Figure B-1 N Radix 2 Fft Butterflies Memory Map - Motorola DSP56800 Manual

ASR A
MOVE
DO #N,END_DOE
MAC Y0,X0,A
MAC Y1,X0,A
ASL A
ASR A
MAC Y0,X0,A
MAC Y1,X0,A
END_DOE
;
;
B.1.6 N Radix 2 FFT Butterflies
This is a decimation in time (DIT), in-place algorithm. Figure B-1 gives a graphic overview and memory
map.
A
k
W
B
; Twiddle Factor Wk= wr + jwi =
; - saved on each pass
; xr = ar + wr * br - wi * bi
; xi = ai + wi * br + wr * bi
; yr = ar - wr * br + wi * bi = 2 * ar - xr
; yi = ai - wi * br -
opt cc
B-6
X:(R0)+,Y0
X:(R0)+N,Y1
Y1,X:(R0)+
A,X:(R0)+
X:(R0)+,Y0
X=A+BW
+
-
Y=A-BW
Figure B-1. N Radix 2 FFT Butterflies Memory Map
cos(2πk/N) +j sin(2πk/N)
wr * bi = 2 * ai - xi
DSP56800 Family Manual
X:(R3)+,X0 ; 1 1
; 1 1
; 2 3
X:(R3)+,X0 ; 1 1
; 1 1
X:(R3)+,X0 ; 1 1
; 1 1
X:(R3)+,X0 ; 1 1
X:(R3)+,X0 ; 1 1
_______
Total: 16 6N+11
r0,r2
k
r3,r1
r1
X0
bi br
k
A
yi/ai/yr/ar
X0=a2/2
Y0=wn-2
y1=wn-1
X0= b2/2
X0=b1/2
X memory
ar/xr
ai/xi
br/yr
bi/yi
cos(2πk/N)
-sin(2πk/N)
Y0 Y1
wr -wi
B
xi/ai/xr/ar
AA0079
pointed by R1