Data Muxing on Spartan3 using the embedded carry chain

Hi All,
Does anyone know how to use the embedded carry chain for data muxing.
The Picoblaze docs state that it can use the MUXCY for this. I am not
sure how to use it for data muxing, if anyone knows, your help will be
greatly appreciated.
Thanks in advance
Sudhir

Sudhir.Singh@email.com wrote:

> Hi All,
> Does anyone know how to use the embedded carry chain for data muxing.
> The Picoblaze docs state that it can use the MUXCY for this. I am not
> sure how to use it for data muxing, if anyone knows, your help will be
> greatly appreciated.
> Thanks in advance
> Sudhir
> 

You use the carry chain as a wide OR gate and place an AND in the LUT at 
each bit.  The AND at each bit has to be fed the decoded select on one 
input and the input bit on the other input.  The muxcys get wired with a 
'1' tied to the DI input and the CI input of the first one in the chain 
is connected to '0'.  One carry chain per bit in your mux, one LUT per 
carry chain for each mux input.  If there are 3 or fewer bits in the 
select, you can do the decoding right in the LUT.  If more than 3 select 
bits, then you need an external decode or partial decode, which can be 
shared between mux bits (ie shared among several carry chains).