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Forums > Digital IC > Synthesis and test

Subject: Driving not connected bus bits
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Stalker
Posts: 24
Online: User is Offline
12/21/2006 7:14 AM  
Hello to all RC users,
I have some hierarchical ports of subdesigns, which are currently unused.
The are not driven inside the subdesign and not connected at the higher level.
RC decided to drive them with 1'b0 - assign output_pinΎ] 1'b0
There is an attribute, that is responsible for such behaviour:
rc:/> get_attr hdl_undriven_signal_value /
none
The options are 1, 0, Z, none.
It seems that RC ignores this attribute.

Any suggestion, how to eliminate driving of the unconnected bit would be appresiated.

Regards,
Maxim R.
sthomson
Posts: 1
Online: User is Offline
1/18/2007 12:44 PM  
There are 2 attributes that control the connectivity of undriven logic. The first that you mentioned is hdl_undriven_signal_value. This attribute controls the drive value of the undriven signals in a module.

attribute name: hdl_undriven_signal_value
category: elab (controls elaboration)
object type: root
access type: read-write
data type: string
default value: none
help: Connects each undriven signal in a module to the specified value unless the none value is specified.
more help: permitted values: { 0 1 x X z Z none }


Another attribute, hdl_undriven_output_port_value, controls how undriven output ports will be handled.


attribute name: hdl_undriven_output_port_value
category: elab (controls elaboration)
object type: root
access type: read-write
data type: string
default value: none
help: Connects each undriven output port in a module to the specified value unless the none value is specified.
more help: permitted values: { 0 1 x X z Z none }


Both of these attributes should be set before elaboration to achieve the desired effect.

There is another user utility that is available if you want to delete unused ports and subports.This command is called delete_unloaded_undriven and is only available after adding the following command to your script : "include load_etc.tcl"

rc:/> delete_unloaded_undriven -h

Usage: delete_unloaded_undriven [-disconnect] [-all] [-force_bit_blast] [] [> file]

[-disconnect]:
disconnects the constant connected subports/ports
[-all]:
deletes/disconnects in the top level also
[-force_bit_blast]:
removes indiviual bits of a bus by bitblasting the module
[]:
specify design name


This command and the attributes listed above should allow you to achieve the desired connectivity in your design.
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Forums > Digital IC > Synthesis and test > Driving not connected bus bits


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