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How to make modgen always treat parallel transistors as m-factor

Grover

I'm using Modgen to create neat matched transistor structures, but there's one thing I don't understand about it. In our design kit, the transistors have no m-factor property, so I create multiples by specifying the instance names like "MN1<0:3>". Now when I add a pair of these to the Constraint Manager and run Modgen, then click on the Pattern Editor, the transistors will be displayed in any of three different ways:

A: MN0<0:3> (4)
B: MN1<0:3> (4)

or

A: MN0<0> (1)
B: MN0<1> (1)
C: MN0<2> (1)
D: MN0<3> (1)
E: MN1<0:3> (4)

or

A: MN0<0> (1)
B: MN0<1> (1)
C: MN0<2> (1)
D: MN0<3> (1)
E: MN1<0> (1)
F: MN1<1> (1)
G: MN1<2> (1)
H: MN1<3> (1)

Obviously, I want the first option, but often (usually?) I get the second or third. The only way to get rid of this is to completely delete all constraints, delete the offending transistors, place new ones with different names, and hope that this time it works correctly. Of course I can still make my matched layout if the transistors are not recognized as belonging together, but it's just extremely annoying to figure out what I should write down instead of ABBA-BAAB.

I noticed that there is an entry in Sourcelink that says that this should just work in 6.1.3.500.3 (I'm using 6.1.3.500.13), so could I be doing something wrong here?

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  • Grover

    Hi Quek,

    If I follow your exact steps, the modgen works as expected: I get the same results as shown in your picture.

    However, I've also tried this:

    1. Place two transistors MN<0:3> and wire them up like a differential pair
    2. Connect pins to all terminals
    3. Select both transistors and create a modgen
    4. The modgen appears, and works correctly: it shows MN0<0:3> as A and MN1<0:3> as B.
    5. Cancel modgen and delete the constraint
    6. Select the right transistor, including its wires and pins, and drag (stretch) it a bit to the right
    7. Select both transistors and create a modgen again
    8. The modgen appears, and shows an incorrect list, as shown in the picture.

    In a real design, I drag my transistors around all the time, so if the problem is caused by moving transistors, then this would explain why it went wrong in my design.

    [edit] Click the picture to show it entirely, the modgen interface is on the right and gets cut off in the forum display

Reply
  • Grover

    Hi Quek,

    If I follow your exact steps, the modgen works as expected: I get the same results as shown in your picture.

    However, I've also tried this:

    1. Place two transistors MN<0:3> and wire them up like a differential pair
    2. Connect pins to all terminals
    3. Select both transistors and create a modgen
    4. The modgen appears, and works correctly: it shows MN0<0:3> as A and MN1<0:3> as B.
    5. Cancel modgen and delete the constraint
    6. Select the right transistor, including its wires and pins, and drag (stretch) it a bit to the right
    7. Select both transistors and create a modgen again
    8. The modgen appears, and shows an incorrect list, as shown in the picture.

    In a real design, I drag my transistors around all the time, so if the problem is caused by moving transistors, then this would explain why it went wrong in my design.

    [edit] Click the picture to show it entirely, the modgen interface is on the right and gets cut off in the forum display

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