Thursday, May 26, 2011

Niblet cards arrive...

Always great to get new boards in the mail... This time it's the niblet cards for the alpha backplane.  I used Laen's service (DorkbotPDX) so I got 3 copies of the design.   Quick electrical tests (resistance between linked pins and unlinked pins) look correct.  This card contains the msp430 board controller and provides termination points for the single ended and differential data lines.  It also provides access to the backplane I2C bus as well as the SPI bus and plenty of access points to breakout the signal and power lines for prototype access.



For the single ended lines there are spaces to add pullup or pulldown resistors (or to couple to ground with a cap if needed).  I put the bulk cap for the 3v3 suppl on the back, but otherwise not many components here...

As usual, the copper layer is excellent and the solder mask is almost as good (spoiled I am by whoever fabs these boards).  No errors on any of the 3 boards, no anomalies in the thickness or placement, no bleeds from the copper fills, holes are easily within spec for position and size.

I have no idea what the actual production limits are but the accuracy and precision is clearly much better than the limits we have to make (which is a very good thing).

On the bottom is the DIN 41612 connector for the backplane (this one is from TE) - the retention clips are nice to have - fit perfectly (never know with a part whether it will really hit the states specs until you do it).  Just above that are the differential termination resistors (I'm starting with them at 150 ohms but this will likely change as testing begins - the alpha backplane is 2 layers with intended characteristic impedance targets for the signal lines between 148 and 152 ohms, differentials closer to 180 (these are far from the ground plane and tightly coupled).  I'm not certain what the actual imp. will be or how much it will change depending on slot population (which is what testing ought to reveal) but eventually after I get some real world data the act/dyn impedance control designs can begin...

On the bottom right is the little ldo (3v3 always enabled) for the msp430 controller - I like to use the micrel MIC5205's for this - low noise, easy, simple, always works.  Above that is a small ceramic filter cap and a standard resistor/led combo (I don't usually put leds on all the power supplies but when there isn't an enable control for them I do - saves trouble shooting and drains the caps once the power is cut).  This 3v3 supply feeds the msp430 controller just above (socket is empty here) and fills the electrolytic cap on the top (but backside) of the board.  Above the controller socket are the filter caps and the pullup resistor for reset (needed by the sbw programmer - just tap a launchpad - works great and cheap).

I didn't populate the pads for the single ended lines yet (because the may end up in many different configurations).  I also left most of the breakout headers for the power supplies and signal lines unfilled.

Now that the termination board is ready I just have to built the apricot board and fun with signal testing can begin...