SN74LV123A:

Hi,

I tried to simulate a circuit using SN74LV123A in orcad. The pspice model was downloaded from  TI site.

I am using two multivibrator models in a circuit.

The first multivibrator consist of:

A high to low pulse given at ANOT, B and CLR is HIGH.

According to our design, Rext = 200k and Cext = 100nF. But when I give these values and simulate, I am always getting a low value at Q (approx 52uV).

I am only getting a pulse at Q when Rext is given 200 MEG and Cext is given 10nF. And, with this combination of Rext and Cext, the width of the pulse should be 200MEG*10nF = 2s. Unfortunately,  I am only getting a pulse width of 32ms. Why is it so? Why I am not getting the correct logic of Rext and Cext? Is there any limitation in the model for Rext and Cext values?

Also, I have to use second same multivibrator in the same circuit with :

ANOT and CLR high and B is connected with Q of the first multivibrator

Rext = 100k and Cext = 100nF, and I am taking the output from QNOT.

And the output of QNOT should be high to low. But, I am sometimes getting a constant high voltage and sometimes getting a low to high pulse.

Why is it so?

Regards,

Sreelekshmi.