Automatic and manual clock managing. Errors with ring counter but not with 4-bit counter

Question

I'm building an 8 bit computer and I've just developed the clock.

I used a NE555 (datasheet) with some other integrated (74 series) to manage it.

• The operator could stop signal from 555 (with a switch) and use it in "manual mode" using another switch.
• No clock signal should reach the output pin if clock enable pin is low.

To achieve this I useed:

• 74HC00 (NAND) to debounce manual switch
• 74HC00, 74HC08 (AND) to debounce auto witch and signal from 555
• 74LS32 (OR) to get the manual clock or the automatic clock (with 555)
• 74HC08 to get an high value on the output pin if both clock and clock enable are high.

I hope this diagram will be more clear than my words:

^{simulate this circuit – Schematic created using CircuitLab}

Now I'm testing it and what I got is really strange!

1. First I tested it with a 74HC161 (datasheet), a synchronous 4-bit counter: what I got is ok: I see 0000, 0001, 0010, ..., 1111 as expected.

2. Then I used a 74HC4017 (datasheet), a 10-bit ring counter with the clock inverted by a 74LS04 as it need to be in my computer:

   • Unexpectedly at "high" frequencies (2Hz), using the automatic mode, the counter becomes active with bot high-to-low and low-to-high tranistion. Using the manual this behavior never occurs.

   • Than if I switch off the automatic clock manager and put clock enable to ground the counter continue advancing the high bit. Note: manual clock is low.

   • On the other hand if I switch off 555 and I use only manual clock the integrated word fine.

So I checked the voltage value between OUT and ground and what I got is 0.07 V . It's a too small amount to be evaluated by the 744017, but it seems to evaluate it anyway.

How to repair my circuit? What is wrong with it?

Any idea/solution/hint will be appreciated.

Answer 1

I think you have issue with signal quality but you need oscilloscope to find that. However it's hard to see from the schematic how the 744017 is connected and what exactly is the chip - are there any letters in the name like HC and HCT (they are totally different!)?.

Few advices:

1. Connect Clock directly to the AND2 input (i.e. isolate entire man/auto part) and see what happens.
2. If this helps - the problem is with 'Automatic Clock Manager' part. Check joints / replace the chips.
3. If this doesn't help - either you are coupling incorrectly TTL and CMOS chips (are you using HC or HCT chip version?) either you have some noise on the signal lines and you need filtering capacitors (but you can see this only with oscilloscope).

Hope this helps. If 2 doesn't help and you fall in 3 - just provide more details on rest of the schematic. Read also this https://www.fairchildsemi.com/application-notes/AN/AN-368.pdf how to connect TTL to HC. (HCT input is directly compatible to the TTL output).