Thursday, May 19, 2011

Happy mail... (DP free pcb drawer - cpld breakout)

Yay... I received another free pcb from Dangerous Prototypes pcb drawer (I got another code for building the first free pcb... neat).  If came taped to the inside of the envelope with a business card - as before, no problems in transport at all.


Overall the board has good quality traces and pads - no bridges - electrically everything looks good - yay...

 Most of the vias were filled with the solder mask - not a problem though...

Overall the accuracy of the mask and the drill holes was a  bit better than the prototype busblaster board from before, easily in spec...

So now that I knew the board was good, I collected the components and since I was reflowing some other boards I decided to do the smd components with solder paste as well... Almost everything went well but my hand jiggled setting paste on part of the board and I didn't want to wipe everything off... This is what it looks like when you get paste a little too far away to find a comfortable place to bind when hot... The little solder schmutz is easy to remove with IPA in a syringe and/or a hot iron tip.  Always look at the board after thoguh because these things can move around, cause shorts and in general make life unexpectedly frustrating if you don't clean them up.

This is what it looks like after the components are placed.  I bumped the Tantalum caps up to 4.7 uF from 3.3 uF because that's what I had on hand.  When I first added power it passed the smoke test, but it didn't work... two minutes of testing revealed that the 3v3 ldo was working fine but the 1v8 ldo wasn't putting anything out - rechecked the lines - no shorts... The problem was that I used a 1v8 ldo from TI (TPS78918) that has an active low enable instead of active high enable... because of the routing I couldn't just cut a trace so I removed the chip - removed the trace from 3v3 to the input and enable on the 1v8 ldo completely - soldered the chip back - bridged the enable to the ground of the 1v8 ldo - soldered a wire from the 3v3 output cap to the 1v8 input... now it worked... yay.

Voltages are right and the coolrunner II is visible by jtag (side note - the 68 ohm inline resistors on the busblaster seem to work nicely).  I didn't have much time to build something to test all the io lines but they ought to be correct.

Overall it's a nice little board - easy to build and the layout is nice.  There are two jumpers to set the voltages for the io banks on the cpld (1v8 or 3v3 - the onboard supplies).  You can also tap the center post of these to supply a different voltage (I use 2v5 often) if needed.  The only problem with the design here is that the only caps between the Vccio inputs of the cpld and these pins are 0.1 uf caps... Most of the time that's not a problem with the onboard supplies since they have reserve caps but using a long wire to connect a different bank supply is going to be noisy...

So, for what it's worth these are the two suggestions if there is a new revision of the board:

First, instead of connecting the enable lines of the ldo's to their supply voltage I would consider routing them to the back of the board (easy via space available) and put two solder joints (like there is on the optional crystal) to either bind the enable to the supply voltage or ground.  This would make it easier to use ldo's with either high or low enable lines - flexibility in sourcing is good.

Second, I would consider adding pads for at least two caps between each of the Vccio bank lines and the jumpers - probably one for a small Tantalum (3.3 to 10 uF ish) and one (two would be better) for additional ceramics (depending on what you want to do with the board 1 to 10 uF).  I think this could fit on the back.   Since the 1v8 ldo is fed from the 3v3 ldo (which has a Tantalum cap on the output),  I think the cap on the 1v8 input isn't really needed... Perhaps, if this one were routed instead to one of the Vccio banks then there is still enough space on the front to add another one by the jumper for the other Vccio bank... then finding space for the other additional filter caps is easier.

Again, just my suggestions (for what they are worth) if there is another revision in the works... It's a nice little board as it is now...

Here and here are a good pages about the board (design and lots of information).  This is one of those tools that's always good to have around...