[Coco] CoCo3 PIA?

[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