AR# 4642


M1.5,bufg,vhdl: Logic trimmed when all buffers instantiated rather than placed by Express.


Keywords: F1.4, F1.5, M1.5, VHDL, clock, BUFG, instantiate

Urgency: Standard

General Description:
In Express 2.x, when the 'Do not insert I/O Pads' option is selected
(FPGA Express), or the 'Insert I/O pads' is de-selected (Foundation Express),
after instantiating clock buffers manually, the buffered clk signal (output of
a BUFGLS) produces the following message from the Mapper (in M1.5):

WARNING: basnu:111-logical net "clk_in" has no driver.

ERROR:baste:314 - NCD was not produced. All logic was removed from design.
This is usually due to having no input or output PAD connections in the
design and no nets or symbols marked as 'SAVE'. You can either add PADs or
'SAVE' attributes to the design, or run 'map -u' to disable logic trimming
in the mapper.

This causes all clocked logic to be trimmed.

The other nets, with instantiated I/O buffers do not generate warnings.



While synthesizing the HDL code, allow Express to insert I/O pads/buffers
(which is the default), even though all the I/O buffers have been instantiated
in the source HDL. The implementation works fine (additional buffers are not
incorrectly added).


For F1.4 (Express 2.03), there are 2 options:
1. Allow Express to 'insert pads' (keep 'Do not insert I/O pads' deselected). Express will produce warning messages such as:
Warning: Existing pad cell '...' is connected to the port '...' - no pads cells inserted at this port. (FE-PADMAP-1)

This warning can be ignored, since it is just notifying you that it already found a buffer for the port in question. No unrequired buffers will be inserted.

2. At the bottom of the Express netlist (design.xnf) there will be a list of SIG records. You can change the SIG record for the clock to an EXT record.
Then save the modified .xnf file and use it for implementation (M1).

Sig -> Ext modification:
Change the SIG record to an EXT record as shown below.
SIG, clk_in, PIN=clk_in

EXT, clk_in, I
AR# 4642
Date 04/03/2000
Status Archive
Type General Article
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