PDSP1601 datasheet(13/18 Pages) ZARLINK

13 / 18 page

PDSP16116/A/MC

The butterfly operation

The butterfly operation is the arithmetic operation which is

repeated many times to produce an FFT. The PDSP16116A

based butterfly processor performs this operation in a low

power high accuracy chip set.

A new butterfly operation is commenced each cycle,

requiring a new set of data for , B, W, WTA and WTB. Five

cycles later, the corresponding results A' and B' are produced

along with their associated WTOUT. In between, the signals

SFTA and SFTR are produced and acted upon by the shifters

in the PDSP1601/A and PDSP16318/A. The timing of the data

and control signals is shown in Fig.6.

The results (A' and B') of each butterfly calculation in a

pass must be stored away to be used later as the input data

(A and B) in the next pass. Each result must be stored together

with its associated word tag, WTOUT. Although WTOUT is

common to both A' and B', it must be stored separately with

each word as the words are used on different cycles during the

next pass. At the inputs, the word tag associated with the A

word is known as WTA and the word tag associated with the

B word is known as WTB. Hence, the WTOUTs from one pass

will become the WTAs and WTBs for the following pass. It

should be noted that the first pass is unique in that word tags

need not be input into the butterfly as all data initially has the

same weighting. Hence, during the first pass alone, the inputs

WTA and WTB are ignored.

Figure 6 - Butterfly Operation

BB'

A' = A + B. W

B' = A - B. W

Figre 7 Butterfly Data and Control Signals

;;;;;

n-2

n-1

n+1

n+2

n+3

;;;;

n+1

n+2

n+3

n+4

n+5

;;;;

n+1

n+2

n+3

n+4

n+5

;;;;

n+1

n+2

n+3

n+4

n+5

;;;;

n-3

n-2

n-1

n+1

n+2

;;;;

n-2

n-1

n+1

n+2

;;;;

n-3

n-2

n-1

n+1

n+2

;;;;;

n-5

n-4

n-3

n-2

n-1

;;;;;

n-5

n-4

n-3

n-2

n-1

CLK

Present Br, Bi,

Wr, Wi to inputs

Present WTA,

WTB to inputs

Present Ar,

Ai to inputs

Output SFTA

Output SFTR

Output Pr, Pi

Output DAr, DAi

Output WTOUT

Output A'r, A'i, B'r, B'i

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