Reset circuit for Amiga 500 Tower

[nscaleworld]

The Atari ST computer has a momentary push button that resets the computer.  Basicly, there is a reset pin on the 68000 processor.  I would like to build a reset circuit for my Amiga 500 Tower.  No, I do not want to build the control-Amiga-Amiga circuit.  I would like to build a circuit that resets the 68000 cpu and that also clears the memory, just like it does on the Atari ST.

[Azryl]

The reset circuit from the A500 keyboard resets the OS vectors and pushes a full system reset which includes the 68000...  why not use it?

If I can find my hardware reference manual, but from memory I'm sure there is a pin on the keyboard/motherboard interface that will allow a push button reset like you want.

Az

[psxphill]

The reset circuit from the A500 keyboard resets the OS vectors and pushes a full system reset which includes the 68000... why not use it?

Resetting the processor allows programs to stay in memory. This is used for recoverable ram drive and virus'. You could clear RAM during reset by changing kickstart or adding a new ROM.
 
Unfortunately the original question is too vague to give a correct answer.

[nscaleworld]

Pin 19 of the 68000 processor is the reset pin.  Just ground it for about one second and the chip resets.  But, I think the memory, and maybe a few other chips need to be reset too.  It was done on the Atari ST computer, I just wonder if it can be done on the Amiga as well.  I would like to be able to use the reset switch on the front of the tower case to reset the Amiga 500 Tower.

[curtis]

Like mentioned above, you can just reset the processor, OR if you'd like to have the CTL-A-A functionality back, check this out:  http://mylinuxisp.com/~jdbaker/oldsite/images/A5h_A2k_CDTV_kbd.gif

I'm working on rigging one of my 500's with this little circuit right now.

Of course, you DO need to know which end of the soldering iron to hold onto!

[Zac67]

No system clears the memory on reset (in hardware). If it happens it's done in firmware.
Reset is required for CPU and chipset, the RAM doesn't care.

There are systems out there that may lose a few bits due to a reset (when the RAM controller's refresh stops for too long) but I wouldn't call that 'erasing'.

To topic: if you connect a switch to the keyboard connector's pins 3 & 6 you'll have your reset button.

[billt]

It's been ages, and I've long ago replaced the 500 with a 2000 in my tower. I thought I'd have put something on aminet but can't find it so perhaps I did not. Long enough ago that my memory is a little rusty.

I'm pretty sure I was able to reset-button my 500 in there. While I could be wrong, I want to say that I did something with the A500 keyboard reset for this purpose. I had also added an A2000/A3000 compatible keyboard connector I think. Though I may have hacked an A500 keyboard into the external keyboard shell with an A2000 style wire/connector. The A2000 I believe pulls one of the keyboard wires low for some amount of time to indicate a reset, though I think A500 keyboard had an individual wire for reset. If I'm right, then connect one side of your reset button to ground and the other to a resistor, and the other side of that resistor to the keyboard or other convenient reset line location. Look around online for an A500 schematic service manual to help. An A2000/3000 one might be good too.

[zipper]

http://www.amiga.org/forums/archive/index.php/t-35586.html
has some speculation about reset.

[psxphill]

http://www.amiga.org/forums/archive/index.php/t-35586.html
has some speculation about reset.

As it's all documented very well by commodore, most of the speculation is not only useless but is also wrong.
 
If it's not running at all then it will get reset straight away.
 
 
If it's running well and cleanup happens in less than 10 seconds then it will get reset as soon as cleanup finishes.
 
If the system is running well enough to respond to the reset warning, but not well enough to finish in 10 seconds then it will get reset anyway.
 
The A500 definately didn't have this. No idea why, they could have done it in the software of the mcu in the keyboard. It's possible that when the A500 keyboard was designed, they didn't realise that this functionality existed.
 
G Keyboard Interface / Reset Warning

   About Reset Warning.   -------------------   This feature is available on some A1000 and A2000 keyboards. You   cannot rely on this feature for all Amigas.The keyboard has the additional task of resetting the computer on thecommand of the user.  The user initiates Reset Warning by simultaneouslypressing the Ctrl key and the two Amiga keys.The keyboard responds to this input by syncing up any pending transmitoperations.  The keyboard then sends a "reset warning" to the Amiga.  Thisaction alerts the Amiga software to finish up any pending operations (suchas disk DMA) and prepare for reset.A specific sequence of operations ensure that the Amiga is in a statewhere it can respond to the reset warning.  The keyboard sends two actual"reset warning" keycodes.  The Amiga must handshake to the first code likeany normal keystroke, else the keyboard goes directly to Hard Reset .  Onthe second "reset warning" code the Amiga must drive KDAT low within 250milliseconds, else the keyboard goes directly to Hard Reset .  If allthe tests are passed, the Amiga has 10 full seconds to do emergencyprocessing.  When the Amiga pulls KDAT high again, the keyboard finallyasserts hard reset .If the Amiga fails to pull KDAT high within 10 seconds, Hard Reset isasserted anyway.

[Azryl]

To topic: if you connect a switch to the keyboard connector's pins 3 & 6 you'll have your reset button.

Thanks Zac... I knew there was a reset available on the keyboard connector, just didn't have time or manuals available to check.

Az