Answers Database

XABEL: How to assign set/reset preload values to registers in an FPGA


Record #1350

Product Family:  Software

Product Line:  Data I/O

Problem Title:
XABEL:  How to assign set/reset preload values to registers in an FPGA


Problem Description:

For an FPGA design, how can I control the preload value of
registers during powerup and Global Set/Reset from my ABEL
file?

Note that this solutions applies to FPGAs only.  For CPLDs,
refer to (Xilinx Solution 1349)


Solution 1:

To assign a preload value to a register, use the following
equation in the EQUATIONS section of the ABEL file:

<data>.ap = 0;	"For Preset (output high)

OR

<data>.ar = 0;	"For Reset (output low)

where <data> is the name of the register declared in the
DECLARATIONS section.

Note that only the XC4000 family actually has both a PRE
and a CLR pin on the registers.  Only one or the other of these
2 pins may be used for a given flip-flop, however.
The XNF file
which is generated by ABL2XNF will include a D-Flip-Flop, whose
INIT value is either 'R' for reset, or 'S' for set/preset.

This will ensure that the flip-flop is in the desired state
when the device is powered-up, and when the Global Set/Resest
is asserted.  Note:  to specify a specific signal as the GSR
signal, the STARTUP symbol must be placed on the schematic with
the desired GSR signal tied to the GSR pin on the STARTUP
block.	There is no way to put the STARTUP block into the ABEL
file.

For the other FPGA families, the registers only have a Clr pin,
and thus really only have the ability to preload to a Reset or
0 state.  However, if you do use the .ap extension shown above
to achieve a preload value of 1, ABL2XNF will simply invert the
input and output of the 'reset' flip-flop to emulate a 'preset'
flip-flop.

In either case, the CLR or PRE pin on the D-flip-flop will be
tied to ground.  XNFPREP will later trim out that signal to
ground, as it is unnecessary.



End of Record #1350