Date   : Mon, 16 Mar 1992 17:47:01 GMT
From   : micro-heart-of-gold.mit.edu!wupost!usc!aero.org!desktalk.desktalk.com!jeg@bloom-beacon.mit.edu (John E. Greene)
Subject: Re: Z80 chrystal upgrade ???

In article <DDVKWAG@irie.ais.org> gravitar@ais.org (Kevin Robert Spears)
writes:
>Is it really that easy to increase the clock speed, just change the
>timebase crystal?  Even assuming all the ICs could handle the increase in
>speed, wouldn't any of the firmware be time-critical andd need to be
>re-written?

Way back when, I was working with a terminal server that used a Z80 as the 
processor.  In order to get 19.2Kbaud, full duplex on both ports, we had to
upgrade to the 8 Mhz Z80 processor.  At the time, it was brand new and we 
received some of the first engineering prototypes.  It was a direct
replacement and none of the code had to be rewritten.  Anything timing related
was handled in hardware and no timing loops in the code were used.  However,
we did run into one major problem.  It seems that although Zilog made a 
processor that ran at 8 Mhz and all of the other supporting electronics could
handle that rate, Zilogs DMA chip would not run any faster than 4Mhz.  This 
was a problem.  In order to make things work we had to develop a sort of 
automatic transmission that would shift the clock rate down from 8Mhz to 4Mhz
everytime we accessed the DMA chip.  It worked liked a dream and everyone was
pretty proud of the accomplishment and when we showed it to Zilog they asked,
"Why would you want to do that?", response "Because you don't make an 8Mhz 
DMA!", ...."oh"...  I guess they never thought of that.


-- 
John E. Greene             Everyone needs something to believe in.  I believe 
Sr. Staff Engineer         I'll have another homebrew!
Desktalk Systems Inc.      
(213) 323-5998             internet:  jeg@desktalk.desktalk.com