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On 3/5/2016 10:20 PM, Daniel Kho wrote: > On Saturday, 5 March 2016 04:08:17 UTC+8, KJ wrote: >> On Friday, March 4, 2016 at 1:20:19 PM UTC-5, Weng Tianxiang wrote: >>> >>> I want to use the (log N) to be the width of a counter: >>> >>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a known largest number N >>> >> >> What exactly is the issue? ieee.math_real defines a log2 function that can be used to define the upper bound. Actually what you want is >> ceil(log2(N)) >> >> Kevin > > Yes, KJ is right. In code, this would look like (I use this very often in synthesizable code): > > signal cnt: u_unsigned(positive(ceil(log2(real(N))))-1 downto 0); Code like this is why people use Verilog. I'm just sayin'. ;) Oh, it's also wrong. -- RickArticle: 158676
On Sunday, 6 March 2016 11:56:54 UTC+8, rickman wrote: > On 3/5/2016 10:20 PM, Daniel Kho wrote: > > On Saturday, 5 March 2016 04:08:17 UTC+8, KJ wrote: > >> On Friday, March 4, 2016 at 1:20:19 PM UTC-5, Weng Tianxiang wrote: > >>> > >>> I want to use the (log N) to be the width of a counter: > >>> > >>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a known largest number N > >>> > >> > >> What exactly is the issue? ieee.math_real defines a log2 function that can be used to define the upper bound. Actually what you want is > >> ceil(log2(N)) > >> > >> Kevin > > > > Yes, KJ is right. In code, this would look like (I use this very often in synthesizable code): > > > > signal cnt: u_unsigned(positive(ceil(log2(real(N))))-1 downto 0); > > Code like this is why people use Verilog. I'm just sayin'. ;) > > Oh, it's also wrong. > > -- > > Rick Yes, but explicit type conversions are also one of VHDL's strengths. :) It's better to be explicit in your code when it comes to what types you're using, instead of the tool (or language) doing the conversions implicitly for you - and later on you find out that the implicitly conversions aren't what you want. - danArticle: 158677
On 3/5/2016 11:23 PM, Daniel Kho wrote: > On Sunday, 6 March 2016 11:56:54 UTC+8, rickman wrote: >> On 3/5/2016 10:20 PM, Daniel Kho wrote: >>> On Saturday, 5 March 2016 04:08:17 UTC+8, KJ wrote: >>>> On Friday, March 4, 2016 at 1:20:19 PM UTC-5, Weng Tianxiang wrote: >>>>> >>>>> I want to use the (log N) to be the width of a counter: >>>>> >>>>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a known largest number N >>>>> >>>> >>>> What exactly is the issue? ieee.math_real defines a log2 function that can be used to define the upper bound. Actually what you want is >>>> ceil(log2(N)) >>>> >>>> Kevin >>> >>> Yes, KJ is right. In code, this would look like (I use this very often in synthesizable code): >>> >>> signal cnt: u_unsigned(positive(ceil(log2(real(N))))-1 downto 0); >> >> Code like this is why people use Verilog. I'm just sayin'. ;) >> >> Oh, it's also wrong. >> >> -- >> >> Rick > > Yes, but explicit type conversions are also one of VHDL's strengths. :) > It's better to be explicit in your code when it comes to what types you're using, instead of the tool (or language) doing the conversions implicitly for you - and later on you find out that the implicitly conversions aren't what you want. Over the years I have lived on both sides of the fence. I program hardware in strongly typed VHDL but when it comes to software I prefer Forth which has no concept of type. People try to tell me how good strong typing is and I agree that it can catch problems early that would not be cause so easily later on, but the verbosity and complexity of some code is hard to accept. Others just side step that theoretical issues by pointing out the number of errors made which tools can't catch such as the one you wrote. You didn't even address that. Your code has a logical flaw that allocates one bit too few for any value of N that is a power of 2. I suppose the tools would flag that when you try to assign a vector of a different size to it... unless you make a similar mistake elsewhere in which case you won't find out until in simulation you exercise with the max value. If that doesn't happen you will have a very hard time trying to figure out why the design doesn't work correctly in the device. I'm not trying to knock your code. I'm trying to point out that strong typing can impact your coding and allow mistakes to be missed because of the verbosity. Much simpler would be to write a small function to calculate a vector index max value that would replace the whole positive(ceil(log2(real(N))))-1 thing. -- RickArticle: 158678
On Sunday, 6 March 2016 14:13:43 UTC+8, rickman wrote: > Your code has a logical flaw that allocates one bit too few for any > value of N that is a power of 2. I suppose the tools would flag that > when you try to assign a vector of a different size to it... unless you > make a similar mistake elsewhere in which case you won't find out until > in simulation you exercise with the max value. If that doesn't happen > you will have a very hard time trying to figure out why the design > doesn't work correctly in the device. Yes you're right there, for values of N that is a power of 2. My earlier code should have been: signal cnt: u_unsigned(positive(ceil(log2(real(N+1))))-1 downto 0); Personally I prefer the simpler approach of flog2(N) rather than clog2(N+1)-1 to get where this needs to go. flog2 is a very simple function to write. If you use floating point routines for log2 and ceiling you need to then convert to integer. Are there integer functions for these routines or do you need to write them? I guess that wouldn't make sense since log2 either returns a floating point number or does some form of truncation or rounding. Well, I don't mind writing (log2(N+1)-1), though yes, writing a function can make your life easier. - danielArticle: 158679
On 3/6/2016 2:26 AM, Daniel Kho wrote: > On Sunday, 6 March 2016 14:13:43 UTC+8, rickman wrote: >> Your code has a logical flaw that allocates one bit too few for any >> value of N that is a power of 2. I suppose the tools would flag that >> when you try to assign a vector of a different size to it... unless you >> make a similar mistake elsewhere in which case you won't find out until >> in simulation you exercise with the max value. If that doesn't happen >> you will have a very hard time trying to figure out why the design >> doesn't work correctly in the device. > > Yes you're right there, for values of N that is a power of 2. > My earlier code should have been: > signal cnt: u_unsigned(positive(ceil(log2(real(N+1))))-1 downto 0); > > Personally I prefer the simpler approach of flog2(N) rather than > clog2(N+1)-1 to get where this needs to go. flog2 is a very simple > function to write. If you use floating point routines for log2 and > ceiling you need to then convert to integer. Are there integer > functions for these routines or do you need to write them? I guess that > wouldn't make sense since log2 either returns a floating point number or > does some form of truncation or rounding. > > Well, I don't mind writing (log2(N+1)-1), though yes, writing a function can make your life easier. I hope I'm not belaboring the point too much, but it's not (log2(N+1)-1), it's (positive(ceil(log2(real(N+1))))-1) vs. flog2(N). I think I developed an aversion to the long windiness of VHDL when I was initially learning it and didn't fully understand the typing. Also it was a bit harder back then when "10011" would not be recognized as an SLV and had to be specified as SLV. Trying to convert types all the time in efficient ways was a PITA. So I learned to write my own conversions that did it in a minimum of layers. VHDL 2008 is better now with a lot of type issues gone. I still like to eliminate long function chains like the floating point calculation used here to count bits. -- RickArticle: 158680
On 3/4/2016 8:25 AM, Weng Tianxiang wrote: > Hi, > > I have a constant N = 27, how to define a counter whose width is big enough to hold integer 27? Have you considered this? signal Count : integer range 0 to N; -- RickArticle: 158681
On Friday, March 4, 2016 at 8:01:50 PM UTC-5, Weng Tianxiang wrote: > Here is the code: >=20 > -- =3D floor of log2(); log2(27) =3D 5. >=20 > function log2(integer: N) return integer is=20 > variable K : integer; > variable M : integer; -- =3D M mod 2 > begin > K :=3D 0; > M :=3D N; > loop1: while M /=3D 0 loop > M :=3D M mod 2; -- it cannot use M :=3D M srl 2, because N is an in= teger > K :=3D K+1; > end loop; >=20 > return K; > end log2; > M :=3D M mod 2 is not correct. You want to divide M by 2 (M :=3D M / 2) in= stead. > -- to be debugged Or, rather than reinventing and debugging the wheel, Google yourself over t= o http://www.vhdl.org/comp.lang.vhdl/FAQ1.html to find the solution. There= are two log2 algorithms implemented, one uses recursive calls to a log2 fu= nction, the other uses a while loop. Both claim to be synthesizable in wor= king with constants, the recursive version I can vouch works with signals a= s well (i.e. Log_Sig1 <=3D log2(Sig1);) Kevin JenningsArticle: 158682
On Friday, March 4, 2016 at 10:48:11 PM UTC-8, rickman wrote: > On 3/5/2016 12:23 AM, rickman wrote: > > On 3/4/2016 8:01 PM, Weng Tianxiang wrote: > >> On Friday, March 4, 2016 at 2:27:55 PM UTC-8, rickman wrote: > >>> On 3/4/2016 5:03 PM, Mark Curry wrote: > >>>> In article <nbcq1j$i1q$1@dont-email.me>, rickman <gnuarm@gmail.com> > >>>> wrote: > >>>>> On 3/4/2016 1:20 PM, Weng Tianxiang wrote: > >>>>>> On Friday, March 4, 2016 at 5:34:42 AM UTC-8, Gabor wrote: > >>>>>>> Weng Tianxiang wrote: > >>>>>>>> Hi, > >>>>>>>> > >>>>>>>> I have a constant N = 27, how to define a counter whose width is > >>>>>>>> big enough to hold integer 27? > >>>>>>>> > >>>>>>>> Or how to get a constant = log N? Where "log" is a logarithm > >>>>>>>> with base 2. > >>>>>>>> > >>>>>>>> Thank you. > >>>>>>>> > >>>>>>>> Weng > >>>>>>> > >>>>>>> The function you want is the ceiling of log base 2. In Verilog > >>>>>>> this is $clog2(). If you don't have a similar function it's quite > >>>>>>> easy to make, since it's generally accomplished by shifting the > >>>>>>> input value right until it becomes zero and counting the shifts > >>>>>>> required to get there. > >>>>>>> > >>>>>>> -- > >>>>>>> Gabor > >>>>>> > >>>>>> Hi Gabor, > >>>>>> > >>>>>> I want to use the (log N) to be the width of a counter: > >>>>>> > >>>>>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a > >>>>>> known largest number N > >>>>> > >>>>> I don't think you need a ceiling function. I think you need a floor > >>>>> function and add 1. log2(8) = 3.0, log2(9) = 3.17, both need 4 > >>>>> bits to > >>>>> represent them in binary. A ceiling function will return 3 for 8 > >>>>> and 4 > >>>>> for 9. So it would be > >>>> > >>>> ceiling(log2(8+1)) = 4; > >>>> > >>>> What's the trouble? > >>> > >>> This can work if you add the 1 first, but the number you want is 3. So > >>> you have to subtract 1 from this result. Which is simpler? > >>> > >>> Personally I prefer the simpler approach of flog2(N) rather than > >>> clog2(N+1)-1 to get where this needs to go. flog2 is a very simple > >>> function to write. If you use floating point routines for log2 and > >>> ceiling you need to then convert to integer. Are there integer > >>> functions for these routines or do you need to write them? I guess that > >>> wouldn't make sense since log2 either returns a floating point number or > >>> does some form of truncation or rounding. > >>> > >>> -- > >>> > >>> Rick > >> > >> Hi Gabor, KJ, Rich and Mark, > >> > >> After your posts, I realized that a user-defined function's returned > >> integer value can be used as boundary limit!!! Before the post I > >> didn't know it. > >> > >> So I decided to accept Gabor's method to write an integer function > >> log2(integer N) in my design so that it can be repeatedly used later > >> for my life. > >> > >> Thank you. > >> > >> Weng > >> > >> Here is the code: > >> > >> -- = floor of log2(); log2(27) = 5. > >> > >> function log2(integer: N) return integer is > >> variable K : integer; > >> variable M : integer; -- = M mod 2 > >> begin > >> K := 0; > >> M := N; > >> loop1: while M /= 0 loop > >> M := M mod 2; -- it cannot use M := M srl 2, because N is an > >> integer > >> K := K+1; > >> end loop; > >> > >> return K; > >> end log2; > >> > >> -- to be debugged > > > > I can't recall the last time I wrote even a simple program that was 100% > > correct the first time. I think when you debug your program it will > > need to be M := M / 2; > > > > You might want to make your comment, "it cannot use M := M srl 1, > > because N is an integer" > > Oh yeah, you also need to change "while M /= 0" to "while M > 1". > Otherwise you will get a return value of 1 for 1, 2 for 2, 3 for 4, 4 > for 8, etc, which are all 1 more than the correct value and not optimal > for your use. > > -- > > Rick Hi Rick, I just want to 1 for 1, 2 for 2, 3 for 4, 4 for 8, the bit count that is big enough to hold N. signal Counter : unsigned(log2(N)-1 downto 0); I don't want to skip "-1", because all unsigned() expressions in my design uses format name(xxx-1 downto 0); for all integer M mod 2 == M / 2, but latter is better than former. So actually my design has no error at all!!! WengArticle: 158683
On Monday, March 7, 2016 at 1:43:59 PM UTC-8, Weng Tianxiang wrote: > On Friday, March 4, 2016 at 10:48:11 PM UTC-8, rickman wrote: > > On 3/5/2016 12:23 AM, rickman wrote: > > > On 3/4/2016 8:01 PM, Weng Tianxiang wrote: > > >> On Friday, March 4, 2016 at 2:27:55 PM UTC-8, rickman wrote: > > >>> On 3/4/2016 5:03 PM, Mark Curry wrote: > > >>>> In article <nbcq1j$i1q$1@dont-email.me>, rickman <gnuarm@gmail.com> > > >>>> wrote: > > >>>>> On 3/4/2016 1:20 PM, Weng Tianxiang wrote: > > >>>>>> On Friday, March 4, 2016 at 5:34:42 AM UTC-8, Gabor wrote: > > >>>>>>> Weng Tianxiang wrote: > > >>>>>>>> Hi, > > >>>>>>>> > > >>>>>>>> I have a constant N = 27, how to define a counter whose width is > > >>>>>>>> big enough to hold integer 27? > > >>>>>>>> > > >>>>>>>> Or how to get a constant = log N? Where "log" is a logarithm > > >>>>>>>> with base 2. > > >>>>>>>> > > >>>>>>>> Thank you. > > >>>>>>>> > > >>>>>>>> Weng > > >>>>>>> > > >>>>>>> The function you want is the ceiling of log base 2. In Verilog > > >>>>>>> this is $clog2(). If you don't have a similar function it's quite > > >>>>>>> easy to make, since it's generally accomplished by shifting the > > >>>>>>> input value right until it becomes zero and counting the shifts > > >>>>>>> required to get there. > > >>>>>>> > > >>>>>>> -- > > >>>>>>> Gabor > > >>>>>> > > >>>>>> Hi Gabor, > > >>>>>> > > >>>>>> I want to use the (log N) to be the width of a counter: > > >>>>>> > > >>>>>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a > > >>>>>> known largest number N > > >>>>> > > >>>>> I don't think you need a ceiling function. I think you need a floor > > >>>>> function and add 1. log2(8) = 3.0, log2(9) = 3.17, both need 4 > > >>>>> bits to > > >>>>> represent them in binary. A ceiling function will return 3 for 8 > > >>>>> and 4 > > >>>>> for 9. So it would be > > >>>> > > >>>> ceiling(log2(8+1)) = 4; > > >>>> > > >>>> What's the trouble? > > >>> > > >>> This can work if you add the 1 first, but the number you want is 3. So > > >>> you have to subtract 1 from this result. Which is simpler? > > >>> > > >>> Personally I prefer the simpler approach of flog2(N) rather than > > >>> clog2(N+1)-1 to get where this needs to go. flog2 is a very simple > > >>> function to write. If you use floating point routines for log2 and > > >>> ceiling you need to then convert to integer. Are there integer > > >>> functions for these routines or do you need to write them? I guess that > > >>> wouldn't make sense since log2 either returns a floating point number or > > >>> does some form of truncation or rounding. > > >>> > > >>> -- > > >>> > > >>> Rick > > >> > > >> Hi Gabor, KJ, Rich and Mark, > > >> > > >> After your posts, I realized that a user-defined function's returned > > >> integer value can be used as boundary limit!!! Before the post I > > >> didn't know it. > > >> > > >> So I decided to accept Gabor's method to write an integer function > > >> log2(integer N) in my design so that it can be repeatedly used later > > >> for my life. > > >> > > >> Thank you. > > >> > > >> Weng > > >> > > >> Here is the code: > > >> > > >> -- = floor of log2(); log2(27) = 5. > > >> > > >> function log2(integer: N) return integer is > > >> variable K : integer; > > >> variable M : integer; -- = M mod 2 > > >> begin > > >> K := 0; > > >> M := N; > > >> loop1: while M /= 0 loop > > >> M := M mod 2; -- it cannot use M := M srl 2, because N is an > > >> integer > > >> K := K+1; > > >> end loop; > > >> > > >> return K; > > >> end log2; > > >> > > >> -- to be debugged > > > > > > I can't recall the last time I wrote even a simple program that was 100% > > > correct the first time. I think when you debug your program it will > > > need to be M := M / 2; > > > > > > You might want to make your comment, "it cannot use M := M srl 1, > > > because N is an integer" > > > > Oh yeah, you also need to change "while M /= 0" to "while M > 1". > > Otherwise you will get a return value of 1 for 1, 2 for 2, 3 for 4, 4 > > for 8, etc, which are all 1 more than the correct value and not optimal > > for your use. > > > > -- > > > > Rick > > Hi Rick, > I just want to 1 for 1, 2 for 2, 3 for 4, 4 for 8, the bit count that is big enough to hold N. > > signal Counter : unsigned(log2(N)-1 downto 0); > > I don't want to skip "-1", because all unsigned() expressions in my design uses format name(xxx-1 downto 0); > > for all integer M mod 2 == M / 2, but latter is better than former. > > So actually my design has no error at all!!! > > Weng Hi, Sorry, everyone who participate in this discussion. After finishing my code, I wrote log2() function and finally found that I didn't use it at all and actually use the following format before having read Rick suggestions. signal Count : integer range 0 to N; instead of signal Count : unsigned(log2(N)-1 downto 0); KJ: he says that "M := M mod 2;" is not equal to: "M := M/2;" I don't know why? Can you list a digital example to show your point? Thank you. WengArticle: 158684
Weng Tianxiang wrote: > KJ: he says that "M := M mod 2;" is not equal to: "M := M/2;" I don't know why? Can you list a digital example to show your point? > No example required; they're completely different operators. mod is the remainder. / is the quotient. That's like asking for an example to prove that / and + are different. -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix.Article: 158685
Weng Tianxiang wrote: > Hi, > > I have a constant N = 27, how to define a counter whose width is big > enough to hold integer 27? > > Or how to get a constant = log N? Where "log" is a logarithm with base 2. > > Thank you. > > Weng And, of course, in VHDL, you can say : signal blat : integer range 0 to 27; And VHDL figures out that you need 5 bits to represent that. You can use this in the port definitions between modules, too. Such as : entity blah is port ( state : out integer range 0 to 27 ); One thing you have to keep in mind is what the logic might do if some value comes in from outside which exceeds the declared range but fits in the number of bits required to represent that. I'm assuming that synthesis tools don't protect against that. JonArticle: 158686
On 3/7/2016 4:43 PM, Weng Tianxiang wrote: > On Friday, March 4, 2016 at 10:48:11 PM UTC-8, rickman wrote: >> On 3/5/2016 12:23 AM, rickman wrote: >>> On 3/4/2016 8:01 PM, Weng Tianxiang wrote: >>>> On Friday, March 4, 2016 at 2:27:55 PM UTC-8, rickman wrote: >>>>> On 3/4/2016 5:03 PM, Mark Curry wrote: >>>>>> In article <nbcq1j$i1q$1@dont-email.me>, rickman <gnuarm@gmail.com> >>>>>> wrote: >>>>>>> On 3/4/2016 1:20 PM, Weng Tianxiang wrote: >>>>>>>> On Friday, March 4, 2016 at 5:34:42 AM UTC-8, Gabor wrote: >>>>>>>>> Weng Tianxiang wrote: >>>>>>>>>> Hi, >>>>>>>>>> >>>>>>>>>> I have a constant N = 27, how to define a counter whose width is >>>>>>>>>> big enough to hold integer 27? >>>>>>>>>> >>>>>>>>>> Or how to get a constant = log N? Where "log" is a logarithm >>>>>>>>>> with base 2. >>>>>>>>>> >>>>>>>>>> Thank you. >>>>>>>>>> >>>>>>>>>> Weng >>>>>>>>> >>>>>>>>> The function you want is the ceiling of log base 2. In Verilog >>>>>>>>> this is $clog2(). If you don't have a similar function it's quite >>>>>>>>> easy to make, since it's generally accomplished by shifting the >>>>>>>>> input value right until it becomes zero and counting the shifts >>>>>>>>> required to get there. >>>>>>>>> >>>>>>>>> -- >>>>>>>>> Gabor >>>>>>>> >>>>>>>> Hi Gabor, >>>>>>>> >>>>>>>> I want to use the (log N) to be the width of a counter: >>>>>>>> >>>>>>>> signal Count : unsigned((log N)-1 downto 0); -- it can hold a >>>>>>>> known largest number N >>>>>>> >>>>>>> I don't think you need a ceiling function. I think you need a floor >>>>>>> function and add 1. log2(8) = 3.0, log2(9) = 3.17, both need 4 >>>>>>> bits to >>>>>>> represent them in binary. A ceiling function will return 3 for 8 >>>>>>> and 4 >>>>>>> for 9. So it would be >>>>>> >>>>>> ceiling(log2(8+1)) = 4; >>>>>> >>>>>> What's the trouble? >>>>> >>>>> This can work if you add the 1 first, but the number you want is 3. So >>>>> you have to subtract 1 from this result. Which is simpler? >>>>> >>>>> Personally I prefer the simpler approach of flog2(N) rather than >>>>> clog2(N+1)-1 to get where this needs to go. flog2 is a very simple >>>>> function to write. If you use floating point routines for log2 and >>>>> ceiling you need to then convert to integer. Are there integer >>>>> functions for these routines or do you need to write them? I guess that >>>>> wouldn't make sense since log2 either returns a floating point number or >>>>> does some form of truncation or rounding. >>>>> >>>>> -- >>>>> >>>>> Rick >>>> >>>> Hi Gabor, KJ, Rich and Mark, >>>> >>>> After your posts, I realized that a user-defined function's returned >>>> integer value can be used as boundary limit!!! Before the post I >>>> didn't know it. >>>> >>>> So I decided to accept Gabor's method to write an integer function >>>> log2(integer N) in my design so that it can be repeatedly used later >>>> for my life. >>>> >>>> Thank you. >>>> >>>> Weng >>>> >>>> Here is the code: >>>> >>>> -- = floor of log2(); log2(27) = 5. >>>> >>>> function log2(integer: N) return integer is >>>> variable K : integer; >>>> variable M : integer; -- = M mod 2 >>>> begin >>>> K := 0; >>>> M := N; >>>> loop1: while M /= 0 loop >>>> M := M mod 2; -- it cannot use M := M srl 2, because N is an >>>> integer >>>> K := K+1; >>>> end loop; >>>> >>>> return K; >>>> end log2; >>>> >>>> -- to be debugged >>> >>> I can't recall the last time I wrote even a simple program that was 100% >>> correct the first time. I think when you debug your program it will >>> need to be M := M / 2; >>> >>> You might want to make your comment, "it cannot use M := M srl 1, >>> because N is an integer" >> >> Oh yeah, you also need to change "while M /= 0" to "while M > 1". >> Otherwise you will get a return value of 1 for 1, 2 for 2, 3 for 4, 4 >> for 8, etc, which are all 1 more than the correct value and not optimal >> for your use. >> >> -- >> >> Rick > > Hi Rick, > I just want to 1 for 1, 2 for 2, 3 for 4, 4 for 8, the bit count that is big enough to hold N. > > signal Counter : unsigned(log2(N)-1 downto 0); > > I don't want to skip "-1", because all unsigned() expressions in my design uses format name(xxx-1 downto 0); > > for all integer M mod 2 == M / 2, but latter is better than former. > > So actually my design has no error at all!!! What???!!! You are thinking of operations on... no, that's not right.... I don't know what you are thinking of. You can define your function any way you want. You can redefine the mod operator to be the same as the divide operator. But if you do that I can't help you. Try coding this up and see if it works. -- RickArticle: 158687
On 3/7/2016 5:37 PM, Jon Elson wrote: > Weng Tianxiang wrote: > >> Hi, >> >> I have a constant N = 27, how to define a counter whose width is big >> enough to hold integer 27? >> >> Or how to get a constant = log N? Where "log" is a logarithm with base 2. >> >> Thank you. >> >> Weng > And, of course, in VHDL, you can say : > signal blat : integer range 0 to 27; > > And VHDL figures out that you need 5 bits to represent that. You can use > this in the port definitions between modules, too. Such as : > > entity blah is > port ( > state : out integer range 0 to 27 > ); > > One thing you have to keep in mind is what the logic might do if some value > comes in from outside which exceeds the declared range but fits in the > number of bits required to represent that. I'm assuming that synthesis > tools don't protect against that. No, but if you use the integer type simulation will catch it. Interesting point though. Something to be careful about in the system design. I seem to recall a rocket falling from the sky from a system design issue because of module reuse without catching an incompatible requirement. -- RickArticle: 158688
On Monday, March 7, 2016 at 4:43:59 PM UTC-5, Weng Tianxiang wrote: > I just want to 1 for 1, 2 for 2, 3 for 4, 4 for 8, the bit count that is big enough to hold N. > > signal Counter : unsigned(log2(N)-1 downto 0); > > I don't want to skip "-1", because all unsigned() expressions in my design uses format name(xxx-1 downto 0); > > for all integer M mod 2 == M / 2, but latter is better than former. > > So actually my design has no error at all!!! > There are three errors in your design, all of which have been pointed out: 1. ceil(log2()) is needed, not log2(). 2. To include N, you need to use ceil(log2(N+1))-1 down to 0, not ceil(log2(N))-1 downto 0. Example: N=8 will require four bits to represent 8, not 3 bits 3. M/2 is not equal to M mod 2 for integers. Example: int(5/2) = int(2.5) = 2; 5 mod 2 = 1. The mod operator gives you the remainder of the division, not the quotient Kevin JenningsArticle: 158689
On 3/7/2016 10:16 PM, KJ wrote: > On Monday, March 7, 2016 at 4:43:59 PM UTC-5, Weng Tianxiang wrote: >> I just want to 1 for 1, 2 for 2, 3 for 4, 4 for 8, the bit count that is big enough to hold N. >> >> signal Counter : unsigned(log2(N)-1 downto 0); >> >> I don't want to skip "-1", because all unsigned() expressions in my design uses format name(xxx-1 downto 0); >> >> for all integer M mod 2 == M / 2, but latter is better than former. >> >> So actually my design has no error at all!!! >> > > There are three errors in your design, all of which have been pointed out: > 1. ceil(log2()) is needed, not log2(). > 2. To include N, you need to use ceil(log2(N+1))-1 down to 0, not ceil(log2(N))-1 downto 0. Example: N=8 will require four bits to represent 8, not 3 bits > 3. M/2 is not equal to M mod 2 for integers. Example: int(5/2) = int(2.5) = 2; 5 mod 2 = 1. The mod operator gives you the remainder of the division, not the quotient Why does everyone keep saying to use the ceiling function of log2(), then they add 1 to N before computing it? Isn't it simpler to just bloody take the floor of log2() and skip adding the 1? Then Weng has a compulsion to calculate a result that he then has to subtract 1 from before using just because that is what he is used to seeing in array declarations. Maybe I've had too much caffeine, but all this seems a bit absurd. BTW, your first two points are based on using real functions, no? So it would need to be to_integer(ceil(log2(real(N+1))))? This really pushes it over the top for me. Weng is not using real routines, but is writing his own log2() function for an integer input and integer output. But in reality it is ceil_log2_of_N+1() as it returns a value 1 too large to be log2. -- RickArticle: 158690
On Monday, March 7, 2016 at 11:18:55 PM UTC-5, rickman wrote: > Isn't it simpler to just=20 > bloody take the floor of log2() and skip adding the 1? >=20 Different yes, not simpler or harder. Either way there is an addition and = a log function to perform. > BTW, your first two points are based on using real functions, no? =20 No they are not. > So it=20 > would need to be to_integer(ceil(log2(real(N+1))))? This really pushes= =20 > it over the top for me. >=20 What I use is the recursive version of the log2 function from the VHDL FAQ = that I posted earlier and I have a separate ceil_log2 function that returns= ceil(log2(N)). Both the ceil_log2 and the log2 functions have natural arg= uments and return natural. I use the log2 function from VHDL FAQ rather than ieee.math_real.log2 becau= se I want something that is synthesizable and works with signals not just f= or computing constants. y <=3D log2(x) can be used to calculate the most s= ignificant bit that is a 1 is one example where log2 can be used with signa= l I/O. KevinArticle: 158691
On 3/8/2016 7:11 AM, KJ wrote: > On Monday, March 7, 2016 at 11:18:55 PM UTC-5, rickman wrote: >> Isn't it simpler to just bloody take the floor of log2() and skip >> adding the 1? >> > Different yes, not simpler or harder. Either way there is an > addition and a log function to perform. I don't follow what you are saying. Using a floor_log2() function means you skip adding the 1 to the input parameter and also skip subtracting the 1 to use it in the range specification. The point is the log2() function is not really the correct function for calculating the range. So why adjust it three ways when you can adjust it just once? 1st adjustment, use ceiling function. Second adjustment, add 1 to the input value. Third adjustment, subtract 1 from the result to get the range high value. ---or--- Just use the floored log2() function which is actually the same as the integer version of log2(). So maybe that's no adjustment at all? >> BTW, your first two points are based on using real functions, no? > No they are not. > >> So it would need to be to_integer(ceil(log2(real(N+1))))? This >> really pushes it over the top for me. >> > What I use is the recursive version of the log2 function from the > VHDL FAQ that I posted earlier and I have a separate ceil_log2 > function that returns ceil(log2(N)). Both the ceil_log2 and the log2 > functions have natural arguments and return natural. I'd like to see your ceil_log2 function. Is it just log2() with a 1 subtracted from the input and a 1 added to result? > I use the log2 function from VHDL FAQ rather than ieee.math_real.log2 > because I want something that is synthesizable and works with signals > not just for computing constants. y <= log2(x) can be used to > calculate the most significant bit that is a 1 is one example where > log2 can be used with signal I/O. Yes, an integer log2 function has been discussed here, it just wasn't written 100% correctly. -- RickArticle: 158692
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On Tuesday, March 8, 2016 at 9:47:08 AM UTC-8, rickman wrote: > On 3/8/2016 7:11 AM, KJ wrote: > > On Monday, March 7, 2016 at 11:18:55 PM UTC-5, rickman wrote: > >> Isn't it simpler to just bloody take the floor of log2() and skip > >> adding the 1? > >> > > Different yes, not simpler or harder. Either way there is an > > addition and a log function to perform. > > I don't follow what you are saying. Using a floor_log2() function means > you skip adding the 1 to the input parameter and also skip subtracting > the 1 to use it in the range specification. The point is the log2() > function is not really the correct function for calculating the range. > So why adjust it three ways when you can adjust it just once? > > 1st adjustment, use ceiling function. Second adjustment, add 1 to the > input value. Third adjustment, subtract 1 from the result to get the > range high value. > > ---or--- > > Just use the floored log2() function which is actually the same as the > integer version of log2(). So maybe that's no adjustment at all? > > > >> BTW, your first two points are based on using real functions, no? > > No they are not. > > > >> So it would need to be to_integer(ceil(log2(real(N+1))))? This > >> really pushes it over the top for me. > >> > > What I use is the recursive version of the log2 function from the > > VHDL FAQ that I posted earlier and I have a separate ceil_log2 > > function that returns ceil(log2(N)). Both the ceil_log2 and the log2 > > functions have natural arguments and return natural. > > I'd like to see your ceil_log2 function. Is it just log2() with a 1 > subtracted from the input and a 1 added to result? > > > > I use the log2 function from VHDL FAQ rather than ieee.math_real.log2 > > because I want something that is synthesizable and works with signals > > not just for computing constants. y <= log2(x) can be used to > > calculate the most significant bit that is a 1 is one example where > > log2 can be used with signal I/O. > > Yes, an integer log2 function has been discussed here, it just wasn't > written 100% correctly. > > -- > > Rick Hi Rick and KJ, > Yes, an integer log2 function has been discussed here, it just wasn't > written 100% correctly. Your are right. The only error "M := M mod 2;" should be "M := M / 2;" KJ saying that it has 3 errors doesn't make sense. WengArticle: 158694
On Wednesday, March 9, 2016 at 7:26:39 AM UTC-8, Weng Tianxiang wrote: > On Tuesday, March 8, 2016 at 9:47:08 AM UTC-8, rickman wrote: > > On 3/8/2016 7:11 AM, KJ wrote: > > > On Monday, March 7, 2016 at 11:18:55 PM UTC-5, rickman wrote: > > >> Isn't it simpler to just bloody take the floor of log2() and skip > > >> adding the 1? > > >> > > > Different yes, not simpler or harder. Either way there is an > > > addition and a log function to perform. > > > > I don't follow what you are saying. Using a floor_log2() function means > > you skip adding the 1 to the input parameter and also skip subtracting > > the 1 to use it in the range specification. The point is the log2() > > function is not really the correct function for calculating the range. > > So why adjust it three ways when you can adjust it just once? > > > > 1st adjustment, use ceiling function. Second adjustment, add 1 to the > > input value. Third adjustment, subtract 1 from the result to get the > > range high value. > > > > ---or--- > > > > Just use the floored log2() function which is actually the same as the > > integer version of log2(). So maybe that's no adjustment at all? > > > > > > >> BTW, your first two points are based on using real functions, no? > > > No they are not. > > > > > >> So it would need to be to_integer(ceil(log2(real(N+1))))? This > > >> really pushes it over the top for me. > > >> > > > What I use is the recursive version of the log2 function from the > > > VHDL FAQ that I posted earlier and I have a separate ceil_log2 > > > function that returns ceil(log2(N)). Both the ceil_log2 and the log2 > > > functions have natural arguments and return natural. > > > > I'd like to see your ceil_log2 function. Is it just log2() with a 1 > > subtracted from the input and a 1 added to result? > > > > > > > I use the log2 function from VHDL FAQ rather than ieee.math_real.log2 > > > because I want something that is synthesizable and works with signals > > > not just for computing constants. y <= log2(x) can be used to > > > calculate the most significant bit that is a 1 is one example where > > > log2 can be used with signal I/O. > > > > Yes, an integer log2 function has been discussed here, it just wasn't > > written 100% correctly. > > > > -- > > > > Rick > > Hi Rick and KJ, > > > Yes, an integer log2 function has been discussed here, it just wasn't > > written 100% correctly. > > Your are right. The only error "M := M mod 2;" should be "M := M / 2;" > > KJ saying that it has 3 errors doesn't make sense. > > Weng Actually if i used integer_bit_width(N) instead of log2(N), there are no confusions here, no ceiling or floor. What I want is the bit width of an integer. My only error "M := M mod 2;" should be "M := M / 2;" for integer 8, return is 4; if 7, return is 3. Very simple. Of cause I learn 2 things from this post: 1. A constant uplimit can be generated by a function with constant input parameters. 2. A void use operator mod that is more complex than what you think. Thank you everyone. Weng The following first post by KJ causes Rick's doubt: What exactly is the issue? ieee.MATH_REAL defines a log2 function that can be used to define the upper bound. Actually what you want is ceil(log2(N)) Kevin I jusu ignored KJ' above info.Article: 158695
On 3/9/2016 12:11 PM, Weng Tianxiang wrote:
> On Wednesday, March 9, 2016 at 7:26:39 AM UTC-8, Weng Tianxiang wrote:
>> On Tuesday, March 8, 2016 at 9:47:08 AM UTC-8, rickman wrote:
>>> On 3/8/2016 7:11 AM, KJ wrote:
>>>> On Monday, March 7, 2016 at 11:18:55 PM UTC-5, rickman wrote:
>>>>> Isn't it simpler to just bloody take the floor of log2() and skip
>>>>> adding the 1?
>>>>>
>>>> Different yes, not simpler or harder. Either way there is an
>>>> addition and a log function to perform.
>>>
>>> I don't follow what you are saying. Using a floor_log2() function means
>>> you skip adding the 1 to the input parameter and also skip subtracting
>>> the 1 to use it in the range specification. The point is the log2()
>>> function is not really the correct function for calculating the range.
>>> So why adjust it three ways when you can adjust it just once?
>>>
>>> 1st adjustment, use ceiling function. Second adjustment, add 1 to the
>>> input value. Third adjustment, subtract 1 from the result to get the
>>> range high value.
>>>
>>> ---or---
>>>
>>> Just use the floored log2() function which is actually the same as the
>>> integer version of log2(). So maybe that's no adjustment at all?
>>>
>>>
>>>>> BTW, your first two points are based on using real functions, no?
>>>> No they are not.
>>>>
>>>>> So it would need to be to_integer(ceil(log2(real(N+1))))? This
>>>>> really pushes it over the top for me.
>>>>>
>>>> What I use is the recursive version of the log2 function from the
>>>> VHDL FAQ that I posted earlier and I have a separate ceil_log2
>>>> function that returns ceil(log2(N)). Both the ceil_log2 and the log2
>>>> functions have natural arguments and return natural.
>>>
>>> I'd like to see your ceil_log2 function. Is it just log2() with a 1
>>> subtracted from the input and a 1 added to result?
>>>
>>>
>>>> I use the log2 function from VHDL FAQ rather than ieee.math_real.log2
>>>> because I want something that is synthesizable and works with signals
>>>> not just for computing constants. y <= log2(x) can be used to
>>>> calculate the most significant bit that is a 1 is one example where
>>>> log2 can be used with signal I/O.
>>>
>>> Yes, an integer log2 function has been discussed here, it just wasn't
>>> written 100% correctly.
>>>
>>> --
>>>
>>> Rick
>>
>> Hi Rick and KJ,
>>
>>> Yes, an integer log2 function has been discussed here, it just wasn't
>>> written 100% correctly.
>>
>> Your are right. The only error "M := M mod 2;" should be "M := M / 2;"
>>
>> KJ saying that it has 3 errors doesn't make sense.
>>
>> Weng
>
> Actually if i used integer_bit_width(N) instead of log2(N), there are no confusions here, no ceiling or floor.
>
> What I want is the bit width of an integer.
>
> My only error "M := M mod 2;" should be "M := M / 2;"
>
> for integer 8, return is 4; if 7, return is 3. Very simple.
>
> Of cause I learn 2 things from this post:
> 1. A constant uplimit can be generated by a function with constant input parameters.
>
> 2. A void use operator mod that is more complex than what you think.
>
> Thank you everyone.
>
> Weng
>
> The following first post by KJ causes Rick's doubt:
>
> What exactly is the issue? ieee.MATH_REAL defines a log2 function that can be used to define the upper bound. Actually what you want is
> ceil(log2(N))
>
> Kevin
>
> I jusu ignored KJ' above info.
Yes, you can invent an arbitrary function to return any value you want.
So why not write it as vector_high_bit and save the subtraction? It
then maps perfectly to the integer function log2(N) and you are done.
function vector_high_bit(integer: N) return integer is
begin
return log2(N);
end vector_high_bit;
Even that name it too wordy for me, but it helps to be descriptive. I
program software in Forth (very efficient and all about simplicity) and
hardware in VHDL (very wordy tending toward the complex). Sometimes the
contrast is very annoying.
--
Rick
Article: 158696I have published the usable version of my scripts for complex sources hierarchy management on the github: https://github.com/wzab/vextproj A copy of current version is also available in alt.sources: https://groups.google.com/forum/#!original/alt.sources/nPIluidTlMU/bv-oEzMrHgAJ With best regards, WojtekArticle: 158697
With Vivado 2015.4, I'm trying to add DDR3 memory to a Microblaze design. I= add the Memory Interface Generator to the block diagram, and double-click = on it to configure it. The windows that comes up says that "It is very impo= rtant that the correct Vivado Project Options are selected", that Design En= try is set to VERILOG (sic), and "If any of these options are incorrect, pl= ease click on 'Cancel', change the Vivado Project Options, and restart MIG.= However, the Vivado Project Options shows that my target language is VHDL= . What do I actually have to do to convince MIG that Design Entry is VHDL?Article: 158698
On Friday, February 26, 2016 at 2:28:12 PM UTC-7, Eric Smith wrote: > The Xilinx packaging documentation for the VQ44/VQG44 package (document P= K012, v1.2, dataed 2004-06-18) doesn't contain a recommended PCB footprint.= In attempting to find such a footprint, Google turned up what appears to b= e a copy of a Xilinx web page, apparently no longer available from the Xili= nx web site: >=20 > http://glacier.lbl.gov/gtp/DOM/dataSheets/Xilinx_pkg_info.htm >=20 > Does Xilinx actually still have this information in an current publicatio= n? In the description of the package footprint in my library I'd like to be= able to cite a current Xilinx document. Finally found the information. It's in Xilinx UG112 "Device Package User's = Guide".Article: 158699
We need to interface 8 to 16 embedded USB 2.0 devices to an FPGA. We could just use external ULPI transceivers but am wondering what FPGAs have transceivers that are USB 2.0 compatible and if the internal USB SIE gate count is low enough to make it cost effective to wire the USB devices directly to the FPGA. (Would save lots of pins ....) Any suggestions ?
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