3V signals when driving a 5V ATmega or PIC device. Modern Device has found that with the Optiboot this can lead to download problems.
Though I do not intend to power my development boards from the USB, for those who do, you should noted that the USB can source a maximum of 500ma at 5V, and a maximum of 50ma at 3.3V from the internal LDO regulator on the FTDI chip. Considering the way I will use these breakouts, it would be more useful to tie the power pin of the breakout to VCCIO. Doing this would make sure the interface voltage of the FTDI chip matches the MCU being used; however, in this configuration the breakout could not power the PCB. From the schematic, the Amicus processor card uses this method, that is VCCIO of the FTDI chip tied to the VCC of the 18F25K20. Since the FTDI chip is on the Amicus PCB, the USB can power the board and, since they have an onboard 3.3V regulator, they do not have the current limit problems.
A final requirement for the USB breakout for use with my boards is that the output connector be mounted on the bottom of the breakout as with the Sparkfun DEV 09716.
Here are my recommendations for USB breakouts:
- Modern Device, BUB 1 @ $14.. This unit makes it possible to connect up just about any way you want. Because of this it is bigger than the competition. This unit has a poly fuse and a ferite filter.
- Modern Device, BUB 2 @ $14. Similar to Friend with addition of an on board 3.3V optional regulator. The output connector is right angle and is shipped unsoldered. Fused.
- Adafruit, USB Friend @ $14.75. For my uses this has all the jumpers, as solder bridges, that I need. No fuses or filters.
- DFRobot, DFR0065 @ $15.95 from Jameco. This seems to be alright but I have asked some question of DFRobot so we will see what their answers are. Poly fuse.