Re: [myhdl-list] fixbv to current version of myhdl

[Christopher F. <chr...@gm...>]

> These are the results I get
> python
> Python 2.7.3 (default, Sep 26 2013, 20:03:06)
> [GCC 4.6.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>  >>> from myhdl import *
>  >>> ww = (26,18)
>  >>> ca1 = fixbv(-1.586134342)[ww]
>  >>> x2 = fixbv(100.0)[ww]
>  >>> print ca1
> -1.585938
>  >>> print ca1*x2
> -158.593750

Edward,

The above results make sense, I need to look into
why the fixed -> float (for the print) gave a
slightly different value and if it is expected.

Explanation:

   >>> ww = (26,18) # 7 fractional bits
   >>> ca1 = fixbv(-1.586134342)[ww]
   >>> print('{}, {}, {}'.format(
             ca1, repr(ca1), bin(ca1, 26)))

   -1.585938,
   fixbv(-1.585938,  format=(26,18,7), ),
   11111111111111111100110101

 From the above we can see the complete word format

   >>> bv = intbv('11111111111111111100110101')[26:]
   >>> bin(~bv+1, 26)

   '00000000000000000011001011'

The fractional part is .1001011 which is:

   >>> 1/2 + 1/16 + 1/64 + 1/128
   0.5859375

Given this value, the actual fixed-point value the
100 * -1.5859375 = -158.59375 makes sense.

Hope that helps,
Chris