ifft.s

dsPIC30F_DSP算法库

;*********************************************************************
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;
; This Complex DIF IFFT implementation expects that the input data is a
; complex vector such that the magnitude of the real and imaginary parts
; of each of its elements is less than 0.5. If greater or equal to this
; value the results could produce saturation.
;
; Also, the program performs an implicit scaling of 1/2 for every stage
; to the intermediate values, so that the output values are scaled by a
; factor of 1/N, with N the length of the FFT (which is desired for the
; computation of the inverse transform).
;
; NOTE: input is expected in bit reverse ordering, while output is produced
; in natural (sequential) ordering.
;
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	; Local inclusions.
	.nolist
	.list

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	.section .libdsp, "x"

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;
; _IFFTComplexIP: Complex (in-place) DIF IFFT.
;
; Operation:
;	F(k) = 1/N*sum_n (f(n)*WN(-kn)), WN(-kn) = exp[(j*2*pi*k*n)/N],
;
; n in {0, 1,... , N-1}, and
; k in {0, 1,... , N-1}, with N = 2^m.
;
; Input:
;	w0 = number stages in FFT (log2NVal)
;	w1 = ptr to complex source vector (srcCV)
;	w2 = ptr to complex twiddle factors (twidFactors)
;	w3 = COEFFS_IN_DATA, or memory program page with twiddle factors.
; Return:
;	w0 = ptr to source vector (srcCV)
;
; NOTE: the complex source vector MUST be modulo 2*N word aligned for
;	the implicit bit reverse operation to succeed.
;
; System resources usage:
;	{w0,w3}		used, not restored
; plus resources from FFTComplexIP and BitReverseComplex.
;
; DO and REPEAT instruction usage.
;	no DO intructions
;	no REPEAT intructions
;
; Program words (24-bit instructions):
;	11
; plus program words from FFTComplexIP and BitReverseComplex.
;
; Cycles (including C-function call and return overheads):
;	15
; plus cycles from FFTComplexIP and BitReverseComplex.
; NOTE that the FFTComplexIP and BitReverseComplex source codes report
; the number of cycles including C-function call overhead.
; Thus, the number of actual cycles from FFTComplexIP to add to IFFTComplexIP
; is 4 less than whatever number of cycles it takes a stand alone call
; to FFTComplexIP. Analogously, the number of cycles from BitReverseComplex
; is 2 less than a stand alone call to BitReverseComplex.
;............................................................................

	; External symbols.
	.extern	_BitReverseComplex
	.extern	_FFTComplexIP

	.global	_IFFTComplexIP	; export
_IFFTComplexIP:

;............................................................................

	; Save working registers.
	; none to save...

;............................................................................

	push	w1				; save return value (srcCV)

;............................................................................

	; Compute IFFT using DIF FFT algorithm.
	push	w0				; save log2NVal
	push	w1				; save pointer to srcCV
	call _FFTComplexIP
	pop	w1				; restore pointer to srcCV
	pop	w0				; restore log2NVal

	; Finally, unscramble results back to natural order.
	call _BitReverseComplex

;............................................................................

	pop	w0				; restore return value

;............................................................................

	; Restore working registers.
	; none to restore...

;............................................................................

	return	

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	.end

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; OEF