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AR# 15179

SYNPLIFY 7.x - Synplify reports that I have used "x out of 8" BUFGs (clocks) in a Virtex-II part (Virtex-II devices contain 16 BUFGs)


Keywords: Synplify, Virtex, II, BUFG, 8, 16

Urgency: Standard

General Description:
In the mapping section of the Synplify log file, the report states that there are a maximum of 8 global buffers (BUFG) for the Virtex-II. Is this correct?


This is a reporting error -- 16 BUFGs are available in a Virtex-II device; however, Synplify will only use up to 8 BUFGs.

To overcome the 8 BUFG limit, use the "xc_global_buffers" constraint in an .sdc file in your Synplify project. The following definition is from Synplify's online help information:

Attribute; Xilinx.
Controls the number of global buffers used in a design. The Synplify Pro synthesis tool automatically adds global buffers for clock nets with high fanout. Use this attribute to specify a maximum number of buffers and restrict the amount of global buffer resources added. Also, if there is a black box in the design that has global buffers, you can use xc_global_buffers to prevent the synthesis tool from inferring clock buffers and exceeding the number of global resources. You can only specify this attribute through a constraint file (cannot be specified in HDL source code).

.sdc File Syntax and Example

define_global_attribute xc_global_buffers {maximum}

For example:

define_global_attribute xc_global_buffers {3}
AR# 15179
Date 04/24/2007
Status Archive
Type General Article