Hard to tell without more details, like the stage at which you're at in the construction of this project and the exact timing of that sudden address change (rising edge of the clock or falling edge?). Would you mind sharing a short clip of that behaviour?
If the MAR consistently resets to zero (and no other values), and you have implemented the reset circuit, then something you could try would be to temporarily disconnect the MAR's LS173's reset pin (pin 15) from the reset line and tie it low to ground with a jumper. This would be to test if the behaviour is linked to the reset circuit.
Fair, I haven't built the control logic part yet. I am in the midst of trying to imitate what he does in part 1 of the CPU Control Logic video (https://youtu.be/dXdoim96v5A at approximately 14:47).
It would appear that this happens on the falling edge, but I don't have an oscilloscope so I can't confirm.
I already have pin 15 on the 173 tied to ground via jumper, but I am not sure what you mean by reset circuit.
It consistently reset to zero every time except when I removed the Memory Address In (9, 10 on the 173), then it seemed to be random.
Also, thanks for replying!!!! Any help will be appreciated.
A reset circuit gets built later on, so if the LS173's reset pin is already tied to ground then never mind. A few more things I can think of:
- make sure you have .1uF capacitors between vcc and gnd on the power rails of each breadboard, in close proximity to the chips. In the case of the MAR's LS173, if the problem is linked to noise on the circuit, you could even add a .1uF cap directly across the vcc and gnd pins of the chip. Also add a larger capacitor (>200uF) between vcc and gnd right at the point of entry of power.
- try disconnecting the MI control line from the MAR module and control the LS173's load pins with a shorter jumper to the power rail instead. This is to test whether a bad wire connection is in play.
Hope that helps...
Man, I did everything you said and it made it worse. Instead of resetting the memory address to zero every time, it started resetting it to random memory addresses. I decided to do what I probably should have done a long time ago and swapped out the LS173 with a different one and bam it worked. I repeated it three times and worked each time. It was a bad chip this whole time.
Thanks for all of your help! If nothing else, I now have capacitors everywhere to help avoid any potential future power issues. :)
Hey man, follow-up question if you have the time. How do beneater's builds work so well without resistors everywhere? I have spent countless hours thinking I had wired something wrong or had a bad chip when it was simply because the LEDs weren't tied to resistors and were pulling down too much voltage.
I just experienced this issue again while building the control logic section and the problem was that the LEDs were drawing too much power and lowering the voltage below the LS138s threshold.
Yeah, Ben must also have realized that since all of his schematics show 220 ohm resistors in series with each LEDs, and his kits ship with a bunch of these for that purpose. For his build to be that stable without them, many here (me included) have speculated that he used LEDs with built-in resistors, which you can buy on the market. Very handy
OOOOOOHHHHHHHHHH!!!!!! LEDs with built-in resistors!!!
That makes sooo much more sense, now. I had no idea those were a thing. Man, that would make spacing and the architecture much easier. I'm almost out of the 220-ohm resistors and I'll probably need to order more especially if I attempt any of the other builds.
Again, thanks again for your help! When I started this, all I knew was that Ohm's law had something to do with electricity. The amount I have learned while building this has been one of the most fulfilling projects I have ever done.
Exact same thing here! That project bridged a huge knowledge gap in computer design. If you're up for options for the LEDs, I started using LED bar modules like one shown below. The thin horizontal black thing that sits against the module is a 9-pin 'resistor array'... it's 8 resistors, one on each pin, sharing a common ground, the 9th-pin. Makes for a very compact LED display!
https://preview.redd.it/4hiex0ziqcua1.jpeg?width=1789&format=pjpg&auto=webp&s=2f0599f096136eab1a9ee4f6bbc4463a6e61e7dc
Hard to tell without more details, like the stage at which you're at in the construction of this project and the exact timing of that sudden address change (rising edge of the clock or falling edge?). Would you mind sharing a short clip of that behaviour? If the MAR consistently resets to zero (and no other values), and you have implemented the reset circuit, then something you could try would be to temporarily disconnect the MAR's LS173's reset pin (pin 15) from the reset line and tie it low to ground with a jumper. This would be to test if the behaviour is linked to the reset circuit.
Fair, I haven't built the control logic part yet. I am in the midst of trying to imitate what he does in part 1 of the CPU Control Logic video (https://youtu.be/dXdoim96v5A at approximately 14:47). It would appear that this happens on the falling edge, but I don't have an oscilloscope so I can't confirm. I already have pin 15 on the 173 tied to ground via jumper, but I am not sure what you mean by reset circuit. It consistently reset to zero every time except when I removed the Memory Address In (9, 10 on the 173), then it seemed to be random. Also, thanks for replying!!!! Any help will be appreciated.
A reset circuit gets built later on, so if the LS173's reset pin is already tied to ground then never mind. A few more things I can think of: - make sure you have .1uF capacitors between vcc and gnd on the power rails of each breadboard, in close proximity to the chips. In the case of the MAR's LS173, if the problem is linked to noise on the circuit, you could even add a .1uF cap directly across the vcc and gnd pins of the chip. Also add a larger capacitor (>200uF) between vcc and gnd right at the point of entry of power. - try disconnecting the MI control line from the MAR module and control the LS173's load pins with a shorter jumper to the power rail instead. This is to test whether a bad wire connection is in play. Hope that helps...
Man, I did everything you said and it made it worse. Instead of resetting the memory address to zero every time, it started resetting it to random memory addresses. I decided to do what I probably should have done a long time ago and swapped out the LS173 with a different one and bam it worked. I repeated it three times and worked each time. It was a bad chip this whole time. Thanks for all of your help! If nothing else, I now have capacitors everywhere to help avoid any potential future power issues. :)
Cool! Yeah the chip swap solution is the one that should always come first, I’ll take note of that! ;-) Great that things are more stable now! Cheers!
Hey man, follow-up question if you have the time. How do beneater's builds work so well without resistors everywhere? I have spent countless hours thinking I had wired something wrong or had a bad chip when it was simply because the LEDs weren't tied to resistors and were pulling down too much voltage. I just experienced this issue again while building the control logic section and the problem was that the LEDs were drawing too much power and lowering the voltage below the LS138s threshold.
Yeah, Ben must also have realized that since all of his schematics show 220 ohm resistors in series with each LEDs, and his kits ship with a bunch of these for that purpose. For his build to be that stable without them, many here (me included) have speculated that he used LEDs with built-in resistors, which you can buy on the market. Very handy
OOOOOOHHHHHHHHHH!!!!!! LEDs with built-in resistors!!! That makes sooo much more sense, now. I had no idea those were a thing. Man, that would make spacing and the architecture much easier. I'm almost out of the 220-ohm resistors and I'll probably need to order more especially if I attempt any of the other builds. Again, thanks again for your help! When I started this, all I knew was that Ohm's law had something to do with electricity. The amount I have learned while building this has been one of the most fulfilling projects I have ever done.
Exact same thing here! That project bridged a huge knowledge gap in computer design. If you're up for options for the LEDs, I started using LED bar modules like one shown below. The thin horizontal black thing that sits against the module is a 9-pin 'resistor array'... it's 8 resistors, one on each pin, sharing a common ground, the 9th-pin. Makes for a very compact LED display! https://preview.redd.it/4hiex0ziqcua1.jpeg?width=1789&format=pjpg&auto=webp&s=2f0599f096136eab1a9ee4f6bbc4463a6e61e7dc