fachat wrote:
Very good article indeed.
In the address decoding page, you could probably add a timing diagram with phi2, address lines and
how the setup times etc add up (what you now explain in words).
Not a bad idea. It'll take more time than I want to put into it this weekend, at least to make it as clear as I would like, so it might have to wait.
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Is it not normally the capital Phi letter? You're using the lower case phi letter, which looks
strange to me. In fact in your diagram it indeed is the capital phi.
I don't like the font on the one that shows which is more-or-less the Times New Roman or the Gentium font. I'll change it when I figure out how. Because of my New Testament Greek study helps, I've been using the SBL Greek font, lower case, looking like a squashed-down "o" with the line going through it slanting a little. The manufacturers, often not having the Greek character set, used a capital O or a 0 with the / through it since the slash would be too far off-center in the lower-case o. Done correctly, the capital has a perfectly vertical line, and usually has serifs on the ends of it, with the ends (and the serifs) sometimes even inside the O for at least one of the fonts.
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anecdotal evidence: that the VIA needs CS before Phi2 is the reason it does not work out of the
box on the C64, where the address bus is shared with the video chip (using phi1). So you had to
delay the rising edge of phi2 a bit.
That's what I put at the bottom of
http://6502.org/users/garth/projects.php?project=7 in my project pages, an I/O expansion board for the C64. I just went and added it as a comment in parentheses to the primer, although it would not be necessary for someone who's making his own 6502 computer.
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On the memory map page you start using 6502 opcodes, but knowing them is not a prerequisite.
Ok, maybe be implicit because why build a 6502 computer without knowing it...
Maybe a kind of side box explaining say that :'"STA" means "STore Accumulator", storing the
general purpose accumulator register to a memory address given by the addressing mode', or
what "LDX" and "TSX" mean. Would introduce some of the basics in a simple way, for those
that just "walk by"
The first line of the intro says, "So you've decided to build a 6502 computer-- or maybe you're still deciding. You've been looking at the 6502 instruction set and [...]" I added a note explaining anyway. There are many 6502 mnemonics throughout the set though, especially in the programming tips section.
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On 74xx timing, for example a Philipps 74ALS138 has 22ns max prop. delay - twice as fast as 'LS.
ACT is faster (12ns max from E3 to On\), but I was aiming to keep the beginning builder from the problems that the accompanying faster rise times can produce if construction is poor.
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On the mistery pins: the /SO pin was used in the commodore floppies [...]
That's the only place I've heard of it, and it's an application where math routines would not be running to be interfered with. I just added a mention of it.
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On the wirewrap page you may want to (and are hereby allowed to
show the picture I
give on this page:
http://www.6502.org/users/andre/csa/vdc/index.html with a comparison
of the very first version of my video card (as not to do it) and how it compares to
today's custom-made board. It DID work though!
Thanks. I added it.
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In the I/O ICs you could mention that it's pretty easy to even add intel-like I/O ICs like
the 16550 UART by creating /RD and /WR from Phi2 and R/-W.
It's there, and refers to the circuit given in the "Clock Generation" section, but I moved it so it won't hide so easily.
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Also Intel-compatible Ethernet
chips have been connected directly to the 6502 bus, as well as USB ICs, for example here
http://www.6502.org/users/andre/csa/usb/index.html (shameless self-plug
Very nice, but too complex for beginners. I think if I try USB, it will be with something like the
Maxim MAX3421E which uses SPI.
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And maybe a warning that the 6551 control lines are internally connected with internal state, which you might
not want, so better look at the datasheet.
That's in the RS-232 primer which is linked: "The
only thing I don't like about the 65c51 is that a couple of the controls are merged into the same control bit, where it would be nice to be able to control them separately instead."
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For the displays, even if you would not recommend it anymore, you could mention that even the
first PCs used 6845 (compatible with 6545) based video cards and those were used in the PET
(and the Apple??) for example.
I added a 1-line paragraph saying so, and that the Commodore 128 used the 8564.