help designing a board - Page 1
Forum section: electronics / projects

author: ultrageek.lloyd
author_rank: Contributor
timestamp: August 16, 2012, 02:31:28 am
content: Hi all,I'm currently designing an EJTAG probe. It's based around a PIC32 microcontroller, an FTDI chip, and two TI sn74avc series bus transceivers. I'm not very experienced with board design, so I wanted to get some feedback on what I've got. If anybody could just give it a look over and give me some tips, I would greatly appreciate it. Schematics and gerbers are attachedThanks

author: arclight
author_rank: Contributor
timestamp: August 16, 2012, 06:08:25 pm
content: This looks pretty good so far. You might want to put a polyfuse on the USB power input. I'm not sure if the chips you're using have decent ESD protection, but you might look into that if this is going to be a high-production item that you'd like to make very robust.I'm sure others will have more insightful comments.Arclight

author: ultrageek.lloyd
author_rank: Contributor
timestamp: August 16, 2012, 11:14:23 pm
content: Thanks for the input. I'm not familiar with polyfuses, so i'll go ahead and get acquainted with them and see how i can integrate them into the board.ultrageek

author: FenderBender
author_rank: Super Contributor
timestamp: August 16, 2012, 11:39:31 pm
content: Arclight, do you mean a PPTC? A polyfuse by definition is part of the memory in a chip. A LittleFuse brand PolyFuse is an actual resetable fuse.I think he is referring to this:http://www.littelfuse.de/data/de/Product_Brochures/EC327-E_Polyfuse_PTC.pdfIf you search "polyfuse" on Google, you might end up with the wrong definition.There's also a ton of other names for PPTCs. PolyFuse, PolySwitch, MultiFuse. There's a bunch. They are all thermal resetable fuses. When a certain amount of current is present, the element heats up and resists current. When it cools down, electrons can flow again.

author: poorchava
author_rank: Super Contributor
timestamp: August 17, 2012, 12:01:17 am
content: I would separate analog supply from digital one. I mean route AVDD through some inductor (might as well be 0805 type) and then decouple with 100nF+optional another parallel capacitor (in case you have some noise problem you'll have a footprint to add appropriate decoupling capacitor for particular frequency). This helps alot if you intend to make use of analog functionality.If that probe is purely digital, then you can omit this.As for pcb design, if you intend to go for any kind of production, I would move mcu and passives above it a bit further from the edge of PCB. Generally you don't want components that close to the edge, because it might be a problem, should boards be assembled one by one (not as a panel). Some clearance also makes it easier to fix the pcb in some 3rd hand/vise/jig for hand soldering. I think unless the design is very space constained a good rule of a thumb is that you don't put components within 3mm from the edge.

author: ultrageek.lloyd
author_rank: Contributor
timestamp: August 17, 2012, 12:41:51 am
content: Quote from: poorchava on August 17, 2012, 12:01:17 amya, it's purely digital, so i guess i can not worry about that?Quote from: poorchava on August 17, 2012, 12:01:17 amit's not for production.... yet. so those are good tips. i'll be sure to move those components, or rather, extend the board a bit upwards.thanks for your responses. go ahead and keep 'em coming if you see anything that could be betterafter having gotten some feedback from here, i'm going to send these out for production.ultrageek

author: krenzo
author_rank: Regular Contributor
timestamp: August 17, 2012, 03:21:29 am
content: Quote from: poorchava on August 17, 2012, 12:01:17 amWhy not just go with a ferrite bead?

author: poorchava
author_rank: Super Contributor
timestamp: August 17, 2012, 08:17:28 am
content: Quote from: krenzo on August 17, 2012, 03:21:29 amI guess ferrite beads are for higher frequencies. Or more likely that's a habit of mine. Usually when doing sensitive mixed stuff i tend to build pi-type 3 pole LC filter for analog supply. 0805's and 0603's are cheap andI've never got any bad results with it. If fact it saved my ass from redesign/bodging 3 or 4 times (and ya, i learned that after having to dremel through analog power traces and hack up supply filter there)