Mod 10 down resets at 8 and doesn't pass 7

[rowebil]

Can someone move this to the Multisim forum? I can only put it into Circuit Design Suite.

Anyhow, I am having issues with Multisim. I've made more complex circuits than this, but I somehow lost my memory and cannot remember this clearly. So I am going over Digital Electronics 1, so I am ready for Digital Electronics 2. 

I am making a mod 15 down counter. It will go from 15 to 0 easily, but I want it to go from 9-0, so the display will display 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 and then 'LOAD' at 9 and count down again.

Without the load, it would go from 15-0 and reset. 

So in class, we were told to NAND 15 (1 1 1 1) to load the ABCD presets, which I used a switch so it is customizable. I use 1001 for 9 to 0, but it is always resetting (load) at 7... So it counts 9, 8, 9, 8, 9, 8, 9, 8, and so forth.  

So it will count down to 0, and go back to 15, but at 15 it will flip 1111 to a low and activate the load and reset to 9. I just cannot get it past 8. I can switch the ABCD to 6, and it will work fine. It will go from 6 to 0 and then load back at 6, and repeat. When I try to 9 to 0, it just will not work. 

Any idea? Someone asked me (since I took the Multisim section of Digital Electronics 1 and he took the breadboard section) if he should use dipswitches and how the professor said no because of bounceback? She also told my partner to use one shot...but I can't remember if it was in place of the clock, OR the switches. I see people on Multisim using buttons in the place of my switch. Is that the problem? 

Also, my 7-segment display will not function correctly. 

[rowebil]

I cannot edit my message, so I apologize for the double post.

I theorized something a week ago. 

It is having trouble going from 1000 (8421) [8] to 0111 [7]

If you look at 8 and 7 and put them in place, you get 1111 (from 1000 and 0111)- so does the 74193 see 1111 when it switches from 1000 to 0111? 

I tried to reset at 14 and it still did not work... Again, it will count 6 to 0, but will not count 9 to 0.  I set it at 8, and it just remains there. Essentially it is just loading at 8. I set it at 7 and it counts 7-0... but at 8 or 9, it will not count past 8. 

Now when I AND 1111 (15) to CLR and have the NAND 1111 (15) to LOAD, it will reset to 0, wait 2 clock pulses, and then 7-0. So I finally got it to go under 8, but now it isn't displaying 9 and 8 (it displays 0 for 9 and 8). 

😞

[rowebil · Accepted solution]

So my professor asked me what was the model number for the NAND and I didn't have it. 

She told me to Google the model number and then use that in Multisim. 

What I did was I placed a NAND4 -- not a 7420N. 

I knew the NAND was messing with the 9 to 0 count. So we replaced it with a 7420N and it now counts 9 to 0. 

For the display, I did it correctly, and so did my classmates -- but the flaw is that the 7446 somehow doesn't work and displays the numbers opposite of what it shoud. So a ONE looks like a backwards three, because it turns on the LED's that should be off.

So we corrected this by replacing the component with a 7447. 

If I would have stuck with my instincts and tried to replace with a 7447, I would have finished this a week ago. However, the LAB handout specified using a 7446, so I didn't think it was an issue.

For anyone else have the issue with a MOD 10 count down with a NAND4, use a 7420N in replace of the NAND4. 

It is fixed.