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AR# 1675

XC5200/XC4000E/EX/XL/XV: How flip-flop initial states are determined for FPGAs.

Description


4000, 4000E, 4000EX, XC4000XL, XC4000XV. devices only:

The initial state of a CLB Flip Flop after powerup, and after

deactivating the Global Set/Reset signal, is determined by the

library symbol that is used to instantiate the flip flop.



5200 devices:

The initial state of CLB flip flops after powerup, and after

deactivating the Global Reset signal, is always reset, but it

is possible to use a library macro symbol which inverts the input

and output of the flop so that the flop (with inverter logic)

acts as asynchronously set flip flop after powerup and after

deactivating the Global Reset signal.

解决方案


4000, 4000E, 4000EX devices:



There are two CLB flip flop primitives the FDCE and FDPE,

all other CLB flip flops are macros which utilize one of these

flip flop primitatives at a lower level.

All CLB flops that use the FDCE primitive will be in a

reset (0) state after powerup and after deactivating the Global

Set/Reset signal.

All CLB flops that use the FDPE primitive will be in a

set (1) state after powerup and after deactivating the Global

Set/Reset signal.



--------------------------

5200 devices only:

The 5200 devices behave just like the 4000 family of devices,

the only real difference is that the FDPE in the 5200 family is

a macro, which utilizes a FDCE and two inverters. Therefore

eventhough all flip flops in the 5200 devices are

asynchronously reset flops, you can get a set

behavior by utilizing a macro that uses the FDPE library symbol.



Xilinx has several "Bussed" Flip Flop macros, but all of

these macros will come up in a predefined state of all reset or

all set (the state is depenedent on the primitive flip flop

that the macro uses).



In order to get a "bussed" flip flop macro to come up in a

specific state, other than all set or all reset, the user will

need to create there own macro. When creating the Flip Flop

macro follow the guidelines in the previous resolution (ie.

use a FDCE flop for all reset flops and a FDPE all set flops)
AR# 1675
创建日期 08/31/2007
Last Updated 01/08/2012
状态 Archive
Type 综合文章