Re: Scanning electron microscope digitizer project completed!
- From: Mike Harrison <mike@xxxxxxxxxxxxxxx>
- Date: Wed, 21 Sep 2005 09:51:38 GMT
On Wed, 21 Sep 2005 07:03:19 GMT, "Jeff Miller" <cornheadorama@xxxxxxxxxxx> wrote:
>Hi, some of you may recall some posts I made several months ago asking about
>common SEM drive circuits, trace rotation, and the limitations of human
>eyesight. The goal was to build a digitizer for my MINI-SEM so I could do
>away with the console.
>
>Just wanted to pop in and say the project is coming along, in fact it works
>very well. Based on a Silicon Labs microcontroller with 2 simultaneous
>sampling 16 bit 1MSPS A/D converters, some glue and transceiver logic, a NI
>IMAQ-1424 parallel digital camera PCI card, and a pair of Linear Tech D/A
>converters, it can do 256*256 through 8K*8K resolution at about 750K
>samples/sec. The other A/D is used to digitize voltage levels from pots and
>slide switched
>
>It took perhaps 45 days of spare time. It was my first microcontroller
>project. The SW is written in assembly language, the code is short but it
>was very challenging. Ideally the heart of the code could only be 25 cycles:
>in fact 32 were needed thus the slightly depressed sample rate. More
>importanly, the IMAQ card is sensitive to clock jitter. I ended up with
>about 6 loops, all exactly 32 cycles long, some padded with NOPS, the code
>jumps around from loop to loop. A code change in any loop required padding
>and entry-point changes in all the others for testing. Another challenging
>aspect was that the SILABS demo board has only 1 8 bit I/O port fleshed out
>and it required extensive multiplexing. I finished it about 6 weeks ago,
>since then I've been focusing on high vacuum stuff but will be setting up
>the MINI-SEM soon.
>
>So far I'm only looking at test images created with signal generators, but
>it's working exactly as expected and I don't think I'll have much trouble
>conditioning the signal for the 'scope.
>
>I'll be adding magnification control soon, followed by focus, astig, dynamic
>focus, condenser etc.
>
>Ultimately I'd like to eliminate the IMAQ card, considering PC parallel
>port, USB 2.0, and Ethernet. USB 2.0 microcontrollers don't really seem to
>be happening yet as far as I can tell.
Cypress do some USB2 micros, but hi-speed USB2 is more than the micros themselves can handle.
For USB2 for a 1-off project, look at
https://www.quickusb.com/index.htm
You will probably need to use a CPLD (e.g. Xilinx 95xx series) to handle the actual data transfer
in hardware. You may need a small amount of buffer RAM to match the USB modules burst clock rate -
you could either hook this to a CPLD, or use internal RAM in a small FPGA - e.g. the Digilant
Spartan-3 starter kit ($99). I think they do a USB2 module for this.
If you can live with USB1.1 speeds, look at the FTDI FT232 or FT245 chips.
.
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