SN74HCS74: SPICE model issue

Hello-

I am trying to use Tina-TI to simulate the push button circuit shown in Figure 9-1 in the SN74HCS74 data sheet.  I believe that there is an error in the TI SPICE model for this part.

Based on the information in the data sheet, you would assume that all of the CMOS inputs are the same super high input impedance.  However it looks like the CLK input in the SPICE model has an input impedance of 5K ohms instead.

Here is a simple Tina-TI simulation schematic that I'm using that shows the issue:

Basically each input is just connected to VCC through an ammeter.  This example circuit has the voltage source VS1 set to 5V.

The PRE, CLR, and DATA inputs all read 50.21nA which is what I would expect.  With a 5V input, this corresponds to an input impedance on the order of 99.5M ohms which again is what I would expect.

However, the CLK input measures 1mA.  With a 5V input, that means that this CLK input looks like a 5K resistor which clearly must be incorrect.

This low input impedance on the CLK pin prevents the pushbutton circuit from simulating correctly unless you use really low value resistors (roughly 1K ohm or less) at the CLK input.

Note that there was a similar post on the E2E forum about 4 months ago titled "SN74HCS74: Device Power Button Circuit", that never seemed to be resolved but that thread is now closed.

Also note that the SN74HCS74 SPICE model does seem to behave correctly in all other respects.  You just need a "strong" signal to drive the CLK pin.

Thanks!

-Randy