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AR# 7337

2.1i 9500/XL: Hitop (CPLD Fitter) not fitting DFF--> INV --> OBUF correctly


Keywords: Hitop, D-FF, inv, asynchronous clear, obuf, fitter

Urgency: Hot

General Description:

The circuit in question is a D-FF with asynchronous clear, the
output of which is inverted at the pin. In addition the non-inverted
output signal (RAS) is fed back into the internal logic.

The effect of RESET is to clear the register, which due to the
inversion causes /RAS0 to be asserted (high). Thus the fitter
must convert the CLEAR into a PRESET when it moves the
inversion to the input side of the FF.

However, the report file shows the following equations for /RAS0:

/"/ras0" := /"B1/LBANK1" * "B1/STATE0" * "B1/STATE2" *

... snip...

"/ras0".RSTF = /"/reset" ;GSR
"/ras0".TRST = OUTENB ;FOE
"/ras0".PRLD = GND

These equations indicate that the output "/ras0" will be _cleared_ (low) when
the reset signal is asserted (the .PRLD should also be high (VCC)).

A similarly implemented signal is correctly reported as:
/"/pciack" := "/ms0" * /"B1/BUSY" * /"B1/RFSH" * /"B1/STATE2" *
... snip ...
"/pciack".CLKF = CLOCK ;FCLK/GCK
"/pciack".SETF = /"/reset" ;GSR
"/pciack".TRST = /"/pcireq"
"/pciack".PRLD = VCC

Note that .SETF is used for RESET and .PRLD is high (VCC) as expected.

The bug will be triggered every time a register with a set/rst signal drives an
inverter which in turn drives an OBUFT. In this case the software will not
swap the set/rst when the inverter is swallowed by the register.


This problem has been fixed in the latest 2.1i Service Pack
available at: http://support.xilinx.com/support/techsup/sw_updates
AR# 7337
Date Created 08/23/1999
Last Updated 08/27/2001
Status Archive
Type General Article