# Logic Quandary for analogue circuit

#### Stretchyman

Joined Sep 10, 2019
16
Thinking how best to describe a problem that I'm sure is simple to some but has confused the hell out of me!

OK, I have an analog circuit that I need to enable and bring up in a certain way.

So here's the truth table. Firstly there's an INIT and both inputs must be low to get an output.

Next the THEN phase when the TUNE input can change but the output must remain.

They must happen in this order. If the enable changes the output will (obviously) go low.

INIT

TUNE ENABLE OUTPUT

0 0 1

0 1 0

1 0 0

1 1 0

THEN

0 0 1

0 1 0

1 0 1

1 1 0

I've mused of logic combinations but cant suss it and I'm sure it's some kind of flip flop related cure.

Any ideas?

Regards

Stretchy.

#### WBahn

Joined Mar 31, 2012
26,398
It sounds like you have two different operating modes, so you need a signal that indicates which mode you are in. You might call that signal INIT and if it is HI you are in the initialization phase and if it is LO you are in the running phase.

What determines that you move from the INIT phase to the THEN phase?

#### Stretchyman

Joined Sep 10, 2019
16

re, What determines that you move from the INIT phase to the THEN phase?

Nothing other than it must start up with the inputs low.

One input is to ENABLE the circuit and the other is Hi/Lo power.

It must start up in Lo power mode, otherwise if there is a fault a great explosion may occur!

When both these conditions are met the circuit output is enabled and then it can be switched to Hi power.

So it's just a safety feature to stop the circuit being powered up in Hi power mode.

Hopefully that makes sense?

Regards

Stretchy.

#### djsfantasi

Joined Apr 11, 2010
7,206
The INIT phase function is a NOR gate

The THEN function can be expressed as A & NOT B.

But what do you mean by this statement?
Next the THEN phase when the TUNE input can change but the output must remain.
As WBahn noted, if you have a signal representing which mode you are in, you can use a couple of AND gates on the outputs of the two circuits above to select which circuit is being used. And then OR those two outputs to pass on the output.

#### Stretchyman

Joined Sep 10, 2019
16
The INIT phase function is a NOR gate

The THEN function can be expressed as A & NOT B.

But what do you mean by this statement?

As WBahn noted, if you have a signal representing which mode you are in, you can use a couple of AND gates on the outputs of the two circuits above to select which circuit is being used. And then OR those two outputs to pass on the output.

The circuits output can never go high unless the two inputs are low, once this condition is met the tune input can be in any state and the output will not change, i.e. remain high, it's that simple. Well simple to explain but not to implement!!

Str.

#### djsfantasi

Joined Apr 11, 2010
7,206
Why can’t you use the NOR output to latch an SR flip flop?

Of course, you’ll have to find a way to reset the latch/flip flop.

#### Stretchyman

Joined Sep 10, 2019
16
Why can’t you use the NOR output to latch an SR flip flop?

Of course, you’ll have to find a way to reset the latch/flip flop.
OK, could you draw it out for me as I'm not sure how it's implemented.

To reset the power will be turned off!

Regards

Stretchy

#### djsfantasi

Joined Apr 11, 2010
7,206
OK, could you draw it out for me as I'm not sure how it's implemented.

To reset the power will be turned off!

Regards

Stretchy
Sure... when I get home

#### Stretchyman

Joined Sep 10, 2019
16
Many thanks! I have a real problem with logic, I just don't get it! No problem with RF or anything wiggly. This will really sort out a problem that's been bugging me for far to long.

Regards.

Stretchy.

#### AnalogKid

Joined Aug 1, 2013
8,805
Assuming the three columns are a TUNE input, and ENABLE input, and the output, here is the logic I see.

After INIT:
The output is the NOR of the TUNE and ENABLE inputs.

After THEN:
Ignore the Tune input. The output is the Enable input inverted.

One quad NOR gate can implement both circuits, since a NOR gate can be used as an inverter. There is another approach that still uses only one quad NOR, but it depends on how the circuit sequences (which is not at all clear).

ak

Last edited:

#### Stretchyman

Joined Sep 10, 2019
16
Assuming the three columns are a TUNE input, and ENABLE input, and the output, here is the logic I see.

After INIT:
The output is the NOR of the TUNE and ENABLE inputs.

After THEN:
Ignore the Tune input. The output is the Enable input inverted.

ak

I know the logic, I need a circuit! Not quite as simple or understood as it looks but will need a latch of some kind I guess. I've designed a 500 W class E RF generator, PWM modulater etc etc but I can suss out a simple logic sequencer for my design. I'm baffled, I really am!

#### AnalogKid

Joined Aug 1, 2013
8,805
What governs the change from the init phase to the THEN phase? Are these two different logic structures that are selected between? What are the two inputs? TTL logic signals, sonar echoes, doorbell buttons? What does the output drive (voltage, current)? Missing a lot of info here.

The circuit is a picture of a NOR gate, but my guess is that you need more than that. For a functional circuit, you need to supply info like the operating voltage, input voltage ranges, output voltage range, output current, input frequencies.

Also, where are you located?

ak

Last edited:

#### AnalogKid

Joined Aug 1, 2013
8,805
Still not clear on how the circuit progresses.

The system powers up with the tune and enable inputs in one of four random combinations. Something (don't know, don't care) changes one or both inputs until they reach the 00 combination. At that time the output signal goes high. If we all agree on that part, then what? Is there a time delay before the THEN logic table is implemented? Is the fact that the inputs spent time at 00 to be latched, and the output of that latch enables the THEN logic circuit?

ak

#### Stretchyman

Joined Sep 10, 2019
16
Simpler than you think....

The inputs are switches.

No delays, other than the time to flip a switch.

The output will drive an npn transistor.

I'm in Bristol, UK if that helps?

Regards

Stretchy.

#### Stretchyman

Joined Sep 10, 2019
16
Is the fact that the inputs spent time at 00 to be latched, and the output of that latch enables the THEN logic circuit?

Essentially, Yes!

The input (switches) must both be low (grounded) for the output to latch and give a high to drive the npn tran (relay driver).

The tune input can then be switched from its initial low state to high with no change in output.

If the enable pin ever goes high the output will be disabled and then both switches must be set low to initialize the output.

I think that's it!

Stretchy.

#### AnalogKid

Joined Aug 1, 2013
8,805
So when the circuit is in the THEN mode, the Enable input resets it back to the INIT mode? yes / no ?

If yes, this was not even close to described or implied in any of the previous seven posts.

ak

#### Stretchyman

Joined Sep 10, 2019
16
I'm confused now, so....

Imagine a box, it has power supplied to it and has 2 switches on the front panel.

There's a light on top of the box.

ONLY when having applied power to said box the 2 switches are placed in the off position will the light come on.

If the switches are in any other configuration the light will stay off.

Having achieved the light on state one of the switches (TUNE) can be toggled in any position and the light will stay on.

However if the other switch (EN) is toggled to the off position the light will go off.

I cant describe it any simpler.

Regards

Stretchy.

#### AnalogKid

Joined Aug 1, 2013
8,805
I'm with you so far, but ...

Once the light is on, if the TUNE switch is turned off the light stays on.

Then, if the EN switch is then turned off, the light goes off

BUT, now both switches are off so the light should be on per the INIT truth table.

?

ak

#### Stretchyman

Joined Sep 10, 2019
16
If the EN ever goes high then the output will go off

Str

#### Stretchyman

Joined Sep 10, 2019
16
"However if the other switch (EN) is toggled to the off position the light will go off"

is an incorrect statement, sorry...

If the EN ever goes high then the output will go off

is correct..

Str