TMS320F28035: Why is there no IQ31 in the C2000 IQmath library?

Pierre Bouchet1

Hello everyone,

I am programming a F28035 using the IQmath library. When I look at the library header (IQmathLib.h with Revision 201) I see that the possible values for GLOBAL_Q are between 1 and 30. Why does'nt it go up to 31? And why are none of the functions defined for IQ31 ? For example there is:

no _IQ31toF
no _IQ31mpy
no _IQ31sinPU
no _IQ31mpyIQX
etc.

When I look at the datasheet of the library, it clearly shows the range for each IQ data type, but the one with the smallest range is _iq30 which goes from -2 to +1.999999999. What is the reason for not having an _iq31 type that would go from -1 to +0.999999999 ?

If anyone can answer these questions I would be very grateful.

Pierre

over 9 years ago

0 Vishal_Coelho over 9 years ago

TI__Mastermind 20850 points

Hi Pierre,

we are discussing this internally. But my guess would be IQ31 doesn't include the identity (1.0) and this makes it of very little use.

0 Pierre Bouchet1 over 9 years ago in reply to Vishal_Coelho

Expert 2615 points

Hi Vishal,

In the meantime I thought about it and I arrived at the same conclusion by considering the trigonometric functions. As you stressed out, an hypothetical IQ31 format wouldn't even be able to properly express, say, cos(0) or sin(pi/2).

Regards,

Pierre

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TMS320F28035: Why is there no IQ31 in the C2000 IQmath library?