[Coco] Re: [Color Computer] Interfacing question.

[Gene Heskett]

On Tuesday 22 March 2005 20:42, Phill Harvey-Smith wrote:
>Gene Heskett wrote:
>> On Monday 21 March 2005 13:27, Phill Harvey-Smith wrote:
>>>Dunno if this is a silly question but here goes....I'm trying to
>>>re-implement the RS dos controler using PLD logic chips (22v10
>>> etc), and have run accross a little problem. I have a 22v10, that
>>> does the address decodde for the FDC and for the
>>> drive/density/halt enable. This chip also has the latch for the
>>> halt enable. However I find that this latch will not work with A3
>>> connected UNLESS I install a pull-up on this line.
>>
>> Have you got good scope Phill?  I'm wondering if somehow your 6809
>> has been banged around via pulling a game cart when things were
>> hot, or worse yet plugging one in hot, which might have blown the
>> internal pullup transistor in that 6809.
>
>Scoped them this evening all seem to be reaching 5V, though some of
> them it's easier to see than others, guess it just depends what's
> on the lines :) Odd thing is though if I keep scope probe on line
> when holding Reset I get 2V, this does seem a little strange to me.

When in reset status, then the outputs are I think, effectively 
tristated and would float at about a TTL logic one.  But what do you 
mean by going up to 5 volts?  Any 6809 that can pull its address 
lines up to 5 volts is a rare bird indeed, I've never seen them go 
above about 3.3 volts myself.  63C09's that I've looked at yes, goes 
all the way to both rails, but I've never seen a 6809 that could pull 
that high.  That 3.3 is .9 volts above the TTL logic ones noise 
margin voltage of 2.4, so anything above that, thats also quiet is 
plenty high enough.

How much pullup does it take?

Silly Q:  How hot is the regulator transistor running, with full 
memory and a 6809, they run finger burning hot.  If its not that 
warm, check its output voltage, it may be shorted and putting out 
6.5-7.5 volts.  That would explain the 5 volts you are seeing if 
thats what you are really seeing.

>>     Checkable if its socketed by trying
>> another 6809, or better yet, a 6309 as its active pullups are
>> quite
>
>Have tried both my CoCo 1 and my Dragon same results. I do have a
> couple of spare 6809s though, so I could indeed try changing one of
> them. Don't have a 6309 unfortunatly.
>
>> strong and go *all* the way to the 5 volt rail, unlike the 6809. 
>> It is a pure CMOS chip, using only a few milliwatts of power at
>> the coco's clock frequencies.
>
>May try andd get hold of a couple it is a pin compatible drop in
> isn't it ?

Yes, and about 10% faster with no other changes because it pipelines 
the next instruction fetch with the last internal cycle.  Running the 
latest nitros9, its about 2x faster than the original os9.  Saddly 
the basic in the roms has not been patched, M$ might take a dim view 
of that.

>> Compare the height of the logic one on A3 with that of one of the
>> other address lines, for TTL compatibility it will need to hit
>> around 2.7 to 3 volts.
>> You might check the expected logoc levels the 22v10 needs, maybe
>> it needs more logic 1 than the normal TTL specs do?  I don't
>> personally know on that point.
>
>Yep, definiatly reaching this high, acording to the data sheet for
> the ATF22v10BQL that I am using anything above 2.0v should be seen
> as logic 1.
>
>>>So I guess my question is, is there anything strange about A3 ?
>>> that can cause this to happen. I am eventually planning to move
>>> over to a CPLD, but I hopefully don't want to have the same
>>> trouble with that.
>>
>> Not that I'm aware of Phill.  A0 and A1 catch a bit more load than
>> the rest, but A3 really shouldn't have any more load on it that A4
>> thru A15 unless something gone foobar.
>
>Humm that's what I figured.
>
>The really odd thing is that A3 is affecting my PLD even if it is
>connected to it but not used in the internal logic which is weird I
> have verified this by temproally disconnecting (power off of
> course) the pin, the damn thing works absolutely fine then, but of
> course I need A3 to select FF40/FF48 :)
>
>Phill.

Thats odd indeed, and would have me searching the docs on that PLD 
looking for some condition they just mention in passing.

>Brought to you by the 6809, the 6803 and their cousins!
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>
>
>

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.


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