Re: The death of combinatorial logic.

Well, I'm not sure if it's okay to distribute the original PDF with
the circuit in it, but it's K179 Unipolar Steppermotor driver and the
site credit is kitsrus.com, so see if you can google it.

If I'm very much mistaken, the 4 power MOSFETS are fed directly from
the outputs of 2 RS flip flops. The FFs are updated in turn once every
clock cycle.

Each FF drives 2 MOSFETS, one connected to Q and the other connected
to Q-bar - (how *do* you type Q-bar?) so that when one MOSFET is on,
the other is off. But when the one is off, the other is on, so at all
times there are 2 MOSFETS on (one for each FF) and you're draining
power with the wheels locked up unless you use another FF with an
output enable line (which I haven't seen), or add some extra logic to
intercept the signals to the MOSFETS so you can turn them all off.

Now you could do that with a high current MOSFET attached in series
with the Motor Power line, but somehow that doesn't seem right. My
first inclination was to plop a set of 2 input AND gates between the
FF outputs and the gate of the MOSFETS, tying one input of each AND
gate together to create and ENABLE line. When this ENABLE line was
low, the signal from the FF would be blocked. Again, probably the
wrong thing to do (?) And since I was creating a composite circuit to
drive 2 stepper motors from the same board, that meant 2 AND chips.

Then I thought of using a tri-state buffer cause I've got piles of 'em
and who cares if I smoke them on the breadboard?

Then I looked at the pin count and thought "nup".

I dunno if my new design is any good but I'll test it out and see how
it goes.

On Sun, 30 Jul 2006 15:39:54 GMT, "Jason S. Mantor"
<jmantor@xxxxxxxxxxxx> wrote:

I'd be interested : ) also can you tell or show us newbies the
modifications to the circuit you mentioned?

Tim Polmear wrote:
Not wanting to spend $40 bucks a robot on UCN5804B controllers I
thought I'd make a controller with FETs and discrete logic, using the
circuit from kitsrus.com. It was only as I started laying out the
tracks for the AND gates to modify the circuit so that it could idle
without running power through the power FETs that I thought "This is
crazy, 6 ICs to do the job of 2. I can run these FETs straight from a
PIC".

So I ditched the XOR gates, the AND gates and the NAND gates with
Schmitt triggers and designed the circuit round a single PIC that I'll
get running with software.

Now what do I do with all my chips?


____________________________________________________
"I like to be organised. A place for everything. And everything all over the place."



____________________________________________________
"I like to be organised. A place for everything. And everything all over the place."
.

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