semi-OT: do you routinely attempt projects out of your comfort
classiccmp at philpem.me.uk
Fri Mar 12 18:01:29 CST 2010
Tony Duell wrote:
> Notice I said 'provide it as a kit', not 'provided it as a kit or ready
> made'. The point being that if _only_ the kit is available, people who
> want it have ot buy the kit, and they have to be clueful enough to put it
Like I said before, though -- the DiscFerret is an almost completely SMD
design. In fact, the only parts of the board that aren't SMD are the USB
connector (a Lumberg thru-hole B-type "device" connector), the power
connector (Switchcraft RAPC722 2.1mm DC barrel socket), and the
interface connector (40-pin right-angled IDC with latch ears).
As a bare minimum, if I made a kit, I'd want to supply it with the power
controller chip soldered down (the aforementioned QFN chip -- TI
TPS75003). That's simply because the leads don't extend up the side of
the package, which makes it close to impossible to solder down without a
hot-air station. Pin pitch is an eye watering 0.5mm, and there are 20
pins to solder, plus the under-chip ground/heatsink pad.
The LM2679 Simple Switcher chip is tame by comparison (1.27mm pin pitch
aka 50 mil or half DIL pitch), and the solder tab extends past the
plastic body, so is dead easy to solder down to the heatsink plane.
The FPGA is narrow pitch (0.5mm QFP), but can be soldered down using
"drag soldering" -- basically cover the pins in gel flux and run a hot
soldering iron down an entire row of pins at once. Same goes for the
J-lead or TSOP-II SRAM chip -- soak it in flux and drag-solder it.
Replacement isn't hard either -- in some ways it's easier than thru-hole
because the through-plating doesn't tend to suck away heat or get filled
with solder that just won't melt. Either hit it with a hot air gun, or
Chipquik works terrifically well on leaded chips. The latter is
basically Wood's Metal and a special flux.
> I really don't see the problem with SMD assembly at home...
I don't -- I've done it, it's a snap once you know the tricks.
The problem I have is that if I sell these things as kits, someone's
going to send one back and RMA it because they f00ked the soldering.
The other problem is that the PICmicro is supplied blank, so anyone
building a DF would have to have a PIC programmer. Admittedly they're
not expensive (a brand-new PICKit2 costs about £30 with the Debug
Express demo board or about £20 without). Once you've flashed in the
bootloader, the chip can have the Usercode (the non-bootloader bit of
the firmware) and Microcode loaded over USB. The trick is getting the
bootloader in there in the first place...
> OK, I am odd, but I have avoided buying some low-production-count designs
> becasue they were only avaialble ready-built. The main reason for this is
> that a kit comes with dcoumentation on how to assembly it and often a
Would this be more to your liking?
User's Guide / Programming Guide
Software installation / basic usage
Hardware pinouts -- disc interface / power connectors
API documentation and USB control protocol
Firmware theory of operation
(possibly useful if you want to play with it at a lower level
than DiscAPI allows)
PCB layout diagram (x2 magnified)
PCB top and bottom layer (x2 magnified)
(useful if a trace gets burned away!)
Assembly instructions (for kits)
Hardware theory of operation (for each major section)
(no sense in building a kit if you don't know how it works!)
> Given the scheamtic and the knowledge that I put it together in the first
> place, it's likely I can fix it.
Unless a failed part is NLA...
classiccmp at philpem.me.uk
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