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In message 85.21 (Tip of the Day, #20) I tell about a subtle programming difference I found between the Rockwell and the CMD parts. I haven't used any WDC ones yet, but I have a hunch they're more likely to be like the Rockwell VIA.
My CMD data book says their VIAs' PA and PB pins present one TTL load in the input mode. This sounds like a horrendous load and I think it's probably a maximum and they usually present a much lighter load; but the WDC data book says theirs presents a CMOS load in the input mode. The latter is far more appealing! WDC's also have bus-holding devices on the inputs. These keep an input in a valid logic-high or -low state when the output driving it goes to high-impedance or is disconnected. Noise or small leakages in the board will not make these inputs wander around in no-man's land between logic states.
On a similar note, the WDC VIA outputs have stronger pull-up capability, affording faster rising edges under real-world circumstances where there is some capacitive loading on the line.
One major difference between WDC's VIAs and all others that I know of is that WDC's have the active IRQ\ pull-up, instead of just the open-drain arrangement. This became more necessary as clock speeds increased, to make sure the IRQ\ line came up before an RTI so you don't get a false interrupt immediately following the end of the ISR that took care of the interrupt and cleared the VIA's IRQ-active status. This design requires combining multiple IRQ\ sources with an AND gate, or at least a diode (Schottky may be required) in series with each IRQ\ output, instead of directly wire-OR'ing them together with a single pull-up resistor. Otherwise you'll have the situation where one part could be trying to pull the IRQ\ line down while other VIAs are trying to pull it up.
The faster VIAs come from WDC. Bill Mensch, the president, has told me that they only test them to 10MHz but he expects they'll run much faster. I couldn't tell you from experience—yet. CMD's are only available to 6MHz. Few on this forum are running this stuff above 4MHz, but I like to leave room for growth and leave the possibilities open.
There are other minor improvements in WDC's VIAs, but these are the major differences. For new designs, I would definitely go for the WDC part.
Garth
_________________ http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?
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