Mike's Hobby Blog

Good, somewhat surprising news. I programmed the AVR that is being used for the USB interface and the device attached to a PC on first attempt. So far I haven’t been able to get serial data through it, but just communicating with a PC is fantastic progress. Next steps…

1) Figure out where serial data is being lost
2) I’ll plug in the Disk ][ interface AVR and get some basic operations working
3) I’ll have to finalize and implement the host/Disk ][ communications protocol.

Disk section of new proto board checks out – I’m able to read/write/format disks with same functionality as the first proto board. Inner tracks don’t seem to read quite as reliably as before, but I’ll look at this later on. Using a crystal for clock seems to be a good thing, because the format function seems to fit a track a little better on to a disk. I couldn’t tune that RC oscillator well enough to suite my liking.

Next step is the USB interface.

Regards,
Mike Willegal

Quick update – holes drilled and components soldered in (except for AVR’s, which go into sockets). Power supply checks out OK. I will be able to bring up USB and Disk interface sections separately. Communications between the two AVRs is through a serial interface and I have added a header which will allow me to interface from a RS232 port on a PC directly to each AVR, during initial bring-up. First, I intend on bringing up the Disk interface to state where I left it on original proto board. Some tweaks to the SW must be made because of change in AVR versions, but this shouldn’t take very long. Once that is working, I’ll bring up the USB interface. Finally I’ll combine the two and work on host protocol, which is the last major activity planned for this project.

I have all the parts and etched the board this morning. Registration between layers isn’t as good as I’d like, but it will do for a prototype. It will take another evening to drill holes and solder in the components. Some of you may wonder why I go through the effort of making a PCB for a prototype. Well I think a PCB results in a vastly more presentable finished board and I like the esthetics of it. The result should also be a little more reliable than the results of other prototype processes. I don’t think that it is that much more trouble than say wirewrap or point to point prototyping. Finally, creating a prototype using the same artwork that a production house would use, makes going for a production run much less risky.

The board is designed to fit an a Serpac A20 enclosure, as used on my shutter tester project – http://www.willegal.net/light/tester.JPG. This is not the cheapest enclosure, but I think it is a pretty good looking off the shelf box. I’ll have to come up with some kind of cool decal to put on top of it.

-Mike W.

PS In this image I haven’t yet removed the ink from the laser printer image that is used to form the resist.

Quick update. i’ve got board lay out complete and parts on order for the second proto board. I may start the etching process today. This promises to be one of the most tricky two sided boards that I have tried to etch at home. When etching boards at home I try to keep them to a one sided design, but connecting the 20 pin header basically is forcing a two sided design. Also starting to work out interface/Disk II protocol. It looks like ADT will be used as a starting point for the interface.

Quick update. Next proto board is starting to shape up. It makes use of two AVRs. An attiny2313 running AVR-USB firmware to support the USB interface. This is connected to a atmega88 running my DISK ][ RWTS firmware via a serial interface. The USB will start with a CDC (communications device class) type device, though I could potentially change this without changing the hardware.

I probably have another couple of weeks to finish the layout, get the parts I need and build the board

With a tweaked RC oscillator calibration setting, the UART is behaving and I can format a disk that can be completely copied by an Apple ][. Read performance seems to be around 1 soft error in 1000 sectors read. This is about 1 soft error in 2.7*10^6 bits. I’m not sure what is expected with one of these old drives using old media, but I’m declaring success (at least for now).

I’m running out of things to work on with this first proto board.

I need to start working on proto ][ with more memory, USB interface and an improved low drop out voltage regulator. I’m likely to switch to a crystal based oscillator since the built in RC oscillator calibration setting seems too course for this application. Seems like there is only 1 calibration value that works reliably. and I’d like a little more precise clock.

I’ve mostly recovered from the power supply disaster. Instead of replacing the ATMEL AVRISP programmer, I purchased a USBtinyAVR kit. I figured that I would be able to repair any damage instead of doing a replacement, if I burned another programmer up. Well the kit works alright, but isn’t nearly as well integrated into AVR studio software as the ATMEL programmer was. Anyway, I’ve been able to get back to work finishing the format and write functions, well sort of.

Now I am working with a regulated bench power supply jumpered to the mother board with alligator clips (I guess I don’t learn). The wall wart is on the back burner until I can find a better 12 volt regulator solution.

Since the great power supply disaster I have had issues with dropping RS-232 characters being sent from my proto-board. I also found that I couldn’t fit all 16 sectors on a track during the format process. Tweaking the speed of the drive solved the formatting issue, but this clearly isn’t an acceptable workaround. What is really happening with both these issues is that, for some reason, the processor clock is now running way too slow. I need to recalibrate the internal RC oscillator that I’m using to clock this device. That will hopefully solve both issues.