Re: 6522 VIA inputs

From: Gerrit Heitsch <>
Date: Sat, 22 Apr 2017 17:44:24 +0200
Message-ID: <>
On 04/22/2017 05:19 PM, wrote:
> On Saturday 22 April 2017, 17:12:10 Gerrit Heitsch <>
> wrote:
>> On 04/22/2017 04:35 PM, wrote:
>>> On Friday 21 April 2017, 21:40:17 wrote:
>>>>> It's perfectly acceptable to ignore the pull ups and drive 0v/5v.
>>>> I am not sure if I am not trying to overengineer but what causes my
>>>> doubts
>>>> is that it's never 0V/5V, and especially never the same between different
>>>> families of devices, etc.  While pushing the (pulled-up) line LO is OK
>>>> because that's what it is meant to be done, the potential of the sourcing
>>>> output and the pulled-up line are almost certainly different so it will
>>>> have to cause some (unnecessary / unnecessarily higher) current flow
>>>> through the line, possibly adding to consumption, unwanted emissions,
>>>> etc.
>>>> ... or are all those possible side-effects fully negligible and I am just
>>>> too paranoid here? ;-)
>>> driving a NMOS i/o line high is a big nono. just dont. its a common thing
>>> to do to connect several outputs together, forming a wired OR - when one
>>> of those outputs is driving high, the one trying to pull low will have a
>>> hard time doing it. even if it still may work, the signal timing will go
>>> poop
>> Eh? So far I though NMOS is very good at sinking current to GND, but not
>> at supplying current from Vcc. Looking at the datasheet for the 6526
>> supports this, the output driver can supply at most 1mA, but can sink
>> something in the range of 3mA.
>> So if you connect 2 NMOS outputs together, the one pulling the line LOW
>> will win. Also, on most NMOS outputs, you cannot disable the 'pullups'
>> (see output driver schematic for the 6522).
>> What you must not do is using a CMOS output set to HIGH and connect it
>> to an NMOS output set to LOW.
> thats what i ment with "driving high". a NMOS output does never "drive high",
> it only ever pulls low.

The output driver for the B ports on the 6522 disagree.


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Received on 2017-04-22 16:02:18

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