AR# 9018: 4.1i XC4000XL MAP - Using the STARTBUF component to drive non-reset type pins causes logic to be trimmed.
4.1i XC4000XL MAP - Using the STARTBUF component to drive non-reset type pins causes logic to be trimmed.
Keywords: STARTBUF, trim, reset, 4000, 4K, XL
General Description: When the STARTBUF is used in a design, much of the associated logic is trimmed by MAP. However, if the STARTBUF is replaced with a regular BUF, the logic is not trimmed.
The STARTBUF is used in 4KXL designs so that a user can use the dedicated GSR network to reset a design and also have the net available for simulation. During implementation, the input to the STARTBUF is routed to the on-chip STARTUP block. The net driven by STARTBUF is removed. Any components that are not directly driven by the dedicated GSR line will be left hanging and will be trimmed.
This commonly happens when a designer places an inverter between the STARTBUF and the CLR pin of a flip-flop -- the undriven CLR pin will cause the FF to be trimmed.