I've done some additional (software only) testing. In short, I'm not having any issues with the SanDisk CF Cards. I even have a very early 96MB Card which shows the manufacturer as SunDisk, so that's about as early as it gets. They work perfectly fine... just the ones that are non-SanDisk have issues... and are consistent.
As is proper, RD and WR signals are qualified with PH2, that's handled by the ATF22V10 as well. My C02 Pocket SBC has a design point of 6MHz, but I have one SCC2691 that runs at 8MHz, so that's my typical dev board setup... as I use the other ones (at 6MHz) for long term testing. I'm very familiar with the original ISA bus.... I was level 2 tech support for the IBM PC group from 1984 through the PS/2 line of machines.... and yes, I was there when we brought out the first machines with IDE drives. But as noted... 8MHz is pretty much the bus limit, albeit I had some modified PC-AT's and some un-announced systems boards that I had running reliably at 10MHz and 12.5MHz, as we were able to get early chips direct from Intel as samples and test.... fun times.
It's also crossed my mind that the STI sourced CF Cards are at their limit with bus timing. I'll switch out the can oscillator to a slower one and see if anything changes with the STI cards. I have noticed some odd bits about the identification block on these... they show LBA support, but show a 32-bit field of zero for available sectors but show the correct number of accessible LBAs. The STI cards don't have DMA support (the SanDisk cards do) but that doesn't matter as I'm using PIO mode with interrupts.
We have the same SanDisk doc.... I also have an older SanDisk doc and recently found a Transcend CF Card doc as well. For the most part they agree on details. I've since updated the BIOS to include sensing the LBA capability... it will provide a message on boot that LBA is not supported. Still some more work to do on the BIOS before it's really solid, but it's still relatively small in size.
My github page has the full schematics and code for the PLD used on the C02 Pocket SBC. I'll be adding the RTC-IDE adapter soon, but will probably get the Version 1.01 PCBs done and tested first. Thanks for the input of course.... I'll post back once I try a slower clock speed. Here's the schematic for the RTC-IDE adapter:
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