Re: [myhdl-list] intbv with max != 2**n. Error or annoyance?

[Per K. <bas...@gm...>]

Hi!
Over the weekend I have come to the conclusion that it is an error.

Checking the value of a combinatorical net before it has settled has to be
wrong. So depending on implementation feasibility and in order of
preference we could:

- Wait until the net has stabilized before checking the range, or
- Disable range checks for combinatorical nets, or
- Assert whenever a signal with range delimiters != 2**n is used as a
combinatorical net.

What do you say guys?
/Per




On Fri, Jan 25, 2013 at 1:08 PM, Per Karlsson <bas...@gm...>wrote:

> Hi!
> Here is something that I can't make up my mind if it is an error or just
> plain annoying:
>
> I have a signal where the max value is not an even 2**n, and I
> combinatorically assign it the value of a signal with a higher max value,
> but only on the condition that the value is within range. This is fine.
>
> But if the condition is moved to a separate @always_comb driving a flag
> and the flag gates the assignment, then there is a ValueError. The
> generated verilog is fine of course.
> /Per
>
> ps. Here is an example:
>
> from myhdl import *
>
> def range_check(capped, clk, rstn, cnt_max):
>     cnt       = Signal(intbv(0, min=0, max=cnt_max))
>     less_flag = Signal(intbv(0)[1:0])
>
>     cnt_max      = cnt.max
>     capped_max   = capped.max
>     capped_width = len(capped)
>
>     @always(clk.posedge, rstn.negedge)
>     def count():
>         if rstn==0:
>             cnt.next = 0
>         else:
>             if cnt + 1 == cnt_max:
>                 cnt.next = 0
>             else:
>                 cnt.next = cnt + 1
>
>     @always_comb
>     def setcapped():
>         if less_flag==1:
>             capped.next = cnt[capped_width:]
>         else:
>             capped.next = 0
>
>     @always_comb
>     def setless():
>         if cnt < capped_max:
>             less_flag.next = 1
>         else:
>             less_flag.next = 0
>
>     return instances()
>
> def tb(capped_max):
>     clk   = Signal(intbv(0)[1:0])
>     rstn  = Signal(intbv(0)[1:0])
>
>     @instance
>     def rstngen():
>         rstn.next = 0
>         yield delay(100)
>         rstn.next = 1
>         yield delay(10000)
>         raise StopSimulation
>
>     @always(delay(10))
>     def clkgen():
>         clk.next = not clk
>
>     capped  = Signal(intbv(0, min=0, max=capped_max))
>     cnt_max = 15
>     dut   = range_check(capped, clk, rstn, cnt_max)
>     return instances()
>
> s = Simulation(tb(4))
> s.run()
> print "4 is OK"
> s = Simulation(tb(5))
> s.run()
> print "5 is not"
>
>