h1

Xmega adapter boards for breadboarding

2010/04/16

I’ve got a number of projects coming up that would be radically simplified by a) using ATXmega parts, and b) breadboarding a prototype.  The problem is that Xmega’s don’t come in anything but surface-mount packages.  The obvious solution is to construct an adapter board.  A straight pin conversion would be a waste of effort, so adding programming headers, clocks, and decoupling capacitors is a necessity.

I posted to the DorkbotPDX list to see if anybody is interested in ordering some of these alongside my own order in the next group PCB buy (April 26th), and got one response almost immediately.  As a result I’m going to tweak the boards up and make sure they’re ready to go, and post them up here for comments.

The first board here is the ATxmega*A4 unit, which is a 40-pin 600-mil DIP.  PDI header on the right, crystal on the left, decoupling and AVCC filter caps scattered around.  Port pins are labeled as are + and -.  PDI pins are accessible on the main header as well as the programming header.  Discretes are all 0603 for ease of assembly, though the chip itself is a QFN (unfortunately).

The second board is a straight extrapolation of the first, designed for the larger A3 chips.  It’s a 60-pin 600-mil DIP with the same basic feature set.

I’ll be tweaking these over the next few days, and hopefully producing a batch of them in the next couple weeks.

About these ads

5 comments

  1. What is the specified load capacitance for the crystal that you expect to use? I see that your board is marked 15pF for the load capacitors. That might be an appropriate value but it seems a bit low to me.

    The load capacitance for a crystal in the 16MHz to 20MHz range is often about 18-20pF. By the usual formula, that would dictate load capacitors of 27pF to 30pF or so, depending on the board layout.

    You might see if you can squeeze in pads for an optional reset switch.


    • The 15pf is just a default value I’ve copied in, and I have it in stock for my own boards. I’ll put in a better value before it gets finalized.

      A reset button’s a good idea, I’ll see if I can cram one in between the PDI connector and the chip. I’m also packing in an alternate crystal mount (HC49) on the other side.


      • The “XMega A” manual states 10pf for 12-16Mhz crystals. See Table 7-5 “Oscillator frequency range selection”.


      • Ah, there it is…. was hunting for that table yesterday and didn’t find it. I’ll update the artwork and the parts list, thanks!


      • I wouldn’t put any stock in the capacitor value given in the Atmel datasheet. There is no way that one can specify the load capacitor value without knowing the specifications of the particular crystal being used.

        To determine the proper capacitor value, take the load capacitance specification of the the particular crystal, double it and then subtract the approximate parasitic capacitance for one of the crystal circuit traces. For a crystal with an 18pF load specification, this will yield a value of perhaps 27pF to 31pF.



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

%d bloggers like this: