MD II overclock variable

Hi i tried to do my new mod form my MD II it's a overclock variable


i used the LTC1799 chip for doing the mod, I wanted to get a variable oscillator of 8Mhz~10Mhz but it's not working

here's my schematic:

http://img571.imageshack.us/img571/3...erclock68k.gif

i use a 6.7k resistor instead the 2.5 Kohms and a 5khom potentiometer instead the 10K to limit the overlock at 10 Mhz and not over this

when i switched on the MD in normal mode it's working fine but when i switch to overclock mode the game freeze and i can't nothing

if i switch on the MD directly on overclock mode the game don't boot

anyone know what to do?

thx in advance

i go it to work!! i got a range of 8.47 Mhz~10.36Mhz and under the 8.47Mhz i have a lot of bugs in the game and if i go down i got an error of the MD ^^

so the value of the pot is 2.85Kohm~5Kohm under 2.85 you will get bugs :D i will try later to optimize the schematic

edit: i need help to do the halt switch

i don't want to use a second switch for this mod so how can i use the same switch and use it also to halt the processor?

and adding a Led to know in which mod i'm

anyone? :? i really want to finish this mod

You cannot use the same switch for halt...

I can't see why underclocking would result in bugs... I have clocked my 68000 to around 70kHz (!!) with no troubles (and like 0.07FPS) :p, but it was a synchronous clocking. It also doesn't require an halt switch - the clock rate could be varied on the fly. I was using a pulse generator.

oops i'm sorry i forgot to edit my messages :? it's not the underclocking that gave me bugs but above 10.36Mhz :?

i can vary the clock into the overclock mode

example: i can adjust to 8Mhz,8.45Mhz etc... without the Halt switch

but when i want to switch from normal mode to overclock mode the MD freeze and i need to reboot it

and i want to include this to my switch but i need a component which can give me a delay, switch on the halt mode, switch to overclock mode and finally switch off the halt mode and all of this with only one switch :?

perhaps if i use flip flop? what do you think? :confused:

The problem is detecting that you flick a switch... you cannot really do it without using some special kind of multi section switch. You could try to use a pull up/down resistor on the CPU side clock line, as when you flick the scwitch, the pin becomes disconnected for a split second, and during that time, it will toggle erratically, and thus can cause major issues.

and with flip flop? if i remember it can detect the flick and change his output no?

you'd need a flipflop, some multiplexer, and some logic chips too.. I don't have a clear idea on how to do it, but its certainly doable.... I'd rather add another switch :P

hmmm so to make it easy it's better to add a switch

thanks you for your help :)

i have a last little question how can i add a bi-color common anode led for showing the status in my schematic?

i have a new idea and i need help finalize it

if i connect my oscillator with a spdt 6 pins i can transform the normal and overclock mode to 0v and 5v

like this:

1 | 2
------
3 | 4
-----
5 | 6

1- pin lifted from the processor
3- the trace that i cut
5- output of my oscilator

2- 0v
4 -input of a component
6- 5v

now when i will be in the normal mode i will get 0v
when i will be in the overclock mode i will get 5v

now i need a component that can take the input and change the state of the halt pin someone can help perhaps?

i know it's easier to put another switch but i want to try ;)

i did it!! with the help of Heero_91 from the forum of Obsolète-Tears

The circuit should work because i did a simulation on my computer with the program Multisim 11.0 if someone know it

here's the schematic:

http://img163.imageshack.us/img163/3...lock68k.th.gif

The circuit work like this:

when i'm in normal mode and i switch to overclock mode i get a negative pulse of 0.5µs on the pin 17 of the processor (Halt pin) from the first Timer, when i switch back to the normal mode i will get a negative pulse of 0.5µs on the pin 17 of the processor (Halt pin) from the second Timer.
The Nor gate is here to let one of the output of the Timers.

Features:

-Overclock variable from 8.33Mhz to 10Mhz (with a potentiometer)
-Halt of the 68000 processor when you switch between the mode (0.5µs)
-Led Bi-color for showing the status

All of of this with only one switch!!

Note: you can use a NE556 instead the two NE555

hope you will like this mod :)

The mod is working on "the edge"... the clock gets switched almost at the same time as halt does, but halt should switch on before clock....

Why not just a linear adjustment from 7.6 to 10? Seems easier to me.

Quote:

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Why not just a linear adjustment from 7.6 to 10? Seems easier to me.

---

because i want to do something new ;)

TmEE you are right i will try to fix this soon :)

You can use a SR-Latch or a JK-FlipFlop and some gates acting as a clock multiplexer. The mulitplexer select port would be the Q of the SR/JK and the S/J and the R/K inputs have to be controlled by the 555 timers. In case of using a JK it's clock can be whatever one (original or the variable one, but dont toggle it).

^ That will make the HALT signal propagate before the clock switching.