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

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.