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| Parallax Forums > Public Forums > SX Microcontrollers, SX/B Compiler and SX-Key Tool > Ambient RGB LED Lamp | Forum Quick Jump
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darco
Registered Member
      
Date Joined Oct 2007
Total Posts : 85
| Posted 10/16/2007 12:13 AM (GMT -7) |   | I started messing around with microcontrollers a few months ago, and after much tinkering I've finally finished my first PCB. I've already sent it off to get fabbed, but I was hoping to get some feedback here that I can perhaps apply to my future boards.
This board will be a part of an Ambient RGB LED Lamp, and uses an SX28-DIP. The LED's will be mounted on the outer corners, pointing slightly toward the center. Each corner can be a different color, with the intention of being able to control the color of different sides of the lamp independently. Here is a mockup:
I had to use two darlington arrays because the LED's would be pulling too much current when all lit. I added two extra connectors (EX1 & EX2) for the unused circuits on the darlington arrays to facilitate future tinkering.
With the pic below, I've superimposed the bottom copper layer, so that you can follow the traces. (Note that the caps and one connector are missing from the image below. Ignore that part)
As you can see, the bottom layer is mostly ground plane.
Here is the schematic:
Any thoughts? | | Back to Top | | |
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Guenther Daubach
Registered Member
Date Joined Jul 2004
Total Posts : 1333
| Posted 10/16/2007 10:19 AM (GMT -7) | | Darco,
a really nice design, here some remarks:
- When possible, the OSC1 and OSC2 traces should be as short as possible to avoid generation of EMI. In your design, you have located the SX-Key header on the opposite side of the SX package. It would be a better idea to place it on the other side, close to the crystal, and the OSC1/OSC2 pins.
- To avoid EMI, it is important to place a filter cap (e.b. 100 nF multi-layer ceramic) as close as possible to and between the Vdd and Vss pins of the SX28.
- Maybe, it is an idea to place a third pad connected to Vss between the two pads for the xtal - this would allow you to alternatively install a ceramic resonator.
- In general, input pins should not remain floating, so you may add an extra pull-up resistor for the RTCC input (10kOhm would be fine).
- You may consider placing a diode (any 1N400?) in series with the input supply (anode to the connector, cathode to the regulator) to protect the board against wrong supply polarity.
- Depending on the type of regulator you plan to use, two additional 100 nF caps between the IN and GND pins, and between the OUT and GND pins may be required to avoid unwanted oscillation.
Greetings from Germany,
Günther | | Back to Top | | |
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PJMonty
Registered Member
Date Joined Jul 2004
Total Posts : 1024
| Posted 10/16/2007 10:56 AM (GMT -7) | | Darco,
First, the board has a really nice looking layout.
Three thoughts:
1 - Don't be surprised if the power connector holes are really big compared to the leads going into them. IT appears that the plug footprint follows the manufacturer's datasheet specs. The first time I used a coaxial power plug, I followed the datasheet for the suggested hole size and found them to be way to big. I have since made them smaller so I don't have to fill the gap with a ton of solder.
2 - You might consider changing the LED silkscreen outline. I use one that has a flat side near one lead so it's easy to get the orientation right when stuffing the board.
3 - Consider putting some text near the SX and Prop headers that indicate which way to install the SX and Prop keys. It's easy to put them in backwards and damage the keys. I always put the text "SX-Key Front" or "Sx-Key Back" on one side of the 4 pin headers so I know how to plug it in weeks, months, or even years from now without looking at the schematic.
Thanks,
PeterM | | Back to Top | | |
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Phil Pilgrim (PhiPi)
Registered Member
Date Joined Feb 2006
Total Posts : 7744
| Posted 10/16/2007 11:27 AM (GMT -7) | | Nice, neat layout! In addition to the above comments, I would also recommend using an LM2940 for the regulator, which has all kinds of built-in protections, including reverse battery protection. It requires a low-ESR filter cap, though, close to the output pin. If you can find room near there to add a 22uF tantalum, that would be great. Also, your schematic shows a 1000uF cap on Vdd. Double check the footprint of the cap you've got on your layout. It looks a bit small to accommodate a 1000uF electrolytic. Finally, if you intend to use your SX-Key port for debugging, use a socket for the crystal, so you can remove it. (I usually design in a 5-pin SX-Key header with a removeable jumper on the top two pins that connects the crystal/resonator to the SX when the SX-Key is not plugged in.)
-Phil | | Back to Top | | |
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RickB
Registered Member
Date Joined Dec 2004
Total Posts : 211
| Posted 11/2/2007 8:54 PM (GMT -7) | | Regarding hole sizes. As someone who spend his days repairing 2 way radios I can tell you that, parts that are accessible to the outside world are the most common thing to break. Volume controls, switches, and jacks. Provide LOTS of clearance around the leads and pins of those and other multi-leaded parts. If possible, use parts that have protrusions that interlock with the board so that external force is not applied to the solder connections. When you or someone else has to repair that board you will be glad you did. It is very frustrating when some bonehead lays out a board with a thou or 2 of clearance around around the pins of a part and also forgets to include thermal relief for the ground and power plane connections. If you have the room, allow for it. The added cost of the solder is irrelevent. Anyone that does repairs on a daily basis knows what I am talking about.
Rick | | Back to Top | | |
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Capdiamont
Registered Member
Date Joined Feb 2007
Total Posts : 161
| Posted 11/4/2007 10:30 PM (GMT -7) | | If I may, and this is more opinion, than maybe fact, but I thought the external oscillator is mainly for timing accuracy, not the speed of the chip. IE if you needed high speed communications, or a RTC, or anywhere timing is critical. So then we have to figure out, if it is needed. If not we could save a bit on parts, and the bit of power needed to power those parts?
How is the design being controlled? Is the control of colors a critical timing issue? | | Back to Top | | |
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Capdiamont
Registered Member
Date Joined Feb 2007
Total Posts : 161
| Posted 11/5/2007 10:19 PM (GMT -7) | | | | |
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Capdiamont
Registered Member
Date Joined Feb 2007
Total Posts : 161
| Posted 11/7/2007 8:37 PM (GMT -7) | | darco: Please understand, I'm not trying to criticize your project at all, just trying to learn. In fact I love it.
Bean, so internal is not accurate enough for serial communications? The crystal is more accurate? What about power consumption? What about cost? | | Back to Top | | |
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darco
Registered Member
Date Joined Oct 2007
Total Posts : 85
| Posted 11/9/2007 12:02 PM (GMT -7) | | I thought you guys might want to see some pictures of the prototype boards and the lamp itself:
A few more pictures are on my blog post. | | Back to Top | | |
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