FTDI chip design/soldering problems - Page 1
Forum section: electronics / projects

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 04:38:38 am
content: Hey guys, I'm currently working on a project that has a ftdi usb - uart converter and am running into some issues. Now this is the first time I've tried to design and solder something like this before so I'm aware it could be a number of reasons things are going wrong. To give you a bit of background, I am using anft234xd chipthat I am using insimilar to the "self powered" mode explained in the datasheet. Firstly, yes I have powered it (checked the voltage with DSO for ripple etc.) and that seems fine. I've done continuity checks between relevant points and everything is connected to where it should be..At first I got help soldering it which was done by a tech staff where I study. They used solder paste and a reflow/heat profile (w/e its called) machine that can do temperature profiles however the first attempt seemed to lack enough solder for the job. After that, they heated it up again, took the chip off, put more paste on and tried again. It looked sloppy but thought it would work so I took it home. When plugging it into my computer, windows says that there is an unknown device in the USB port and the FT_PROG utility from ftdi cant find anything connected to the computer. After that, I attempted to remove the chip with a hot air gun (did it by feel until the chip came loose making sure to warm everything up before I got it very hot) , clean up the pads, tinned the pads and reflowed the chip again. Same as before, continuity and voltage seems fine but the computer doesn't recognise the device.Considering all that, whats likely to have happened here. Did I brick the chip through overheating? Or possibly have something shorted that I haven't thought of? Or is my schematic wrong/do I need more things on the board to make it "work"?Help a newbie out

author: franksanderdo
author_rank: Contributor
timestamp: March 28, 2015, 05:00:55 am
content: Hi SilenusJust a guess:Windows says unknown device. Can you check VID and PID that windows is detecting and verify this is the standard VID/PID the chip has coming from FTDI?If so there are good chances that your drivers are not installed / got kind of weird. In that case a manual install of the drivers might help you out.All the BestFrank

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 05:14:21 am
content: Quote from: franksanderdo on March 28, 2015, 05:00:55 amThe hardware Ids section of the "unknown device" just says "USB/UNKNOWN"

author: David_AVD
author_rank: Super Contributor
timestamp: March 28, 2015, 05:39:31 am
content: 1) What are R1 and R2 in your schematic? On the PCB they look like ferrite beads.2) I don't see a capacitor on Vcc close to the chip.3) Are you sure you have the USB D+ and D- in the right places on the USB connector? Swapped D+ and D- will cause the "unrecognised device" message.

author: amyk
author_rank: Super Contributor
timestamp: March 28, 2015, 05:43:26 am
content: Fake chips?Good chance to mention that there are alternatives available from other companies...

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 06:24:40 am
content: Quote from: David_AVD on March 28, 2015, 05:39:31 amR1 and R2 are varistors, U6 is a ferrite bead.You are right, the closest one is near the power supply, opposite corner on a 10x10cm board, could possibly be a reason.I double checked the D+ and D-, they are in the right place.

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 06:53:51 am
content: Okay, I hooked up the D+ channel to the DSO and captured some of the serial when I plug the device in.. I am pretty sure it's not supposed to look like that. Any idea what's going on here?

author: zapta
author_rank: Super Contributor
timestamp: March 28, 2015, 06:57:18 am
content: Wrong capacitor values? Try to remove them.

author: David_AVD
author_rank: Super Contributor
timestamp: March 28, 2015, 07:00:45 am
content: I'd also remove the varistors too.Got a picture of the other side of the board in that area?

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 07:15:45 am
content: Quote from: David_AVD on March 28, 2015, 07:00:45 amremoved the varistors, computer now recognises it on com portthank you a lot. I'm guessing I picked the wrong varistors in that they were shunting current away from the D+/D- bus or perhaps coupling noise on the usb frame *shrug* I'll have to have a better read on the subject later.

author: Monkeh
author_rank: Super Contributor
timestamp: March 28, 2015, 08:15:28 am
content: You may have meant well by putting varistors on the data lines, but which varistors specifically did you place?

author: David_AVD
author_rank: Super Contributor
timestamp: March 28, 2015, 08:20:35 am
content: It usually pays to reduce the circuit down to the simplest form in situations like this.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 28, 2015, 08:36:05 am
content: The varistors can add capacitance and lowpass filter a square wave into a sawtooth without edges to sync on. Look for some low capacitance TVS instead.

author: Silenus
author_rank: Contributor
timestamp: March 28, 2015, 08:42:06 am
content: Quote from: Monkeh on March 28, 2015, 08:15:28 amWent withhttp://www.digikey.com.au/product-detail/en/0/478-2498-1-ND, it was my first time using something like that and I saw something similar on an arduino design so I tried to replicate it. My understanding was that they were essentially open circuit unless the voltage spiked over ~6V.

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 28, 2015, 08:50:25 am
content: If you are not shure what you are dooing, it tremendously helps populating your board in stages.Populate the bare minimum to make it work, then test it.If it works put on the next few parts wich could make it go bad, test it.Then put the rest on and test it again.

author: Howardlong
author_rank: Super Contributor
timestamp: March 28, 2015, 09:11:18 am
content: Those varistors are expensive too!It's common to use low capacitance protection diodes: I use this devicehttp://www2.mouser.com/Search/ProductDetail.aspx?R=CM1213A-02SRvirtualkey65230000virtualkey748-CM1213A-02SRfor full speed USB applications in production, but for one off USB own-use designs, including with FTDI devices, I usually don't bother.Does the FT234XD already have enough integrated diode protection? I couldn't see it mentioned on the datasheet, but equally there is no external protection in the examples given.

author: Monkeh
author_rank: Super Contributor
timestamp: March 28, 2015, 07:44:54 pm
content: Quote from: Silenus on March 28, 2015, 08:42:06 amUntil such time as it begins to conduct, that part is a 750pF ceramic capacitor.

author: Howardlong
author_rank: Super Contributor
timestamp: March 28, 2015, 08:02:03 pm
content: By the way what is U6 to the USB connector shield?Be careful if it's just a ferrite, you may have a problem with ground loops if so between the host and the serial target if it's connected to AC mains and not isolated.For this, I use a 4.7nF in parallel with a 1Mohm. This gives both a DC path for ESD protection and an AC path for EMI compliance, but if the ground levels of host and target device are slightly different that will not be a problem either. If you left the ferrite in there, there could potentially be a large current flowing through it, even though it may only be a few hundred mV.