Subject:
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bus loading
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 21 Nov 1996 01:38:27 GMT
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Original-From:
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Randy Sargent <RSARGENT@NEWTONLABS.COMihatespam>
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Viewed:
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1209 times
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Hi Scott,
> I'm going to have another 4 devices on the data bus, 2 x 74HC574 & 2 x 74HC541.
>
> Will this exceed the "fan out" of the cpu ? (I don't have the "A" series ref.
> manual - is there one ? I have the E & F series books.)
>
> I think that it probably will, so I'm looking at buffering the data bus using
> a 74HC245.
Since everything's CMOS, there's no trouble with static loading (CMOS
inputs require essentially zero current in static operation). You will,
however, be increasing the capacitance on the bus, which slows down the
transitions a fairly small amount. But I don't think we're talking about
even 10 NS slowdown, so this isn't an issue at all with a 6811 circuit.
Don't bother with the buffer. In fact, the buffer would slow down the
signal a lot more than the added capacitance, not that even its slowdown
would break the 6811 circuit.
> Are there any problems using HCT devices with the HandyBoards HC
> components ?
No. HC and HCT parts differ only in their input logic level threshold, and
have no difference on outputs. HCT was built for (and is required for) the
case where you have older family TTL parts, with outputs that swing from
maybe .6v to 3v, driving the inputs of CMOS parts, which originally had
inputs that required below 1.5v or above 3.5v for logic levels (rough
numbers). HCT inputs are designed compatible with TTL outputs, but are in
fact compatible with any CMOS outputs as well (CMOS outputs are beautiful
-- full swing from 0v to 5v).
If you're ever working with (much) faster circuits, you'll probably want to
stick with just HC since it's somewhat more immune to noise.
> I don't have much data on the '541, does anyone know of any reasons why
> the '541 can't be used ?
I've seen folks use either the '541 or the '245 as replacements to the '244
that are easier to wire. (Just tie the direction line of the '245 low or
high to turn the bidirectional part into a unidirectional one).
Hope this helps.
-- Randy
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