Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>
Oh here we go ;-)
Ok, with the PRESET and CLEAR inputs, you always have to cover your
behind and toggle them, then you can leave them where you want them.
I usually use a VPULSE on each one with a delay of 1uS (usually plenty
of set up time) and going from a 0 to 1 or a 0 to 1 to 0 (1
representing 5V or whatever, and note the delay takes care of the
first 0 in the sequence). Try not to set both inputs at the same
time, either :P. I remember always having to "prime" my flip flop
simulations with a few uS of getting them all ready. If you dont do
this, you will either get X (dont care) outputs, or on occasion ive
had the flip flop behave strangely.
Its quite annoying to add all sorts of extra voltage pulses in the
simulation, but i dont know of any other work around (i wonder if
there is some "initial condition" for logic chips in pspice?).
Steve Ward
On 1/12/07, Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: Finn Hammer <f-h@xxxx>
Microsim/Orcad aficionados:
Attempting to create synchronization between. 2 digital signals, I
have started to test a 74HC109 flipflop against it`s truth table in
Orcad 9.2.
The first problem I stumble upon is that when I put a voltage pin on
Q and notQ, the probe program shows traces that are without a voltage
scale. I was expecting it to show 0V and 5V. Why is that so?
At times, it shows 2 red lines instead of a Low or a High, what
does that mean?
But worst of all, I cannot make it follow the truth table:
If I tie PRE and CLR to positive, then tying J high and K low should
make the outputs toggle by each clock transition.
They don`t. I get the dual red lines.
So I am stuck here, seeing little sense in progressing further before
the model conforms with the truth table.
Any help is appreciated.
If I configure it as a T type FF by tying J and K together it does
perform as it should, but that is not good enough.
The model I use is available for download here:
http:/home5.inet.tele.dk/f-hammer/flipflop.sch
Data sheet is here:
http://focus.ti.com/docs/prod/folders/print/sn74hc109.html
Cheers, Finn Hammer