

14mhz Accelerator for A500 -Les Ayling
--------------------------


     Welcome once again to cheapskates corner, where we try and buy the world
for a song.  This time around we shall increase the speed of the processor to
just over 14Mhz, double the standard speed, for under $25 !!!!

     You will need the following -

1. an MC68000p12A 16MHz processor. These are available in Sydney from VSI 
   electronics, 16 DICKSON ave ARTARMON 2064 ph (02)4398622  cost is $18.90
   plus tax ($22.68 tax inc.)
2. a 74F74 D type flip flop. Geoff wood at Lane Cove or Rod Irving should
   keep them.
3. a few short lengths of hook-up wire.
4. a SPDT switch.

Refer to the accompanying circuit diagram

Note: Keep the leads to the switch as SHORT as possible.

   The circuit is relatively straight forward. the 74F74 is wired as a
positive edge triggered divide by two circuit, which provides the alternate
clock frequency of 14.18Mhz approxiamately. The old 68000 8Mhz processor is
removed and replaced with the 16Mhz version which has is clock pin (15) bent
out horizontally so we can connect our new circuitry to it. The SPDT switch
either connects the normal 7Mhz signal or the new 14Mhz signal to the
68000's clock input.

   The switch can be changed while the machine is on, however it will simply
lock up.  Just reset as usual from the keyboard once you have changed speed.
(Check the mod further down for info for changing on the fly!)

I have a revision 5 board with new 1.3 roms which cope with the new speed
quite happily. Using the program CRAMDEN-SI from MEGADISC 10 to compare the
difference the mod makes :

The standard position gives-

performance relative to amiga 1000 : 1.0
performance relative to ibm pc/xt  : 3.2

On high speed the performance is-

performance relative to amiga 1000 : 1.7
performance relative to ibm pc/xt  : 5.2

so roughly a 70% increase in performance.

   Even though the clock speed is doubled to the processor, the performance
dosen't increase to 2.0 because the agnus and the other support chips are
still running at the same speed as before thus holding the processor up for
a slightly higher percentage of the total time than before.

   This mod basically gives the same capability as the CMI accelerator board
priced at $399 in Sydney, with the exception that it isn't software
controlled. However when it's only costing 1/16th the price one cant really
complain!!.

   The machine should work on the 1000 and 2000 however the pin numbers may
be different.  I suggest you compare the a500 schematics with that of your
machine and make note of the differences.

#addendum#

   One machine this mod was carried out on had trouble with the one of the
disk drives at high speed. It appeared to be trying to step the head of the
internal drive faster than the drive could physically move and refused to
boot.  When I connected a drive with a 5ms track-track access time and
booted off of it the problem dissappeared.  I have a patch for this problem
for normal DOS disks to slow down the step rate which sits in the boot
sector.  It may be a problem however for protected software or non-standard
boot sector type programs.  Don't despair however the patch is very small
and should fit in somewhere anyway.  Just drop me a line.  If your machine
exhibits the problem, the patch disk is available DIRECT from me with
various usefull disk speedup utils and a Trackdisk step-rate alteration
program, plus loads of other priceless information on gurus, reset program
flow, diagnostics all on an ARP1.3 system disk for the princely sum of $5 !!
There should be no problem with HARD disks however if anyone encounters any
please let me know........thanks.


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14mhz Accelerator
-----------------
The hack to accelerate your Amiga to 14MHz has been published in countless
articles, but I may as well include a brief description of it here. The hack
involves removing the original 68000 from your Amiga and substituting a
version rated at 16MHz (confusingly called the 68000P-12; the Motorola chips
have "16MHz" stamped on them, but the SGS-Thompson clones do not). The clock
pin is bent out and soldered to the output of a divide-by-2 counter whose
input is connected to the base 28MHz clock. THIS MODIFICATION SHOULD BE
INSTALLED WITH A SWITCH !! If not, you WILL (not may) find that some games
will refuse to load because they use timing-dependent loaders.
        Aside from the problem mentioned above, the only compatibility
down-side to this modification is that some floppy drives will not handle the
increased step rate. If you find that your drives give read/write errors at
the high speed, you can either replace them or run a program called "DFDelay"
to slow down the step rate. (If df0: will not work, you have a problem. Try
swapping df1: for df0: - to do which you will need a Commodore df1:. The
mechanisms used in the old A1010 drives are EXTREMELY reliable and much
quieter than the new rubbishy things; if you have an A1010, you can be fairly
certain that it will support the new speed).
        To perform the mod, remove your old 68000 and embed it in a perspex
block for future reference. Now rig half of a 74S74 dual D bistable as a
divide-by-2 counter. Connect the input of this to a convenient take-off point
of the 28MHz clock line on the motherboard (I use pin #34 of Agnus, but
anywhere will do). Now bend out pin 15 of the new CPU and solder a wire to
this pin. Take a piece of stiff telephone wire, strip approx. 5mm of
insulation from one end, and stick this end down pin 15 of the 68000 socket.
Insert the new CPU in the socket and verify that pin 15 is well clear of the
wire in the socket. Now wire a SPDT switch as follows :

                                            BACK VIEW
                                     +---+  When held as shown UP=7.14MHz,
   To output of divide-by-2 counter -+-o |  DOWN=14.28MHz
                 To pin 15 of 68000 -+-o |
  To wire in pin 15 of 68000 socket -+-o |
                                     +---+

The 74S74 should be mounted somewhere convenient; I break off all the unused
pins, bend the remaining pins out, and stick the chip on top of one of the
data path IC's; this has the advantage of allowing me to take power directly
off the existing components rather than stringing yet another pair of wires
through the machine.
        If you only have chip and/or slow memory then your machine will run
at approximately 150% normal speed (on CPU-intensive operations; DMA rates
will be unchanged). If you have true fast-RAM then code running from
fast-RAM will execute at close to 200% of normal speed. Try placing a large
perpective brush with anti-aliasing HIGH in DeluxePaint III (or IV !) and
you will see the speed increase.


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ENHANCED ACCELERATOR
by Ray Taylor

    This is an enhancement of the cheap accelerator project published in
   Megadisc 14.  That project had a problem that caused the system to
   crash if you tried to change speed.  The extra chips will act to delay
   the change of speed until the two clock signals are at the same logic
   level.  This should overcome the problem of crashing.  The circuit also
   overcomes the problem with the length of leads between the clock
   signals and the switch. I have not been able to test this because
   everyone else must have beaten me to VSI Electronics and bought all
   the 16Mhz 68000 processors.  I'm waiting for new stocks.

    Keep up the good work with your useful magazine.  I've learnt more
   from this magazine than I have from all the manuals, books and magazine
   articles I've read.


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HEAT PROBLEM ON 16MHZ 68000 - Ian Mills

    Quite a while after I upgraded to 14Mhz I was pottering around with
   Amy's internals and my hand brushed the 68000. It came as quite a
   shock to find it hot enough to fry an egg near the clock input pin. I
   quickly powered off suspecting I had dropped something in causing a
   short. Upon inspection, it seems this is normal, to run at 16Mhz the
   68000 must use more power hence more heat dissipated. I rectified this
   by mounting a small heatsink directly on the 68000 near the clock
   input. It is held in place by a blob of heatsink compound.

    In the two months mine ran without the heatsink, I didn't experience
   any problems. Too much heat can cause computer chips to behave
   erratically, and ultimately, cause irreparable damage. Now the weather
   is warming up, it may raise the temperature high enough to cause
   problems. Due to the fan and forced air cooling in a 2000, the 68000
   should be cooler than a 500, but the few cents spent on a heatsink is a
   good investment when you consider a 16Mhz 68000 costs about $24.

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             FURTHER MODIFICATIONS to Les Ayling's 7 - 14 Mhz Upgrade.
                 Changing Speeds 'On the Fly' without a re-boot.

                                                          Ian Mills
                                                          PO Box  429
                                                          Artarmon  2064
                                                          NSW

Brief :

   It will allow you to change speeds without rebooting. It can also be
   used in a slightly different way, as described at the end.

Disclaimer :

   The same as always ! Your warranty is already void if you did Les's
   original upgrade ! I take no responsibility for any modification
   you may do resulting from this article.

Background :

   In his article, Les stated that the CPU must be reset after changing
   speeds, as the 68000 got upset when its speed was changed inelegantly.
   This got me thinking, as he also said the commercial version supported
   a change in speed 'on the fly'. It must be possible, but how ......

   Most CPUs have a number of 'Non Maskable Interupts'  (NMI) that are used
   for low level communications between other chips or CPUs, and our 68000
   is no exception. It has a NMI called HALT that is connected to pin 17.
   Normally it is high, but by pulling it low, the 68000 will finish its
   current instruction and wait until HALT goes high again.

   The HALT signal on the Amiga 68000 is connected to GARY's pin 42. GARY
   pulls HALT low during the keyboard reset sequence (Ctrl, Amiga, Amiga).
   Halt also is connected to the expansion edge connector on the left hand
   side of the Amiga 500 (pin 55).

Parts:

   1 x SPST switch
   Hook-up wire

   Modification:

   Connect a wire from ground to one side of the switch. Connect the other
   side of the switch to pin 17 on the 68000 or 55 on the expansion bus.
   Mount the switch near the 14 - 7 Mhz switch.

   Now to change speeds, halt the CPU by throwing our new switch, select
   the other speed via the original switch, then allow the CPU to continue
   by turning off the halt switch.

   Variations to this Modification :

   The fact that pulling HALT low causes the CPU to stop and wait may be
   a usable feature for some users.  It would be the ultimate "Control P"
   for gamers or for a close look at a fast animation.
   Another use could be a key-switch rather than a toggle switch to keep
   unwanted meddlers out of your system.


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ACCELERATOR NOTES
by Leslie Ayling

    Here are a few problems which some people had which don't appear
   obvious at first.

      1. If you are using ribbon cable to the switch, make sure to
   separate the conductors as some ribbon cables exhibit a cross-conductor
   capacitance. Hence the ground wires separating the various signals on
   the disk-drive connector for example. This could stop the system
   working at all.

      2. Make SURE that you are supplied with the 16MHz version of the
   processor. The 12.5MHz version carries a similar part number but is
   not up to our demands. The 16MHz version should have "16MHz" printed
   on its case underneath the Motorola part number.

      3. I have recently purchased an A590 hard drive and have found that
   the accelerator works fine with it, with or without 2 meg of ram
   installed.

      4. Since there have been many enquiries from A1000 users, next issue
   I will probably describe the adjustments needed for that computer.

