Arduino Forum

Using Arduino => General Electronics => Topic started by: evanrich on Apr 04, 2013, 12:00 am

Title: Questions regarding FT232RL USB->UART and schematics
Post by: evanrich on Apr 04, 2013, 12:00 am
I just finished building a project I was working on to control Sure 3208 displays, and it worked, however I had a few questions regarding the FT232RL IC that I'm using.

First, my issue.
Occasionally when I plug the board into my computer, my PC says "Unrecognized device", and I have to unplug, wait a few seconds, and plug in...sometimes it takes 2 or 3 times to get it to work.   I'm thinking it might be loose solder connections on the package, but that leads me to my questions:

1.) On some schematics I see on Google, there is a voltage divider on the reset pin, on others the reset pin is disconnected.  Why is this, and should I put one on my schematic?

2.) I've seen resistors on the USB data pins, usually around 33 ohms, but on others the lines are connected directly to the USB connector. And then I've also seen 22-47 pf capacitors decoupling the data lines to ground.  This seems to be mixed as well, so why do some people use them and others don't?  According to the datasheet, the FTDI chip already contains series resistors as well as pullup resistors on the data lines, so these should not be needed, but what about the decoupling caps?

3.) In FTDI's documentation, it states that for "Normal Operation" the "TEST" pin must be tied to ground, or "The device will appear to fail".  Yet in almost every schematic I've seen, the test pin is left disconnected/floating (not tied to anything).   Should I tie this to ground?


Thanks!
Title: Re: Questions regarding FT232RL USB->UART and schematics
Post by: cmagagna on Apr 04, 2013, 07:44 am
The data sheet for the ft232rl has example circuits for the chip in USB powered, self powered, and either/or power (like the Arduino itself does).

I know those designs will work, because I've built things that use them.

Anything other design may or may not work, but why not just save yourself the hassle of speculating and troubleshooting and follow the manufacturer's recommendations?

Good luck!
Title: Re: Questions regarding FT232RL USB->UART and schematics
Post by: CrossRoads on Apr 04, 2013, 03:07 pm
I've followed the design arduino uses, and also read the datasheet.
Reset connected to +5, Test to Gnd, no resistors, no caps on data pins.
Title: Re: Questions regarding FT232RL USB->UART and schematics
Post by: evanrich on Apr 05, 2013, 01:27 am
Thanks guys!