What is a pin default value, and how does it work?

Description

The pin default value sets a default value of GND or V_CC to a input pin in a Text Design File (.tdf) or Graphic Design File (.gdf) subdesign. For a TDF, there is no assumed default. The user must specify a value of GND or V_CC to the pin in the SUBDESIGN declaration of the input:

SUBDESIGN top

(

 foo, bar, clk :    INPUT = VCC;

 a[7..0]         :    OUTPUT;

)

For a GDF, the pin default value is indicated on the input pin primitive graphic by GND or V_CC in the lower right corner of the input pin symbol. Each input pin is given a pin default value of either GND or V_CC.

For a GDF or TDF, the pin default value is used when an input port to a lower level file is left unconnected in an upper level instance. Default values are only recognized in lower-level files—they are ignored for top level files. If a pin in the top level design has a default of V_CC, it does not mean that if the device input pin is unconnected, it will default to V_CC. This only has an effect on a lower level design pin.

Here is an example of how it works:

1. Instantiate a DFF in a GDF and connect the D, CLK, and Q ports to input (default value = V_CC) and output pins respectively.
2. Compile the design and create a default symbol.
3. Instantiate the default symbol you created in a new GDF.
4. Connect the CLK port to an input pin, the Q port to an output pin, and leave the D port unconnected (no input pin).

When you compile and simulate the design, the design acts as if the D port is connected to V_CC even though there

is no "real" input to the design. In this case, the MAX PLUS^® II software uses the pin default value of the input pin

in the lower level of the design as input to the upper level design.

The pin default value does not work if it is placed on a pin attached to a device's physical pin.