Re: 250466 PCB curiousness

On 7/14/19 6:06 PM, smf wrote:
> On 13/07/2019 21:47, Gerrit Heitsch wrote:
>> But that's not possible, either an adress bit is 1 or it is 0, so any
>> address will select exactly one cell.
>>
> That is the mirage you want to project when designing a computer, but
> it's really all just dirty analogue.
> 
> On 14/07/2019 11:03, Gerrit Heitsch wrote:
>> Correct... But we also have a latch between the SRAM and VIC, at least
>> for A0 - A7 which gets locked by /RAS going LOW. The /CS for the SRAM
>> goes low once /CAS and /RAS are both LOW. From what I understand about
>> the VSP bug is that the address lines change very close to /RAS going
>> LOW. Since the SRAM is not active at that time (/CS still HIGH), it
>> doesn't care what the address lines do as long as they are stable for
>> long enough before /CAS goes LOW.
> 
> I thought VSP bug changed the address bits while RAS was low.

No, it changes the bits around the time when /RAS goes low. As soon as 
/RAS is low for a short amount of time, the DRAM doesn't care what you 
do with the address bits anymore, they are latched internally. The next 
step is then to put the column adresses on the lines and latch them into 
the RAM with /CAS. On normal page mode RAM, you can then change the 
address lines all you want, it won't make a difference. If you want to 
address a different cell, you have to take /CAS back to HIGH and then 
LOW again. Static column RAM is where you don't need to do that.

As for the latch, with the 74LS373 and '573, the outputs follow the 
inputs as long as the /LE pin is high. As soon a /LE becomes LOW, the 
state of the outputs gets 'locked' and remains like this as long as /LE 
remains LOW.



> I'm sure someone tried an SRAM conversion and posted here that they
> still had issues with VSP.

That would be surprising since the SRAM is not selected until /CAS goes 
LOW and the problem happens around /RAS going LOW.



> I don't see the point in doing an sram conversion, unless the dram is
> dead (which is possibly true).

You can still get 4164 and 41256 (*) easily, but 41464 are getting 
harder to get, so boards 250466 and 250469 with dead RAMs are the ones 
where a SRAM conversion begins to make sense.

(*) You can use a 41256 as a 4164 if you tie pin 1 to either GND or +5V. 
Simplest way is to connect pin 1 and pin 16.

  Gerrit