MFM IC's (Was RE: Any AMIGA users?)

see below, plz. Dick ----- Original Message ----- From: "Pat Finnegan" <pat(a)purdueriots.comTo: <classiccmp(a)classiccmp.orgSent: Sunday, January 06, 2002 8:39 AM Subject: MFM IC's (Was RE: Any AMIGA users?) The only "catch" in many PC-based controllers is that the FM mode is suppressed by the hard strapping of one of the controller pins. There are two problems here. (1) the "available today" part is always iffy. (2) no controller chipset I've ever seen can do the MFM and PLL functions without additional support for a couple of reasons. (a) Though the PLL provides "bit-synchronization," byte-framing is established by the use of "address marks" in a soft-sectored environment. A system that writes essentially one sector per revolution of the disk, which is what I've gathered the AMIGA does, is an exception, since it's essentially hard-sectored. These address marks signify the nature of the data to follow, i.e. is it an ID address mark, and, if so, what sort, e.g. track or sector, or is it a data A.M. This practice resolves the ambiguity in which phase of the extracted clock is "clock" and which is "data." Some MFM chipsets used in HDC's, notably the WD1100 chipset, generated signals that were used to indicate the PLL was locked and that, having acquired synchronization, the PLL filter bandwidth should be reduced to retain lock. All of these functions were based on assumptions described in the overal structure of the format scheme, and, by omitting some of these features, as was done in the AMIGA, replicate the functions of the AMIGA's hardware/software scheme with hardware and software other than theirs. There is hope! With a little effort in cooking up a firmware set for the extremely inexpensive UBICOM SX28-100 microcontroller, which is very similar to a small PIC, it should be quite straightforward, at that MCU's 100 MIPS execution rate, to bit-bang the bitstream in order to incorporate both the PLL function and the byte-framing functions in a single part. Since these cost well under $10 each, and are in-circuit programmable using hardware you can easily build yourself and software available at no cost, this can be done at minimal expense. Another alternative would be the somewhat more costly, somewhat larger, and somewhat slower (not much, though, since the instruction set is more diverse and the program/data store on board is larger) DALLAS 89C420. That's a 50-MHz core on the 805x model, with a modified clock circuit that generates one instruction cycle per clock cycle. Either of these, and a wide range of others can easily handle floppy disk interface by conventional "bit-banging" techniques, while the faster ones I've mentioned can easily process a hard disk too. All that's required is a thorough understanding of the task requirements and thorough understanding of the controller's operation. This amounts to what is essentially building your own custom chip, but since the devices are generic and the tools are readily available, I'd say it's an attainable goal. Write precompensation is definitely necessary. With floppy disks, it required imposing a bit shift of about between 1/16 and 1/8 the bit rate. With MFM hard disks, the most common amount was about 60 ns against a 200 ns bit window. The most common compromise I observed in FDC's intended for both 500 and 250 MHz bit rates was about 160ns, or, essentially a 6 MHz clock. For hard disks of the '80's a 16 MHz clock would have worked just fine. Wherever current-generation single-chip microcontrollers are sold, though you might do better requesting a sample. If your experiment works out, you can write them an application note that they can publish. That, if you can convince them you're likely to succeed, will help get you the samples you need. That Dallas part is being sampled currently. The part is available in PLCC-44 on short notice with about a 6-week lead time on the DIP-40 package. ISTR that Mouser, among others sells the UBICOM (formerly SCENIX) parts in onesies if getting samples is a problem. You should have no trouble doing this with a single part, though it's a big job for evenings and weekends. ... good luck! Beware of trying to expand the range of capabilities too much, though!