colon expression to use integer operands
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The following code does not work because the colon operands are not integer values
stepSize = (pi/512);
thRadDbl = (0:stepSize:(2*pi - stepSize));
Question :How to rewrite the colon expression to use integer operands ?
采纳的回答
Andy Bartlett
2021-1-29
3 个投票
1)
For embedded designs, if at all possible consider changing your design to work with revolutions instead of radians.
1 revolution = 360 degrees = 2*pi radians
Many angle sensors are perfectly matched for recording angles in revolutions. For example, a spec. sheet may say that each angle sensor pulse equals 360 degrees / 4096. This is equivalent to 2^-12 revolutions per pulse. Equivalently, 4096 pulses equals one revolution. For this sensor, great data types for the angle would be
numerictype(0,12,12) % 0 to just under 1 revolution
numerictype(0,16,12) % 0 to just under 16 revolutions
Updating this angle would be lossless and would require a simple increment or add. In contrast, trying to record the angle in degrees or radians would always involve precision loss for representing 360/4096 or 2*pi/4096, and would require more math and bigger types.
To compute things like sine or cosine, it is generally useful to do a modulo first. A huge advantage to angles in revolutions with binary scaled fixed point is that modulo 1 revolution is lossless and trival. Just a simple masking operation to keep all the fraction bits and drop any integer bits. In contrast, attempting to do module 360 or 2*pi requires more costly calculations and can introduce precision losses.
2)
To create your evenspaced vector of n elements, covering the closed open interval [0, valueMax).
2a)
You gave an example of n = 1000.
If possible consider changing to an exact power of two like n = 1024 = 2^10. It will generally make the math nicer, and if using revolutions, then the math will be lossless too.
2b)
You need to pick one fixed-point data type for the entire vector. You cannot use different data types for each element of the vector. First determine, the level of accuracy required. For example, if the angle will be coming from the sensor example above with 4096 ticks per revolution, then a Fraction Length of 12 is a perfect choice. Based on the desired FractionLength, the rest of the data type attributes can be determined
2c)
The attached script
evenAngleVecCalc.m
shows how to set the output data type and compute the vector.
-Andy
6 个评论
Life is Wonderful
2021-2-1
编辑：Life is Wonderful
2021-2-1
Andy Bartlett
2021-2-1
Hi
Keep in mind that
sin
cordicsin
expect the input to be in radians, not revolutions.
I'm guessing you're trying to get baseline by calling sin(vec).
I suggest you create the baseline using doubles math rather than fixed-point math.
See the attached script.
The math will not be perfect. There will be some inaccuracies due to fixed-point approximations.
-Andy
Life is Wonderful
2021-2-2
Life is Wonderful
2021-2-2
编辑：Life is Wonderful
2021-2-2
Makes sense
For
cordicsine - Input in Radains
sin - Input in Radains ( corrected typo was degree)
- Please see my conversion in below figure and it makes sense to have have errror in fixed point stuff.
- Since cordicsin does contain smaller LookUp table but with bigger latency /computational , I wanted to make comparision and please see below plot.
- Please comment, Can I use cordicsin instead of sin function ?
Walter Roberson
2021-2-2
sind() would expect input in degrees. sin() would expect input in radians.
Life is Wonderful
2021-2-2
编辑：Life is Wonderful
2021-2-2
更多回答（2 个）
Image Analyst
2021-1-28
0 个投票
Not true. That code runs fine.
Maybe you want to consider linspace though. Or if you're going to use it as indexes, then round the values.
Seems to work fine for me.
stepSize = (pi/512);
thRadDbl = (0:stepSize:(2*pi - stepSize));
thRadDbl(1:5)
There's no way you're going to be able to use thRadDbl as a vector of indices since arrays in MATLAB have neither an element 0 nor an element pi/512.
One thing you could do is to wait to multiply by pi until after you've constructed the vector.
step = (1/512);
v = pi*(0:step:(2-step));
v(1:5)
Or perhaps the sinpi and/or cospi functions would be of interest to you.
7 个评论
Life is Wonderful
2021-1-28
编辑：Image Analyst
2021-1-28
Life is Wonderful
2021-1-28
编辑：Image Analyst
2021-1-28
Image Analyst
2021-1-28
编辑：Image Analyst
2021-1-28
Both of your last two code snippets run without error. They do different things.
I don't know what variable type 'embedded.fi' is - there must be something you have not told us, like the entire error message. A complete error message will have additional information, like the actual line of code that threw the error as well as a traceback for all prior lines of code that you stepped into before you got to the line of code that threw the error. What is "init_data_fixpt Line: 13 Column: 15"?
Next time, include ALL the red text, not just some of it.
Your v will still be a floating point array with a starting value of 0.
FYI embedded.fi is a class in Fixed-Point Designer. Instances of that class are created using the fi function.
This seems to work when I tried it in release R2020b.
p = fi(pi)
step = (1/512);
v = 0:step:(2-step);
pv = p*v;
pv(1:5)
I don't know if this is exactly what you're looking for, but I don't work with Fixed-Point Designer really at all, mainly just occasionally reading its documentation to answer questions on Answers.
Life is Wonderful
2021-1-28
编辑：Life is Wonderful
2021-1-28
Steven Lord
2021-1-28
We've about exhausted my knowledge of the fi data type so I don't think I can provide any more assistance.
Image Analyst
2021-1-29
Call tech support.
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