7201 (was Re: Z8530 (was Re: Navtel 9460 Protocol Analyzer

Tony Duell ard at p850ug1.demon.co.uk
Sat Apr 16 18:47:05 CDT 2005

> >Serial chips, I get post-traumatic-stress just thinking about
> >them.
> I'll remember that one.  Though compared to serial chips many disk 
> controllers and their associated stuff that wraps around them 
> programtically are pretty horrid too.

The 8255 parallel interface chip was bad enough. It has an amazing 
mis-feature : Any write to the mode control register sets all output 
lines to 0's. 

After a reset, all lines are inputs (which was sensible, it avoids 
possible contentions). If you drive TTL devices from them, it's 
conventional to pull them high (or the TTL inputs will float high). You 
then write to the mode control register, configuring some lines as 
outputs, at which point they all go low. You then write to the output 
registers setting the lines to the states you want. You have to design 
your hardware to deal with this.

Of course you also can't reverse the direction of a port under software 
control without clearing all the outputs.

Sensible parallel chips (6821, 6522) don't do this. The output bits are 
not affected by writes to any other register. And those chips let you 
define the direction of each port line.

I want to insert an 8255 up the rear end of its designer...


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