[Discuss-gnuradio] Help with Cordic.v

[Ronald Jetli]

Hi All,

I needed some help with Verilog Cordic code.(cordic.v and
cordic_stage.v) I do understand the Cordic Logic, but was having some
trouble with the pipelined code implementation of cordic.v . My
questions start with Q --.

In cordic.v:
------------------
16 bit vector xi, yi and zi are passed as input.  Then cordic
constants are pre-defined.

Q -- Not sure what "wire [bitwidth+1:0] xi_ext =
{{2{xi[bitwidth-1]}},xi};" statement is doing. Is it just normal
concatenation and assignment?

Now, @posedge  clock, if reset is set, then x0, y0, z0 is initialized
to '0'.  Next, when enable is set, z0 is assigned all bits except the
first two. And the first 2 bits are used for the case statement.

Q -- My understanding is that in this case statements, the first 2
bits are used to determine the quadrants and accordingly adjust the
quadrant (angle) for  xi_ext and yi_ext by 90 or 180 or 270 or 360
degree.. But I am not sure. Any comments ?

Then many instances of cordic_stage are defined and they are passed
input values x0, y0, z0 and constants c00 etc.

Q -- I am not sure what "#(bitwidth+2,zwidth-1,0)" in this instance
definition does. My understanding is that  # value is used to define
some delay.

In cordic_stage.v
-------------------------
if reset is set then xo, y0 and zo are set to 0. And when enable is
set, value of z is checked. My understanding of cordic logic is that,
if say that current angle is 45, and desired is 20, then subtract from
the current angle, and make corresponding adjustments on x axis and y
axis variables.

Q -- This statement checks if "z_is_pos" and does next operation
respectively. I am not sure what "xi -
{{shift+1{yi[bitwidth-1]}},yi[bitwidth-2:shift]}" is actually doing.
My understanding of cordic is that based on angle adjustments,
operations of multiples of 2 are carried out on x-axis and y-axis
variable. And {} statement are used for concatenation in verilog.

Again, in cordic.v
---------------------------
xo, yo and zo are in continuous assignment mode and are updated
everytime a x12, y12 or z12 value changes respectively.

Q – Not exactly sure how the exact desired angle is obtained in
cordic.v. The algorithm that I read, made continous adjustments on
'current angle' so that it converged to the desired angle. I don't see
that step happening in cordic.v

I am extremely sorry if I might have annoyed you by asking so many
questions. But, any help with the above questions will immensely help
me.

Thank you.

Ronald.