Re: Building a 6502 peripheral - timing

Den Thu, 15 Mar 2018 09:49:43 +0100 skrev Gerrit Heitsch
<gerrit@laosinh.s.bawue.de>:
> On 03/15/2018 09:21 AM, Baltissen, GJPAA (Ruud) wrote:
> > Hallo Patryk,
> > 
> > 
> >> No, it can't. PHI2 is nowhere to connect to there so it has to be
> >> taken care of externally.
> > 
> > I meant: the moment you disable CS, the state of WE doesn't matter
> > anymore. In my designs I always make sure that PHI2 is part of CS
> > selection circuit. Using R/W could be dangerous: what if WE becomes
> > (H) before CS? In that case a bus clash can occur, even it is just
> > a very short time.
> 
> In most systems you have bus clashes, but yes, short ones. Someone
> said that this is the reason why a 7501 uses less power than a 8501
> (20mA less from, what I measured). The 8501 is faster, so it gets its
> lines active before the other side becomes inactive.

The most important thing here is to study the data sheets /
specifications for the involved ICs to make sure that there is no harm
when a short bush clash occurs.

As 74LS139 were a topic here, I looked in the data sheet and it says
that not more than one output is allowed to be shorted to ground at the
same time, and the maximum time a short is allowed is one second. This
kind of information should be in the data sheets for the actual ICs in
use.

You don't explicitly need to have a short bus clash for a fast bus to
work, it's just that it were probably easier to implement back in the
days.

> > Regarding the use of Intel and Zilog ICs: I always use a 74LS139 to
> > create the needed /RD and /WR signals using PHI2 and R/W as inputs.
> 
> A 74LS00 should be able to fulfill the same role with one NAND left
> over for other purposes.

But the 74LS139 has one 2-to-4 decoder left for other purposes. A
matter of taste IMHO.

I would btw study the timing diagrams to make sure that it's not too
slow.

Also study the data sheet wisely. A 74LS139 has different delay times
for different inputs and there is also a difference between rising and
falling edges.

Data sheet for reference:
https://www.uni-kl.de/elektronik-lager/417705


-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.