fftoop.s

dsPIC30F_DSP算法库

;*********************************************************************
;                                                                    *
;                       Software License Agreement                   *
;                                                                    *
;   The software supplied herewith by Microchip Technology           *
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;   is intended and supplied to you, the Company's customer,         *
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;                                                                    *
;   THIS SOFTWARE IS PROVIDED IN AN "AS IS" CONDITION.  NO           *
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;   BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND    *
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;   (c) Copyright 2003 Microchip Technology, All rights reserved.    *
;*********************************************************************

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;
; This Complex DIF FFT 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.
;
; NOTE: input is expected in natural ordering, and output is produced also
; in natural order.
;
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	; Local inclusions.
	.nolist
	.list

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	.section .libdsp, "x"

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;
; _FFTComplex: Complex DIF FFT.
;
; 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 destination vector (dstCV)
;	w2 = ptr to complex source vector (srcCV)
;	w3 = ptr to complex twiddle factors (twidFactors)
;	w4 = COEFFS_IN_DATA, or memory program page with twiddle factors.
; Return:
;	w0 = ptr to complex destination vector (dstCV)
;
; NOTE: the complex destination vector MUST be modulo 2*N word aligned for
;	the implicit bit reverse operation to succeed.
;
; System resources usage:
;	{w0..w4}	used, not restored
; plus system resources from VectorCopy, FFTComplexIP and BitReverseComplex.
;
; DO and REPEAT instruction usage.
;	no DO intructions
;	no REPEAT intructions
; plus DO/REPEAT usage from VectorCopy, FFTComplexIP and BitReverseComplex.
;
; Program words (24-bit instructions):
;	17
; plus words from VectorCopy, FFTComplexIP and BitReverseComplex.
;
; Cycles (including C-function call and return overheads):
;	23
; plus cycles from VectorCopy, FFTComplexIP and BitReverseComplex.
; NOTE that the VectorCopy, FFTComplexIP and BitReverseComplex source
; codes report the number of cycles including C-function call overhead.
; Thus, the number of actual cycles from VectorCopy to add to FFTComplex
; is 3 less than whatever number of cycles it takes a stand alone call
; to VectorCopy. Analogously, 4 cycles need to be discounted from the
; total number of cycles of a stand alone FFTComplexIP, and 2 from a
; call to BitReverseComplex.
;............................................................................

	; External symbols.
	.extern	_VectorCopy
	.extern	_FFTComplexIP
	.extern	_BitReverseComplex

	.global	_FFTComplex	; export
_FFTComplex:

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

	; Save context.
	push.d	w0				; {log2N,dstCV} to TOS
	push	w3				; {twidFactors} to TOS
	push	w4				; {twidPage} to TOS

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

	; Find number of real plus imaginary componets.
	mov	#0x2,w3				; to be shifted...
	sl	w3,w0,w0			; w0 = 2*N (2<<log2NVal)
						; w0 = N*sizeof(complex)

	; Copy (complex) source vector into (complex) destination vector.
	; w1,w2 already set to destination and source, respectively...
	call	_VectorCopy
						; now w0-> dstCV

	; Restore context.
	pop.d	w2				; {twidFactors,twidPage}
						; from TOS
	pop.d	w0				; {log2N,dstCV} from TOS

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

	; Save context.
	push	w0				; log2N to TOS

	; Apply in-place operation to destination vector.
	call	_FFTComplexIP
						; now w0-> dstCV
	mov	w0,w1				; w1-> dstCV

	; Restore context.
	pop	w0				; log2N from TOS

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

	; Bit reverse vector so that frequencies are in natural order.
	call _BitReverseComplex
						; now w0-> dstCV

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

	return	

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	.end

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; OEF