Re: Hacking the Laser 128EX/2

[Charles Stephens <devnull@dobbs.eng.sun.com>]

>>>>> "ST" == Supertimer  <supertimer@aol.com> writes:

They probably have a divisor on the 3.6 clock, so it probably runs at .9 MHz.

A scope or logic probe would be more revealing.

cfs

 ST> Rubywand <rubywand@swbell.net> wrote:
 >> Rubywand wrote:
 >> 
 >>> 
 >>> Getting back to the oscillators, 3.6864MHz is an interesting frequency.
 >>> It seems to be interger divisable to obtain all of the major telecom baud
 >>> rates.
 >>> 
 >>> This could explain the choice of "3.6MHz" (instead of 4MHz) as the uP
 >>> clock frequency. My guess is that the 3.6864MHz osc serves as both the uP
 >>> clock and the base for generating serial port baud rates.
 >> 
 >> Here's another possibility which, really, fits things better. Maybe the
 >> 3.6864MHz osc is entirely devoted to baud rate generation. The "3.6MHz" number
 >> for the Laser's speed comes from 3.5795MHz. This, of course, is the color
 ST> burst
 >> frequency which comes from dividing down 14.318MHz.
 >> 
 >> Since Laser needs to be Apple II compatible and it needs color burst,
 >> using color burst for the uP clock cuts costs and also links/syncs the uP
 ST> clock
 >> to other important A2 frequencies. (And, if Laser ran at 3.6864MHz, it seems
 >> like marketing would advertise speed at "3.7MHz" rather than "3.6MHz".)
 >> 
 >> If the above is correct, you may be able to go to "7MHz" (7.16MHz) by
 >> using the 7M signal for your system clock. Otherwise, it looks like messing
 >> with the 3.6864MHz crystal will not help and you are pretty well locked into
 >> your current speed.

 ST> Shucks, I was afraid you'd say that.

 ST> Still, I might try to change the crystal just to see what happens.  What's the
 ST> worst that could happen?  I don't use the serial ports anyway.  ;-)

 ST> XTAL3 is pretty close in location to the CPU, but I guess that does not really
 ST> say much.  What I'm curious about is the labels.  If there is an XTAL1 and
 ST> an XTAL3, there should be an XTAL2.  I'll have to look more closely.  It may
 ST> be that XTAL2 would illuminate things.

 ST> Another thing is that even in "1Mhz mode" the Laser is not exactly 1Mhz
 ST> vis-a-vis the Apple IIe.  It is slightly different.  This might be what is
 ST> throwing
 ST> off the ZipChip as the ZipChip does not work on the EX/2.  It totally bombs
 ST> while booting up in 3.6Mhz mode.  In 1Mhz mode it works for a little bit,
 ST> then bombs.  I figure if the CPU clock is derived from XTAL1 (14Mhz), then
 ST> the timing should be exactly like an Apple IIe (in 1Mhz mode), but it is not.

 ST> The ZipChip works in the plain Laser 128, though.  Perhaps if a Laser 128
 ST> user would take a look at their motherboard and see if XTAL3 is present,
 ST> the mystery would be solved.  Meanwhile, I'll have to hunt for XTAL2.

-- 
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