[Coco] CoCo3 PIA?

[Mark Marlette]

james, Robert and Phill,

So noted and thanks for the info.

Mark

Quoting jdaggett at gate.net:

> On 4 Jan 2007 at 9:12, Mark Marlette wrote:
>
>> james,
>>
>> Where did you get the dated pricing information from? :) I was being
>> sarcastic on my $0.01 price reference. Where you? :)
>>
> *******************
>
> Price is loosely based on w hat I remember buying reisitors in large  
>  quantity price for carbon
> film resitors. They were in the 3 to 5 cent range for quantities of   
> 1000 range.
>
>> If the LSTTL is not being used as the pull up and the 68x21 doesn't
>> have configurable pullups. How does the circuit guarantee a hi level
>> for the CoCo3 keyboard interface?
>>
> ***************
> You are correct that the 6821 does not have configurable pullups.   
> That does not conclude
> that there are "no" internal pullups. Motorola does not readily make  
>  the internal circuitry of
> their ICs available but they do give the designer an equivalent   
> circuitry. Even this is not
> totally complete. On each port pin is connected to the output and   
> the input section of each
> port. The Data Direction bit essentially enables which section is   
> used. By disabling the
> output section leaves the input NMOS invertor with a very weak   
> pullup. In the order of 50
> nA. The input of an NMOS FET does not need much more than this to turn on.
>
>> They also wire OR'd the joy ports in as well. Again proving that the A
>> side port must be pulled hi. What am I missing?
>>
> *************
> What you are missing is that there is essentially an internal pullup  
>  that is activated when the
> port pin is configured as an input. For the key board interface Port  
>  A is configured as input
> for all 8 bits.
>
> james
>
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