Let's say you work for FTDI Chip... - Page 1
Forum section: electronics / projects

author: KE5FX
author_rank: Super Contributor
timestamp: May 26, 2018, 10:06:10 pm
content: How does something like this happen? Is it poor recruiting practices? Drug use in the workplace? Somebody's idea of a joke? Or just institutional indifference?

author: CM800
author_rank: Frequent Contributor
timestamp: May 26, 2018, 10:34:40 pm
content: All a matter of perspectiveAnd it fits well with the general style of USB slim products (e.g. on a USB cable)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2018, 11:30:20 pm
content: The added 2mm of trace length is utterly immaterial, FYI.Tim

author: KE5FX
author_rank: Super Contributor
timestamp: May 26, 2018, 11:53:21 pm
content: Yeah, obviously it's immaterial... but, again, why do it this way? Cypress did the same thing on the QFN version of their FX2 chip, good luck turningthatone 90 degrees. I don't buy the cable argument but I agree it's the only halfway-plausible excuse.The best way to do it would have been to bond out one of the lines to two pins, one on each side. The resulting stub would also have been immaterial, but I guess they couldn't afford to sacrifice a pin elsewhere.

author: chris_leyson
author_rank: Super Contributor
timestamp: May 27, 2018, 12:08:42 am
content: Somebody has got their panties in a twist, it's not like it's microstrip at GHz only USB 2.0. Their job is to layout and package Silicon not to layout your PCB. If you put the chip on the bottom side of the PCB it wouldn't be a problem. Put your toys back in the pram and deal with it.

author: CM800
author_rank: Frequent Contributor
timestamp: May 27, 2018, 12:10:09 am
content: Quote from: blueskull on May 27, 2018, 12:05:36 amOut of curiousty, what is your reason for always using bottom mount connectors on top side?

author: KE5FX
author_rank: Super Contributor
timestamp: May 27, 2018, 12:11:31 am
content: Quote from: blueskull on May 27, 2018, 12:05:36 amI'll confess I had no idea there was such a thing as a "bottom mount" USB connector. The connector I used here isthis one, perfectly standard part with no mention of what side of the board it's meant to be mounted on.I've used the QFN version of the CY7C68013 (FX2) before, but in that case I actually did mount this connector on the bottom of the PCB. It was only in a recent project with the FT245 that it occurred to me to wonder why they chose this particular pin order.Quote from: chris_leyson on May 27, 2018, 12:08:42 amFound the FTDI employee...

author: DaJMasta
author_rank: Super Contributor
timestamp: May 27, 2018, 12:13:41 am
content: QuoteBut USB is a differential pair, you don't want them to be split up on the chip they're going into...Maybe they think routing towards the interior of the chip instead of out from the pins is less likely to get in the way of using its other IO and bypassing and such in single sided load designs. Then it's just a matter of whether you want the chip slightly on the left or the right of the connector, and they chose left.

author: Monkeh
author_rank: Super Contributor
timestamp: May 27, 2018, 12:13:56 am
content: I'm pretty sure the use of a mini connector is far more suspect than their pinout.

author: KE5FX
author_rank: Super Contributor
timestamp: May 27, 2018, 12:18:24 am
content: Quote from: Monkeh on May 27, 2018, 12:13:56 amBut I like mini connectors.I hate micro connectors, and there's not enough room for the ruggedized Amphenol full-size 'B' connector that Iusually use. That leaves mini-B.

author: CM800
author_rank: Frequent Contributor
timestamp: May 27, 2018, 12:21:31 am
content: Quote from: KE5FX on May 27, 2018, 12:18:24 amWhat about the glorious new USB-C masterrace?

author: KE5FX
author_rank: Super Contributor
timestamp: May 27, 2018, 12:27:56 am
content: Quote from: CM800 on May 27, 2018, 12:21:31 amNo experience with those, even as a user.They seem to cause a lot of confusion, though, and I don't have any of those cables around here.

author: CM800
author_rank: Frequent Contributor
timestamp: May 27, 2018, 12:39:12 am
content: Quote from: KE5FX on May 27, 2018, 12:27:56 amI have them on my phone and laptop.Great connector, sturdy and reliable.you can also plug it either way around.(no flipping it back and forth until the superposition collapses) You don't need to worry about all the excess wires. You can use it legacy on a 2.0 device easily enough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2018, 01:19:17 am
content: Quote from: DaJMasta on May 27, 2018, 12:13:41 amJust how much splitting up do you think is important here? No, seriously -- not being pejorative, it's exactly the question we should be asking ourselves all the time. How much does it matter?Do we really need that bypass cap? Do we really need (or even want) differential-routed traces? Do we really need length-matched traces, or controlled-impedance traces?How much?'Tis a question left sorely unasked, resulting in people often making very bizarre priorities, that only hurt the project overall. Test! Measure! Ask!Tim

author: Mr. Scram
author_rank: Super Contributor
timestamp: May 27, 2018, 01:23:39 am
content: Quote from: CM800 on May 27, 2018, 12:21:31 amThe USB C connector seems to be quite a bit more complex.

author: amyk
author_rank: Super Contributor
timestamp: May 28, 2018, 12:15:47 am
content: Yes, this pinout was probably designed for putting things on the other side of the board.I remember reading some datasheet from a different manufacturer, for a USB device which had automatic D-/D+ detection.On the other hand, MCUs with the GPIO pins for one port scattered on all 4 sides of the QFN is a bit more of awhy?situation...

author: cyberfish
author_rank: Regular Contributor
timestamp: May 28, 2018, 11:28:40 am
content: Here is MC33883, a H-bridge gate driver chip.C1 and C2 go to an external capacitor for the integrated charge pump. They have no other function.

author: CM800
author_rank: Frequent Contributor
timestamp: May 28, 2018, 12:07:07 pm
content: Quote from: cyberfish on May 28, 2018, 11:28:40 amA little bit confused as to your point?I'm reading it like:A car has a steering wheel to steer it, it has no other function.

author: cyberfish
author_rank: Regular Contributor
timestamp: May 28, 2018, 12:38:41 pm
content: Quote from: CM800 on May 28, 2018, 12:07:07 pmIt's the fact that they are on opposite corners of the chip. A more sensible design would put them next to each other. If those pins had other functions, those other functions might have justified putting those pins the way they are.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 28, 2018, 03:13:42 pm
content: This C1/C2 pin assignment looks funny at first sight, and is certainly an annoyance for routing purposes.I'm guessing it may have something to do with the way the power rails are routed inside the chip die though, imposing a pad placement. Then you can't really make bonding wires cross one another to make the final package pins easier to route. Or you would need an interposer, which would make the package costly.Just a thought.