RFM12-Baseboard: Suggestions? - Page 1
Forum section: electronics / projects

author: Dom
author_rank: Newbie
timestamp: June 09, 2011, 08:42:47 am
content: Hello everyone!I'm a student who has to make a project for this years EE-Lab class and I am designing a board which interfaces the RFM12-BP module.After my first attempt failed very hard (PCB layout) I am now in trouble because I have to get it to work in 1+1/2 weeks.I would really appreciate if someone has some suggestions or even spotted some mistakes in my second attempt board and/or schematic.Here's the schematic:.. and a quick screenshot of the layout:Also I'm attaching the EAGLE files in this post.Thanks,Dominic H.

author: Neganur
author_rank: Unknown
timestamp: June 09, 2011, 11:40:24 am
content: What exactly failed? Did you mill and assemble the board and it doesn't operate?Does your oscillator work? Are the fuses of the PIC set properly (disable internal 8 MHz oscillator etc)?In case you assembled the board, check all electrical connections, check the solder joints. Make sure that what you wanted to build and what your schematic tells the software are the same thing.How are you getting 3.3V from a 78L05 by the way? (or are you just using the same footprint, better create a new component)

author: Dom
author_rank: Newbie
timestamp: June 09, 2011, 02:06:52 pm
content: My first board failed because of the tight SMD parts on both sides as well as the massive amound of vias.The board itself is etched (quality varies because students make them) and was a double layer, vias had to be made by hand with wire.At our SMD lab we can only assemble one side of the board via reflow soldering, therefore I just did the side with the Step-Up, rest was done by hand. Also I ordered the wrong package variant of the MCU (SSOP instead of SOP), I tried to solder it with the help of an improvised adapter.It looked reasonably, but the best result I got was an unsupported device error from the PICKIT2, so I couldn't upload my code. After my presentation which was Monday my teacher said that "it looked like a mess" i.e I should make a new PCB and better get it working until the 20th of June.So it mainly failed because the MCU was either dead or not solderable to the board.I already ordered 2 new ones from a local distributor to replace the 18F13K50 which are 18F2550 in DIL, which surprisingly fit better in the application than the other did.Also, I was indeed lazy with the voltage regulator, it's actually the L78L33 which has the same footprint. I gotta rename that now before I get in trouble, thanks for pointing that out.EDIT: It seems that I missed to tie the | POWER >-- signal to an interrupt pin, did it with a zener diode protection and 680 Ohms serial resistor.

author: A-sic Enginerd
author_rank: Regular Contributor
timestamp: June 09, 2011, 03:45:23 pm
content: I hate to sound cold hearted but, welcome to college and a taste of the real world.This is where you begin to develop debugging skills, and the realization that no matter how good of a designer you are and no matter how long you've been doing it mistakes will be made and the true test of your metal will be in your ability to debug it.Personally, if I were your prof and found that you were coming to a place like this to get help on debugging a class project - I'd fail you on the spot. but that's just me........

author: Nermash
author_rank: Frequent Contributor
timestamp: June 09, 2011, 04:56:12 pm
content: Your schematic is hard to read, but I've noticed that you are using 3,3V supply for PIC18F2550. Check the datasheet again (I've made the same mistake), if you plan on using 3,3 as VDD you must use 18LFxxxx part. I've tried the same mcu on 3,3V, PICKIT2 recognizes it in 50% of attempts, but I didn't try to program anything...

author: Dom
author_rank: Newbie
timestamp: June 09, 2011, 08:03:47 pm
content: As far as I know you can operate the F-parts with 3.3V, but it isn't recommended for commercial use.I just looked it up in this datasheet and it says "2.0 to 5.5V" (here)Anyway big thank you for sharing your experience, I'll add some voltage dividers and jumpers to get it right for both Vdd's.

author: johnmx
author_rank: Frequent Contributor
timestamp: June 09, 2011, 08:44:55 pm
content: Quote from: Dom on June 09, 2011, 08:03:47 pmThe minimum supply voltage of the PIC18F2550 is 4.2V! Document: DS39632E, Fig. 28-1, page 368. The LF part can operate with 3.3V but in this case the maximum clock frequency is 25MHz.

author: johnmx
author_rank: Frequent Contributor
timestamp: June 09, 2011, 09:00:26 pm
content: C7 should be at least 220nF (±20%). This capacitor is required for stability of the internal regulator.Parameter D324: External Filter Capacitor Value (VUSB to VSS) -> Min: 0.22uF, Typ: 0.47uF, Max: 12uFNote: Ceramic or other low-ESR capacitor recommended

author: Dom
author_rank: Newbie
timestamp: June 10, 2011, 10:09:47 am
content: Thank you all for the hints, I guess I learned to always check the whole datasheet even when it gets stressful.Now I can submit the layout and hope that I'll get it in time./closed