Daku
Guest
Mon Jan 23, 2012 9:43 am
Please pardon me if this is a very stupid question. My Verilog
is rusty. Could some Verilog guru please explain what the
following means ? I have seen it before, and it seems very
complicated. Does it mean that if sum[25] == true, normalshift is
assigned 0, and so on down the chain.
Thanks for your help.
assign normalshift =
sum[25] ? 0 :
sum[24] ? 1 :
sum[23] ? 2 :
sum[22] ? 3 :
sum[21] ? 4 :
sum[20] ? 5 :
sum[19] ? 6 :
sum[18] ? 7 :
sum[17] ? 8 :
sum[16] ? 9 :
sum[15] ? 10 :
sum[14] ? 11 :
sum[13] ? 12 :
sum[12] ? 13 :
sum[11] ? 14 :
sum[10] ? 15 :
sum[9] ? 16 :
sum[8] ? 17 :
sum[7] ? 18 :
sum[6] ? 19 :
sum[5] ? 20 :
sum[4] ? 21 :
sum[3] ? 22 :
sum[2] ? 23 :
sum[1] ? 24 : 25;
glen herrmannsfeldt
Guest
Mon Jan 23, 2012 1:36 pm
Daku <dakupoto_at_gmail.com> wrote:
Quote:
Please pardon me if this is a very stupid question. My Verilog
is rusty. Could some Verilog guru please explain what the
following means ? I have seen it before, and it seems very
complicated. Does it mean that if sum[25] == true, normalshift is
assigned 0, and so on down the chain.
Thanks for your help.
Quote:
assign normalshift =
sum[25] ? 0 :
sum[24] ? 1 :
sum[23] ? 2 :
(snip)
This is the conditional operator that came from C. (At least that
is where I first knew it in this form.)
So, yes, if sum[25] is true the value is zero (the left side of the : ),
otherwise it has the value of the expression on the right side of the :.
Whether that is a good way to implement a priority encoder, I don't
know.
The logic, as written, would be very slow, propagating all the way
through in the low sum[], high return value cases. The tools are very
good at optimizing much logic, and reasonably likely this one, too.
Even so, I probably wouldn't write it this way.
-- glen
Gabor
Guest
Mon Jan 23, 2012 9:29 pm
glen herrmannsfeldt wrote:
Quote:
Daku <dakupoto_at_gmail.com> wrote:
Please pardon me if this is a very stupid question. My Verilog
is rusty. Could some Verilog guru please explain what the
following means ? I have seen it before, and it seems very
complicated. Does it mean that if sum[25] == true, normalshift is
assigned 0, and so on down the chain.
Thanks for your help.
assign normalshift =
sum[25] ? 0 :
sum[24] ? 1 :
sum[23] ? 2 :
(snip)
This is the conditional operator that came from C. (At least that
is where I first knew it in this form.)
So, yes, if sum[25] is true the value is zero (the left side of the : ),
otherwise it has the value of the expression on the right side of the :.
Whether that is a good way to implement a priority encoder, I don't
know.
The logic, as written, would be very slow, propagating all the way
through in the low sum[], high return value cases. The tools are very
good at optimizing much logic, and reasonably likely this one, too.
Even so, I probably wouldn't write it this way.
-- glen
Because the ? operator can be used in a continuous assignment it is
often used to generate multiplexers and encoders. The alternative
is to use a reg instead of a wire, and then in an always @* block
use a case statement. Normally I would not use the ? operator
for building very large encoders or multiplexers, however it is likely
that the synthesis tool will build the same hardware as if you
used a reg and a case statement.
I think this is more a matter of style than substance. Some people
like to keep combinatorial equations in continuous assigns rather
than using always @*, and may go to extreme lengths to make it happen
as you can see here. Still, once you know how it works, the code
is readable enough, I guess...
-- Gabor
