Article ID: 000081083 Content Type: Troubleshooting Last Reviewed: 09/11/2012

Can a single logic element (LE) implement both a registered and unregistered output in APEXTM devices? (APEX, QuartusTM)

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Description Yes. A single LE in APEX devices can drive an unregistered and registered output at the same time. For example, the following code uses only one LE even though there are two outputs:

input a,b;

output o1,o2;



reg o2;



assign o1 = a&b;



always@(posedge clock)

o2 = a&b;

To implement this code in only one LE, both outputs must have the same expression on the right hand side of the assignments (like a&b in this case). Additionally, turn the Registered Packing Option on in the Quartus software for this implementation.

However, if your design uses carry chains, then the number of LEs cannot be minimized this way. See the following code:

input [3:0] a,b;

output [3:0] outa,outb;

reg outb;



assign outa= a b;



always@(posedge clock)

outb = a   b;

Here, the design must use carry chains, and will be implemented in eight LEs and not four.

A future version of the Quartus software may allow register packing and carry chains to be implemented in the same LE.

Related Products

This article applies to 1 products

Apex™ 20K

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