Close

Control simulation

A project log for One-instruction TTL Computer

A breadboard-able computer which uses only a single instruction - MOVE

justin-davisJustin Davis 10/30/2017 at 12:580 Comments

I am never comfortable sending out a PCB without simulating first.  I decided I had to simulate a few components which I had not.  The control chip was one and it's a good thing I did.  I found I had a combinational loop that oscillated.  Spent a while, but got that fixed without adding more chips.

I also noticed in Ben Eater's videos that he drives a full bank of LEDs without current limiting resistors.  I figured I'd just do it.  And then the good engineer in me decided I need to double check it.  TTL does have an internal pull-up resistor, so it's possible to do it.  I measured the draw is 32mA and the datasheet maximum is 24mA.  I think I should probably test to failure - it might be at room temperature it can handle more.  Or I just put resistors on all 30+ LEDs which takes a lot more board space.

I also discovered some clock glitches while testing.  My clock looks like it bounces on the falling edge, and I only caught it when stepping my shift register one clock at a time - it moved twice for every push.  I still have the old wired-OR logic hooked up, so I need to put the TTL OR in place to see if that's the problem.  More work before ordering PCB...

Discussions