# single stepping TS68000 microprocessor

I recently got TS68000 microprocessor. My final goal is to build a simple computer based on that CPU.

For now I am trying to find a way to single step it.

I found schematic below by John Tsiombikas and I tried to introduce it but I failed...

Below is run using 10MHz clock. I de-bounced my switch (used with pull-up resistor to revert the logic) and connected it to SN74LS93N (4 input NAND - Schmitt Trigger) - 3 pins are set HIGH and 1 is driven by button so when I press the button the output goes HIGH

Output of this gate is connected to MM74C74N (Dual D Flip Flop) - D-Flipflop - output(Q) is connected to second D-Flipflop Second Flip Flop Q' output are both connected to - dual Input NAND gate - output from this gate is what I am after(DTACK).

However when I press the button is stays HIGH for longer than 1 clock cycle as explained in John's video

Not sure what am I doing wrong...

P.S - I hope you can read my writing... I am not the best drawing schematics...

• I'm confused by what you're trying to do with those 2 pull-up/pull-down resistors in your hand-drawn schematic. What you have there does not correspond with what is drawn in the 'How To' image above. – brhans May 6 at 16:12
• In my TTL databook, the 74LS93 is a decade counter. The dual 4 input Schmidt NAND gate is a74LS13 - which are you really using? Your schematic shows a 560 Ohm resistor between Vcc and ground at the switch. If that resistor is intended to be a pull-up it needs to be on the other side of the switch - where it is now just wastes power. – Peter Bennett May 6 at 16:14
• Or just set the Trace-bit, which is meant for single stepping – pipe May 6 at 16:26
• @brhans - i didn't have a simple Schmitt Gate so I had to use NAND gate so I inverted the button logic. Pressed LOW – aberforth May 6 at 17:18
• @Peter Bennet - yes it is 74ls13 – aberforth May 6 at 17:18

• That is perfect! thanks a lot!. The only think I can't get is page 7 top left corner. Is that some kind of resistor on Chip? there is a 4.7k value so I guess it's 4700Ohm? – aberforth May 7 at 7:42