Yep, it's me again... :)

  Well we have successfully accessed pin 6 of the games port in READ mode.

  We are now going to switch the direction and WRITE 1s and 0s to pin 6 of
  the games port. This will generate a train of 'Square Wave' pulses at TTL
  level that can be used as a variable speed clock. The same hardware can
  be used for both input and/or output so as to protect the port from damage.
  Pin 6 is now going to be used in WRITE mode so beware!.

  The registers CIAAPRA and CIAADDRA are both going to be changed this time
  as they are not set up during a coldboot or reboot. See the previous issue
  of the magazine for the details of CIAAPRA and CIAADDRA.

  Ensure NOTHING, EXCEPT the home built hardware, is connected to the games
  port of the AMIGA to prevent possible damage. Read the 'WARNING' below.

  As will be seen there is an Assembler version which was compiled using
  ~a68k~ and linked using ~blink~. These are FREE on AMINET, EXTREMELY
  SMALL in size, easy to use from the CLI/Shell and require nothing special
  to compile basic machine code programs. This generates an executable
  called ~TTL-Clock~ and when run produces a high speed square wave at pin
  6 of the games port. This is PROBABLY the fastest REAL TIME external user
  clock pulse via a port on the AMIGA WITH the ability to stop it using a
  left mouse click only. To attain this speed interrupts and tasking are
  BOTH DISABLED during running to ensure a constant output at pin 6 of the
  games port so be VERY AWARE of this fact. The ~_LVO?~ offsets are shown
  as comments with the actual 'exec' offsets placed into the code directly.
  Because there is NOT likely to be any more changes to the classic AMIGA
  hardware then why waste time and effort writing complex assembler code
  using 'equates', 'offsets' etc... just to access, the now, well
  established hardware registers.

  REMEMBER!!! If a hardware resource is there, grab it for your own use as
  only one item at a time can be connected to it! Don't bother to notify
  the OS that you need it. You know whether it is free for your use or not!

  To give some idea of the generated clock rates for each of these three
  coding methods; then here are the APPROXIMATE frequencies for:-

  1) An A500 with 0.5MB upgrade at $C00000, AmigaBASIC and OS1.3x...
     >8Hz.
  2) An A1200 with 4MB FastRam, AmigaBASIC, KS3.0x and Workbench 1.3x...
     >50Hz.
  3) An A600 with 2MB ChipRam, ARexx and OS2.0x...
     >3Hz.
  4) An A1200 with 4MB FastRam, ARexx and OS3.0x...
     >15Hz.
  5) An A600 with 2MB ChipRam, OS2.0x and the 'TTL-Clock' executable...
     >25KHz!!!
  6) An A1200 with 4MB FastRam, OS3.1x and the 'TTL-Clock' executable...
     >75KHz!!!!!

  To prove that these events are happening then a 'Frequency Counter'
  AND/OR 'Oscilloscope' needs to be attatched to the Blue Wire, (OUTPUT),
  and the Black Wire, (GND/RETURN)...

----------------------------------------------------------------------------

  AmigaBASIC Code:-
  -----------------

REM ***********************************************************************
REM
REM Games Port WRITE access using STANDARD AmigaBASIC only.
REM
REM Original idea and copyright, (C)1996 B.Walker, (G0LCU).
REM
REM Now PD 2007 via:-     http://www.thecryptmag.com
REM
REM This is ready for AmigaBASIC, an old style A500 and OS1.3x.
REM
REM ***********************************************************************

REM Written so that youngsters can inderstand it!
REM $VER: TTL-Clock.b_Version_0_78_00_(C)2007_B.Walker_G0LCU.
REM Set up any string variables.
  LET a$="(C)2007 B.Walker, G0LCU."
REM Set up any numerical variables.
  LET n=0

REM ~CIAA~ address for register ~pra~.
  LET pra=12574721
REM ~CIAA~ address for direction of port ~pra~, (1=OUTPUT).
  LET ddra=12575233

REM Store single byte values of these addresses for future use.
  LET valpra=PEEK(pra)
  LET valddra=PEEK(ddra)
REM Bit 0 of port ~ddra~ DO NOT CHANGE...
REM Bit 1 of port ~ddra~ switches the ~LED~ ON or OFF, (1=ON).
REM Bits 2 to 5 inclusive of port ~ddra~ DO NOT CHANGE...
REM Bits 6 and 7 of port ~ddra~ alter PIN 6 of the mouse/games ports, (1=OUTPUT).

REM Generate a simple setup screen.
  COLOR 1,0
  CLS
  LOCATE 4,18
  PRINT "Open the games port for WRITE access..."
  LOCATE 6,18
  PRINT "Data direction address is at 12575233..."
  LOCATE 8,18
  PRINT "Press almost any key to exit..."
  LOCATE 12,18
  PRINT "Data tansfer address is at 12574721..."
  LOCATE 14,18
  PRINT "Value at pin 6 of the games port is:- 1."
  
REM Write ~1~s and ~0~s to PIN 6 of the games port.
REM Force bit 7 of ~pra~ as an OUTPUT.
REM It is assumed that it is ALREADY set as an INPUT!!!
  POKE ddra,(valddra+128)
REM Enter the ~Clock Mode~ loop...
loopit:
  LET n=PEEK(pra)
REM Set bit 7 of ~pra~ to a value of ~1~.
  IF n<=127 THEN POKE pra,(n+128)
  LOCATE 14,56
  PRINT "1"
  LET n=PEEK(pra)
REM Set bit 7 of ~pra~ to a value of ~0~.
  IF n>=128 THEN POKE pra,(n-128)
  LOCATE 14,56
  PRINT "0"
REM Press almost any key to Quit...
  LET a$=INKEY$
  IF a$="" THEN GOTO loopit:

REM Exit routine.
REM Reset all of the registers back to their original state.
  POKE pra,valpra
  POKE ddra,valddra
  END

----------------------------------------------------------------------------

  ARexx Code:-
  ------------

/* ------------------------------------------ */
/*  TTL-Clock.rexx, (C)2006 B.Walker, G0LCU.  */
/* ------------------------------------------ */
/* Direct games port programming using AREXX. */
/* ------------------------------------------ */

/* This simple program generates a square wave at approximately 5Hz on */
/* a stock A1200(HD) with 4MB of FastRam at TTL level. */
/* Use an Oscilloscope to check that this works. */
/* Written so that youngsters can understand it! */

/* Set up any constants or variables. */
  mousetrap=1
  copyright='9'x'$VER: TTL-Clock.rexx_Version_0.92.00_(C)2006_B.Walker_G0LCU.'

/* Set up the games port screen. */
  ECHO 'c'x
  ECHO '9'x'9'x'Open the games port for WRITE access...'
  ECHO 'a'x'd'x'9'x'9'x'Data direction address is at $BFE201...'
/* Click mouse button to exit. */
  ECHO 'a'x'd'x'9'x'9'x'Click left mouse button to exit...'
/* Access the games port directly. */
  ECHO 'a'x'a'x'd'x'9'x'9'x'Data transfer address is at $BFE001...'
  ECHO

/* Set up games port pin 6 for write only. */
  EXPORT('00BFE201'x,'83'x,1)
  ECHO '9'x'9'x'Value at pin 6 is:- 1.'

/* This is the main loop. */
  DO FOREVER
/* Set pin 6 of the games port to 0. */
    ECHO 'b'x'9'x'9'x'Value at pin 6 is:- 0.'
    EXPORT('00BFE001'x,'7C'x,1)
/* Click left mouse button to exit. */
    mousetrap=IMPORT('00BFE001'x,1)
    IF BITTST(mousetrap,6)=0 THEN CALL getout
/* Reset pin 6 of the games port to 1. */
    ECHO 'b'x'9'x'9'x'Value at pin 6 is:- 1.'
    EXPORT('00BFE001'x,'FC'x,1)
  END

/* Exit the program safely. */
getout:
/* Reset pin 6 of the games port back to TTL logic 1. */
  EXPORT('00BFE001'x,'FC'x,1)
/* Set pin 6 of the games port back to READ mode. */ 
  EXPORT('00BFE201'x,'03'x,1)
  ECHO 'c'x
  SAY copyright
  EXIT(0)

----------------------------------------------------------------------------

  This Is A Special Extra Assembler Code:-
  ----------------------------------------

  To compile from the command line:-
  ----------------------------------

  AMIGA-Shell:> a68k -n TTL-Clock.asm<RETURN/ENTER>
  <Some reports here...>
  AMIGA-Shell:> blink TTL-Clock.o<RETURN/ENTER>
  <some more reports here...>
  AMIGA-Shell:> TTL-Clock<RETURN/ENTER>
  <Awaiting a left mouse click here to return back to the Shell/CLI...>

  NOTE:-
  ------
  This also compiles and runs under DevPac2/3 successfully... :)


	;$VER: TTL-Clock.asm_Version_0.10.00_(C)2007_B.Walker_G0LCU.
	;-----------------------------------------------------------
					;
					;This is an example of direct HW
					;access using plain assembler. It
					;is the fastest means of generating
					;a TTL level clock pulse for your
					;usage.
					;
					;Assembled with a68k.
					;
	;	http://main.aminet.net/dev/asm/A68kGibbs.lha
					;
					;Linked with blink.
					;
	;	http://main.aminet.net/dev/misc/blink67.lha
					;
					;START...
	movem.l	d0-d7/a0-a6,-(sp)	;Save all registers just in case!
					;
	movea.l	$4,a6			;Set the 'ExecBase' pointer.
	jsr	-132(a6)		;Offset -132 == [_LVO]Forbid()
					;Disable tasking and interrupts
	jsr	-120(a6)		;Offset -120 == [_LVO]Disable()
					;
	bset	#7,$bfe201		;Set pin 6 of the games port to
					;WRITE mode.
lmb:					;
	bchg	#7,$bfe001		;Toggle bit 7 of the games port.
	btst	#6,$bfe001		;Check to see if the left mouse
					;button is pressed to quit.
	bne.s	lmb			;IF NOT then loop until it is.
					;
	bclr	#7,$bfe201		;Reset pin 6 of the games port to
					;READ mode.
	movea.l	$4,a6			;Set the 'ExecBase' pointer again.
					;NOTE:- This is a double check only!
	jsr	-126(a6)		;Offset -126 == [_LVO]Enable()
					;Enable interrupts and tasking.
	jsr	-138(a6)		;Offset -138 == [_LVO]Permit()
	movem.l	(sp)+,d0-d7/a0-a6	;Restore all registers.
					;
	clr.l	d0			;Ensure return code is 0.
					;
	rts				;Return to calling routine.
					;
	END				;END... SIMPLE EH!


  A brief explanation of what is going on in the assembler version...

  IMPORTANT!!! THIS IS DIRECT PROGRAMMING OF THE HW SO BEWARE!!!

  At the START of the program all standard registers are saved just in case,
  to ensure they are preserved for later. This is always good practice
  anyhow. Then a pointer to the 'ExecBase' is loaded into register a6.

  IMPORTANT! Next the interrupts and tasking are DISABLED to ensure that
  the resultant square wave at pin 6 is not corrupted. This WILL give the 
  impression that the computer has locked up completely - IT HASN'T!!!

  Next the 'CIAADDRA' register $BFE201 has bit 7 switched to a '1' for
  OUTPUT mode. You now enter the continuous loop for the clock generation.

  Bit 7 of 'CIAAPRA', (pin 6 of the games port), is toggled from its present
  state to its inverse state, that is from say a '1' to a '0'.
  Bit 6 of 'CIAAPRA', (pin 6 of the mouse port), is checked to see whether
  the left mouse button is pressed; if NOT then carry on toggling pin 6 of
  the games port. If the LMB IS pressed then exit the loop.

  Upon exitting the loop immediately reset the games port back to READ mode.

  Ensure that the 'ExecBase' pointer has not been corrupted and then ENABLE
  the interrupts and tasking to make the computer usable again.

  Return the registers back to where they were before running this program.

  And finally give a return code of '0', (OK), and return back to the Shell
  or Workbench and normal computer usage.

  SIMPLE EH!... :)

  As a final note on this version, the 'TTL-Clock' executable was tested on
  an A600(HD) in 2 and 6MB modes using trapdoor and/or CC0: PCMCIA 4MB
  memory expansions and OS2.0x and an A1200(HD) in 2, 6 and 10MB modes using
  trapdoor and/or CC0: PCMCIA memory also and OS3.0x/OS3.1x.... :)

============================================================================

                                IMPORTANT:-
                                -----------


    The Legal Stuff:-
    -----------------

    These programs are Freeware and no profit will be made from them,
    also all of the files must remain unaltered and intact including this
    one. The author is not responsible for any damage to, or loss of, or
    failure of equipment or data caused in any way by the use of these
    programs. There is NO warranty with the use of these software releases
    and YOU USE THEM AT YOUR OWN RISK.

----------------------------------------------------------------------------

    Testing Evaluation:-
    --------------------

    An A1200(HD) in 2MB, 6MB and 10MB modes using trapdoor memory AND/OR
    PCMCIA memory expansions. Also tested on an A600(HD) in 2MB and 6MB
    modes. All test conditions were/are running STANDARD KS2.0x to 3.1x,
    and using standard ~topaz 8~ fonts throughout.

    I have no idea what a strange configuration setup will create so refer
    to the ~The Legal Stuff~ above.

----------------------------------------------------------------------------

                                 WARNING.
                                 --------

  1) DISCONNECT any faulty equipment under test from the MAINS supply.
  2) If a DC supply is used do NOT reverse polarity the connections.
  3) Do NOT power up any electronic item until it is safe to do so.
  4) CHECK and RECHECK all of your construction and repair work thoroughly.
  5) Handle ALL tools used with care.
  6) Beware of ALL types of solvents, glues and etching fluids.
  7) NEVER leave a soldering iron switched on unattended.
  8) KEEP everything OUT of the reach of small children.
  9) Switch OFF the AMIGA before disconnecting or connecting any hardware.
 10) And finally read 1) to 9) again.

----------------------------------------------------------------------------

   Contact:-
   ---------

   Mr Barry Walker, G0LCU.

   Email:-     wisecracker@tesco.net
   URL:-       http://homepages.tesco.net/wisecracker/G0LCU.HTM

   Author of the ~TestGear?~ projects in the ~docs/hard~ drawer of AMINET.

----------------------------------------------------------------------------

   A very useful HardWare related site, (C) Anthony Hoffman, for
   modifications, schematics, repairs and the like is:-

                          http://amiga.serveftp.net/

============================================================================