Re: Strange 8255 behavior

From: Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de>
Date: Sun, 10 Jun 2018 15:07:02 +0200
Message-ID: <22efc6fc-489a-2789-579d-e7f32763229a@laosinh.s.bawue.de>
On 06/10/2018 02:49 PM, Clockmeister wrote:
>   I know nothing of such things, but would it be easier and cheaper to 
> simulate a 6526 using an Atmel controller(s) of some kind?

Too slow. At 20 MHz you'd have less than 10 cycles to see what the host 
CPU wants, deliver that data and get off the bus again.


  Gerrit





> 
> On 10/06/2018 8:33 PM, Gerrit Heitsch wrote:
>> On 06/10/2018 01:59 PM, Michał Pleban wrote:
>>> Hello!
>>>
>>> smf wrote:
>>>
>>>> I assume for cost purposes they don't keep the last output value 
>>>> when in
>>>> read mode, so when you switch back to output mode then zeros is 
>>>> probably
>>>> the lesser of two evils.
>>>
>>> That might be the case when you switch from input to output, but when
>>> you "switch" from output to output (i.e. you leave the direction of the
>>> port unchanged, only modifying some other port) it's just ridiculous.
>>>
>>>> Trying to replace a mos chip with an intel chip is blasphemy :-)
>>>
>>> Guilty as charged :-) But it's for a good reason: there are only very
>>> little 6525's laying around, and if we want to make the 8088 card
>>> accessible to everyone, we had to change to a chip that's readily 
>>> available.
>>
>> How about two 6522 instead? With some clever logic and mapping of the 
>> address bits, you might be able to come up with a register map that 
>> comes close to what the 6525 has, limiting the amount of change in the 
>> code.
>>
>>  Gerrit
>>
>>
>>
>>
>>
> 
> 
Received on 2018-06-10 16:01:10

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