Beginner design question - Page 1
Forum section: electronics / beginners

author: Ment11
author_rank: Contributor
timestamp: June 13, 2012, 06:01:27 pm
content: Hello everyone, this is actually my first post on the forum, but I have been watching EEVblog for a while now, and falling more and more in love with electronics ( I have a computer science background, but only got into electronics about a year ago).I have a few questions about a project I am working on. To summarize the project, it is a device that sniffs data from a GPIB (IEE488) bus decodes the data that the instrument sends out and sends it out to PC through USB port. The GPIB bus can transmit the data in short bursts of up to 1MB per sec (bursts are under 1KB in size) so I am using a FIFO buffer to store the data while the MCU (PIC32MX795 running @ 80mhz) decodes the data and sends it to USB. I have already built the prototype circuit (Thanks Dave for recommending the Altium prototype boards, they are great.) and all the logic works, but the problem I have is powering the project, and keeping the noise down. The device will be used in high RF environment, and I am worried of something failing due to noise.Basically my questions are if my power solution is going to work, and how should I go about laying out the PCB. I have never done a PCB layout before, so any help will be appreciated.I attached the schematic for anyone who is interested.P.S. this is also my first schematic, and my first full design, so any criticism will be appreciated.

author: Kremmen
author_rank: Super Contributor
timestamp: June 14, 2012, 05:35:01 am
content: Your regulators look OK. Both voltages are produced by a basic buck configuration that should work just fine. However, laziness prevented me from verifying the component values but they look reasonable at least.The schematic looks OK as far as drawing is concerned. While individual styles vary and i would have done some parts differently, i see no real problems with this one.If this will be your first PCB ever, then there are 2 things: 1) it will be quite a challenge to get right and 2) you have learned a lot when (if) you get it actually working. I cannot but recommend that you study the subject of PCB layout for some time before naively rushing to create something hopeless. (I assume that you have studied or are in the process of doing so already).Some elementary points to consider:Power, ground and signal current propagation:- always keep in mind that what goes up must come down, i.e. all signal currents have a return current, because electrons travel in a loop. So don't forget the other half of the signal; that the current returns via the ground connection. "Old fashioned" tree-like ground paths are crap - they almost guarantee maximum length of the signal loop - bad. If this will be a 2 layer board where you do not really have dedicated plane layers, aim for a grid-like ground arrangement where the length of return paths are minimized.- After the power and ground, pay special attention to the high speed signal paths and their return grounds. Minimize length as far as reasonably possible and make sure not to introduce crossing tracks in the ground that block return currents.- Make sure that all power bypass caps are as close to the chip power pins as reasonably possible. For processors the usual thing is to lay the caps on the bottom side directly under the chip and connect using vias to minimize track length.General:- Keep the switching regulator board layouts as tight as possible, with minimal track lengths and optimal component placement. Plan the component group layout first and then move the entire group into place, making last adjustments as needed. That way you minimize losses and EMI radiation. Especially see to it that the inductor, freewheel diode and controller pin 2 create as small a loop/area as reasonably possible.- at these clock speeds you don't really need to worry about transmission line effects so signal line terminations or length equalization is not necessary.- do use copper pours / flood fills connected to ground (and maybe positive rails as well, where it makes sense). I usually do 2 layer boards so that bottom is ground flooded and top is rail flooded to increase the bypass capacitance. One thing though: you should not fill areas near (under) heavy EMI sources to avoid noise pick-up. So it may not be clever to flood the area directly under the above mentioned junction in the power supply.- do not trust autorouters and autoplacement too much. Those may be useful tools for initial layout and connections but in practice there is always room for improvement. In fact i never use either myself.You can find much more in the net and literature, but this was my 2c worth on the subject.

author: free_electron
author_rank: Super Contributor
timestamp: June 14, 2012, 01:24:35 pm
content: I have one thing to add : respect the current order !Meaning you go from supply , to decoupling cap , to chip. Not the other way around !

author: Ment11
author_rank: Contributor
timestamp: June 14, 2012, 02:32:34 pm
content: Thanks a lot for the replies.After reading them I have a few more questions. Would it be worth it to do a 4 layer layout for the PCB, and also as far as the decoupling capacitors, is it worth using the ones that are specifically designed for the task? (such ashttp://www.digikey.com/product-detail/en/CX0603MRX7R6BB104/311-1237-1-ND/1219088) or will regular ceramics do the job?

author: T4P
author_rank: Super Contributor
timestamp: June 14, 2012, 04:26:20 pm
content: Quote from: Ment11 on June 14, 2012, 02:32:34 pmThat ... looks weird, use a standard 0603 X7R and you'll be fine

author: Ment11
author_rank: Contributor
timestamp: June 14, 2012, 04:52:13 pm
content: Quote from: DaveXRQ on June 14, 2012, 04:26:20 pmK, thanks. Ya those have extra ground connections from what it looks like, just didn't know if it would make any significant difference. Probably gonna stick with standard 0603 X7R ones the decoupling ones also look like more of a pain to work with.

author: Kremmen
author_rank: Super Contributor
timestamp: June 14, 2012, 05:28:54 pm
content: Quote from: Ment11 on June 14, 2012, 02:32:34 pmNaah, those caps would be just a curiosity in this kind of application. As already stated, regular ceramics will do fine.As to a 4 layer board... myself i would only consider it if there was some specific reason to go that way (space constraint, maybe EMI suppression although i am not sure how much you gain there just by using a 4 layer board). A 4 layer board will be much more expensive than a 2 layer design at least as long as you use something like iTead, Seeed or some such as the source. I'm not sure it would be worth the extra expense. If the processor were a fast ARM or similar, or an FPGA then sure, multilayer would be the thing but imo not necessary for this kind of application.

author: Ment11
author_rank: Contributor
timestamp: June 14, 2012, 06:30:23 pm
content: I am actually not using any dev boards for this project, the prototype was built from scratch on the Altium prototype board (fried 2 PICs while building it lol, put 5v in to one by accident, and then overheated the other one with the rework station, but then I finally got a hang of it). The main reason I was looking at the 4 layer PCB is because I am using about 50 pins on the PIC, and I thought it was going to be annoying to route all of them on a 2 layer. The cost is also not an issue, as long as it is going to work, and be reliable.I am going to play around with the PCB and see what fits better, I was just wondering how much better the noise reduction will be on 4 vs 2.If anyone is interested and wants a good laugh I attached the picture of the prototype (surprisingly it worked on the first try other then frying the 2 chips)

author: codeboy2k
author_rank: Super Contributor
timestamp: June 14, 2012, 07:26:19 pm
content: a prototype is a prototype. Yours looks pretty good in fact, and worked the first time. thumbs up.Here's a picture of Bob Pease's prototype of the LM131/231/331 V-to-F converter. It's on the cover of his book,Troubleshooting Analog Circuits.You can also see it here, at about 29:00 ...

author: free_electron
author_rank: Super Contributor
timestamp: June 14, 2012, 08:21:29 pm
content: ah, Bob Pease's In-Beard-Bread-Board ( try saying that 5 times fast in a row )It's hard to tell where Bob's beard ends and where the Bread-Board begins...I met him a couple of years ago at the De-Anza Fleamarket in Cupertino. He frequented that place bargain shopping electronics parts and selling his books. Had a nice talk and he was kind enough to sign some of his books for me.Sad he is gone. Bob Pease and Jim Williams were the absolute guru's ( after Widlar died, that was the guru's guru ... )

author: Ment11
author_rank: Contributor
timestamp: June 14, 2012, 08:28:09 pm
content: Quote from: codeboy2k on June 14, 2012, 07:26:19 pmthanks, now I can say that my prototype looks like that for a reason. The video was really good, a lot of good info, I wish I saw it a year earlier, I want the metcal soldering station now.

author: T4P
author_rank: Super Contributor
timestamp: June 15, 2012, 05:18:19 am
content: ... Metcal's i promise you, you will miss the adjustable temperature REALLYI really hate to keep another identical set of tips around for leadfree as it is

author: codeboy2k
author_rank: Super Contributor
timestamp: June 15, 2012, 11:29:11 am
content: Quote from: free_electron on June 14, 2012, 08:21:29 pmI used to work on Page Mill Rd, that was about 10 years ago. No I didn't work at HP. Wished I didI got out to the swap meets a few times, but sadly not enough times and I never got to meet either of the gurus.Widlar was gone long before I got to the bay area.Now I am retired and back in CanadaI think I am ready to come out of retirementretirement sucks ... lol.