Indexing the bits of an Integer?
I wanted to output a 1 or 0 signal
based on bit number 2 of a 4 bit counter. i.e. if bit 2 is high, set the line high and if it is low, set the line low. Using the below code where my_counter is a subtype of integer (4 bit counter). i.e. subtype counter_ty is integer range 0 to 15;  4 bit counter, variable my_counter : counter_ty := 0;  Next line compiles but will not sythesize some_signal <= conv_std_logic_vector(my_counter, 4)(2); The code compiles OK but refuses to synthesize with the message something like "complex indexes not supported". How can I get at the bit 2 of the counter so as to output a hi or low signal?? It looks like you cannot directly index the bit of an integer i.e. some_integer(2); Thanks Tak 
Re: Indexing the bits of an Integer?
assign it to a temp signal and then use bit2 of that signal.

Re: Indexing the bits of an Integer?
On Fri, 04 Mar 2005 05:07:14 GMT, "Takuon Soho" <Tak@somwhere.net>
wrote: >I wanted to output a 1 or 0 signal >based on bit number 2 of a 4 bit counter. >subtype counter_ty is integer range 0 to 15;  4 bit counter, >variable my_counter : counter_ty := 0; >How can I get at the bit 2 of the counter so as to output >a hi or low signal?? It looks like you cannot directly >index the bit of an integer i.e. some_integer(2); You can't. To extract a bit from something indexable, you have to use an indexable data type, such as one based on a vector of bits. Best is to include standard libraries std_logic_1164 and numeric_std, and to make "mycounter" of type unsigned. Then any ranges extracted from it are also unsigned; single bits will be of type std_logic. Arithmetic between unsigned (or signed) and integer works; and you can easily convert to the underlying std_logic_vector type whenever you need to.  Brian 
Re: Indexing the bits of an Integer?
OK, thanks.
I was trying to save space by using 4 bit integer subtype. Will use unsigned instead. Tak "Brian Drummond" <brian@shapes.demon.co.uk> wrote in message news:tugg21lsrri1or1qcho9ev6gibcv8fd41c@4ax.com... > On Fri, 04 Mar 2005 05:07:14 GMT, "Takuon Soho" <Tak@somwhere.net> > wrote: > >>I wanted to output a 1 or 0 signal >>based on bit number 2 of a 4 bit counter. > >>subtype counter_ty is integer range 0 to 15;  4 bit counter, >>variable my_counter : counter_ty := 0; > >>How can I get at the bit 2 of the counter so as to output >>a hi or low signal?? It looks like you cannot directly >>index the bit of an integer i.e. some_integer(2); > > You can't. To extract a bit from something indexable, you have to use an > indexable data type, such as one based on a vector of bits. > > Best is to include standard libraries std_logic_1164 and numeric_std, > and to make "mycounter" of type unsigned. Then any ranges extracted from > it are also unsigned; single bits will be of type std_logic. > > Arithmetic between unsigned (or signed) and integer works; and you can > easily convert to the underlying std_logic_vector type whenever you need > to. > >  Brian 
Re: Indexing the bits of an Integer?
There may sometimes be reasons for using an integer subtype as type for
a counter (indexing etc.). With a small function you can get the bit position you were looking for: Library IEEE; use IEEE.std_logic_1164.all; use IEEE.numeric_std.all; .. . . . sig1 <= get_bit(your_int, your_pos); .. . . . .. . . . function get_bit(iv : integer; pos; integer) return std_logic; .. . . . function get_bit(iv : integer; pos; integer) return std_logic is constant size_c : integer := 32;  An arbitrary value larger than  the highest bitpos you will  look for begin  assumes you count from 1 up to MSB if ((to_unsigned(iv, size_c) and to_unsigned(2 ** (pos 1), size_c)) = to_unsigned(0, size_c)) then return '0'; else return '1'; end if; end get_bit; Hope this helps, Charles 
Re: Indexing the bits of an Integer?
On Fri, 04 Mar 2005 15:29:38 GMT, "Takuon Soho" <Tak@somwhere.net>
wrote: >OK, thanks. > >I was trying to save space by using 4 bit integer subtype. > >Will use unsigned instead. That shouldn't save any space over using a 4bit unsigned. There may be a slight difference in simulation speed, but that shouldn't matter until you are doing really large simulations.  Brian 
Re: Indexing the bits of an Integer?
Thanks, this seems quite useful.
Tak "Charles Gardiner" <gardiner.charles@munich.de> wrote in message news:d0aiqn$kf$04$1@news.tonline.com... > There may sometimes be reasons for using an integer subtype as type for a > counter (indexing etc.). With a small function you can get the bit > position you were looking for: > > Library IEEE; > use IEEE.std_logic_1164.all; > use IEEE.numeric_std.all; > . . . . > sig1 <= get_bit(your_int, your_pos); > . . . . > . . . . > function get_bit(iv : integer; pos; integer) return std_logic; > . . . . > function get_bit(iv : integer; pos; integer) return std_logic is > constant size_c : integer := 32;  An arbitrary value larger than >  the highest bitpos you will >  look for > begin >  assumes you count from 1 up to MSB > if ((to_unsigned(iv, size_c) and > to_unsigned(2 ** (pos 1), size_c)) = > to_unsigned(0, size_c)) then > return '0'; > else > return '1'; > end if; > end get_bit; > > Hope this helps, > Charles 
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