From: André Fachat (afachat_at_gmx.de)
Date: 2007-05-28 16:13:47
Hi Bil, many thanks for the engineering lesson :-) While I am able to make a coprocessor, or a second processor that takes over the bus on some condition work almost on the first try, I tend to miserably fail with dRAM due to timing problems. So maybe I am one of those you mentioned, who get everything right but... - but then again, I do it for a hobby :-) Currently I am working on a RAMdisk, re-cycling old 30-pin SIMM memory modules, and I am desperate because I cannot make it work. Probably because the dRAM has much stricter timing requirements. Compared to this, 65xx and 74LSxxx parts seem to be much more forgiving. Also the SIMM modules draw a lot of power, and I had to add an extra power supply connector to the board to keep the voltage monitor (7705) from asserting RESET on a low voltage condition. Maybe this power hungryness also puts strains on signal lines... I will try the Phi0-approch you mentioned, though. Thanks again André P.S.: my system is here: http://www.6502.org/users/andre/csa/index.html (the ramdisk is not yet there, though) -------- Original-Nachricht -------- Datum: Mon, 28 May 2007 09:33:03 -0400 Von: "Bil Herd" <firstname.lastname@example.org> An: email@example.com Betreff: RE: How to design non-trivial cartridges for c-64? > Hi Andre > > Bear in mind that I was from the production world where I had to pay > strict attention to specs or get thousands of failures, not to mention the range > of temperatures, voltages and related part specs that we had to account > for. (as it was parts didn't always pay attention to specs and we would get > thousands of failures). You can often get designs to work just fine in low > quantities at room temperature under a stable voltage... I.E. some things I > couldn't do will work just fine for experimenters. > > Using a buffer to hold data valid longer was not a useful approach back > then as there generally were no valid minimum times on a chip, I.E. your best > off to assume that the output goes invalid almost immediately after the > input goes invalid. I think in terms of data valid propagation and invalid > propagation, hoping a buffer reliably holds the exact same input after > currents start dumping and nodes start discharging is a bad bet when multiplied > times 100,000. Signetics buffers used to Hi-Z with a vengeance as an > example. > > A variation on this is where someone will but a buffer between the clock > and data lines of a latch hoping to create hold time, problem is that > variations in layout, pin capacitance, etc can result in -1 ns hold time worse > case. (not only no delay but negative delay) > > You also slow things time by the TProp of the buffer so at best you pick > up a hold of something like 3ns but slow things down by an additional 20ns. > > This was a recurring stickyness to 6502 designs, I used PHI0 (clock source > instead of clock output) a lot and thought about using long traces on > data lines (joke) among other things... more than one engineer got everything > else right except for this part. > > Bil Herd > > > > > -----Original Message----- > From: firstname.lastname@example.org [mailto:email@example.com] > On Behalf Of "André Fachat" > Sent: Monday, May 28, 2007 8:27 AM > To: firstname.lastname@example.org > Subject: Re: How to design non-trivial cartridges for c-64? > > > > Bil Herd wrote: > > > The area you have to watch is that the data is valid for only a short > > > period after Phi goes low, known as the hold time. Holdtimes are as > > > short as 10-20 ns (shorter if using Phi2) which means that if you have > > > too much logic in line to create the strobe, the data goes away before > > > the strobe (took you longer than 10ns to decide to do something). > Rule > > > of thumb is the strobe has time for only one level of TTL type logic > on > > > anything trying to capture data. > > > > > So, for something a bit more time intensive, are there any suggestions? > > Delay after Phi2 going Hi to start the work? > > You can delay the data by feeding it through a buffer (e.g. 74ls245) > first. > > André -- Der GMX SmartSurfer hilft bis zu 70% Ihrer Onlinekosten zu sparen! Ideal für Modem und ISDN: http://www.gmx.net/de/go/smartsurfer Message was sent through the cbm-hackers mailing list
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