Re: Reading 74HC165 through parallel port? (schematic included)
From: Chris (chris2k01_at_hotmail.com)
Date: 08/05/04
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Date: Thu, 05 Aug 2004 16:19:07 GMT
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Bart T. wrote:
> tronnort@yahoo.com (David L. Jones) wrote in message
> news:<894aa0b3.0408040021.5a856735@posting.google.com>...
>
>> Have you checked the input rise and fall times with a CRO?
>> 74HC chips have a maximum rise/fall time spec, if you exceed it you
>> can get all sorts of WEIRD problems.
>> The capacitance of your connecting cable can cause problems like
>> this. Keep your cable as short as possible and check the lines with
>> a CRO.
>
> I don't have access to an oscope, unfortunately :( But I did make
> some progress today:
>
> I built a voltage divider out of 10K and 2.2K resistors I had handy
> to bring the Vcc down to just under 5V. However, as soon as the
> parallel port is connected, the voltage drops to 2.3V. I never
> experienced a noticable voltage drop when just using the full
> potential of the batteries.
>
> But on the plus side: The chip works! I didn't even have to add RC
> filters to the clock and parallel load lines, though future designs
> will use them.
>
> When I unplugged the batteries completely, the circuit still
> functioned! The 2.3V Vcc seems to be coming from the parallel port
> though I have no idea how. The outputs of the port are only
> connected to the clock line and the data lines and the only
> connections to the power rail are through data input lines (because
> I'm keeping them high.) It's as if the Vcc is being supplied by 4 of
> the 165's data inputs! I know the voltage divider idea isn't really
> suitable for a final design but I'm still wondering why it isn't
> working as it should. I'll try rebuilding the circuit from scratch
> tomorrow. I might just have to buy a 5V battery or use the PC as a
> power source for my prototypes.
>
> Thanks for the help so far! I'll keep plugging away at it to see if
> I can make it work the way it ought to with external power.
Hi,
You might also try using a 7805 (a voltage regulator) to get a perfect
5V with up to 100mA (in the case of 7805) or 1A (in the case of
78H05) available. Just chuck a couple of capacitors between Ground
and Vout for filtering. Vin (according to the data in front of me)
can be anywhere from 8V to 40V.
Chris
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