Wednesday, May 11, 2011

Happy mail... (DP free pcb drawer - bus blaster)

Something nice about getting things in the mail... When I opened this letter I found that I had received my free pcb from Dangerous Prototypes pcb drawer (I got a code and used it to ask for a bus blaster pcb).  If came taped to the inside of the envelope with a business card - no problems in transport at all.

Overall the board has good quality traces and pads.  It's always interesting to see the differences in the ways people make routing decisions.  Things got interesting when I was checking the traces against the schematics and started finding mystery pads on the pcb... (mysteries are fun).  It's not unusual to put current limiting resistors to protect against shorts (or bad designs)... (I confirmed this with an email to Ian - it's actually a newer prototype than the board I was expecting - cool)  I was originally thinking of 20-30ish ohms but since I don't use resistor arrays often I don't have any on hand...  Rather than having to wait for them to arrive, I scavenged some from an old graphics card with fire (Yes I did it safely in a chemical hood with a gas burner...)  I ended up selecting 68 ohms for these.

One section appeared to have a bad trace (there was an intentional cut between two pads).  Near this region I found some other bridges and trace anomalies nearby...

Those were easy to clean up... (always always always check the pcbs before you assemble - even if they pass electrical tests it's worth taking a little time up front than a lot of time later).

This is what it looks like after populating the components... (Yay it passed the smoke test) I made a couple of changes (I bumped one Tantalum cap up a little bit in size because of what I had on hand, I only have 0603 caps so I used them instead of 0805's, I used a different usb connector than the original design because that's what I had on hand, and I rigged a different type of 12 MHz crystal because again... that's what I had on hand).  Not the most beautiful soldering job on my part...

I also left out the diode for the target voltage connection... Usually if I have to connect to a device with higher voltages than the jtag programmer can handle I inline a zener on the flylead, or I make a voltage translator so that I can connect directly to a lower voltage port.  It's probably a good idea to put a zener on the programmer to prevent accidental damage, but I didn't have one so I just left it out.

I was expecting to use a different jtag programmer to setup the board, but since the chip they used (FTDI2232) has multiple ports it wasn't needed.  The documentation for the bus blaster is around here and from that there is an nice explanation of how to self program the device here. 

I also found this page in particular to be very helpful...  They to do a functional jtag test against a logic sniffer (also Dangerous Prototypes... and also something I have on hand (on the top here) - very very useful - small simple quick and dirty and efficient tool - I highly recommend it).  Chain scan worked perfectly the first time... yay...  I'll test it more completely when I get some time later, but if it passes these tests then it ought to work without problems.  I'll check the signal quality on a scope later but 68 ohms seems to be fine for the new part of the design.

Dangerous Prototypes makes the neatest stuff, and I love the openness of their development... hopefully one day I will have something worthy to contribute...