PDSP16116

8

MBFP

Mode select. When high, block floating point (BFP) mode is

selected. This allows the device to maintain the dynamic range

of the data using a series of word tags. This is especially useful

in FFT applications. When low, the chip operates in normal mode

for more general applications. This pin is intended to be tied

high or low, depending on application.

WTOUT1:0 (BFP Mode Only)

Word tag output. This tag records the weighting of the output

words from the current cycle relative to the current global

weighting register (see Table 6). It should be stored along with

the A

WTB, for each complex word during the next pass. In normal

mode, WTOUT1:0 are not used and should be left unconnected.

Weighting of the output relative to

the current global weighting register

WTOUT1:0

One less

The same

One more

Two more

00

01

10

11

Table 6 Word tag weightings

Function

WTB1:0

Shift complex product 1 place to the left

No shift applied

Shift complex product 1 place to the right

Shift complex product 2 places to the right

11

00

01

10

Table 7 Normal mode shift control

SFTA1:0 (BFP and Normal Modes)

In BFP mode, these signals act as the A-word shift control.

They allow shifting from one to four places to the right, (see

Table 8). Depending on the relative weightings of the A-words

and the complex product, the A-word may have to be shifted to

the right to ensure compatible weightings at the inputs to the

PDSP16318 complex accumulator. The two words must have

the same weighting if they are to be added.

In normal mode, SFTA0 performs a different function. If

WTB1:0 is set to implement a left shift, then overflow will occur

if the data is fully 32 bits wide. This pin is used to flag such an

overflow. SFTA1 is not used in normal mode.

Function

SFTA1:0

Shift A-word 1 place to the right

Shift A-word 2 places to the right

Shift A-word 3 places to the right

Shift A-word 4 places to the right

00

01

10

11

WTA1:0 (BFP Mode Only)

Word tag from the A-word. This word records the weighting

of the A-word relative to the global weighting register on the

previous pass. Although the A-word itself is not processed in the

PDSPl 6116, this information is required by the control logic for

the radix 2 butterfly FFT application. These inputs should be

tied low in normal mode.

WTB1:0 (BFP and Normal Modes)

In BFP mode, this is the word tag from the B-word. This is

operated in the same manner as WTA but for the B-word. The

value of the word tags are used to ensure that the binary weight-

ing of the A-word and the product of the complex multiplier are

the same at the inputs to the complex accumulator. Depending

on which word is the larger, the weighting adjustment is per-

formed using either the internal shifter or an external shifter con-

trolled by SFTA. The word tags are also used to maintain the

weighting of the final result to within plus two and minus one

binary points relative to the new GWR. (On the first pass all

word tags will be ignored). In normal mode. these inputs per-

form a different function. They directly control the internal shifter

at the output port as shown in Table 7.

Table 8 External A-word shift control

GWR4:0 (BFP Mode Only)

Contents of the global weighting register. The GWR stores

the weighting of the largest word present with respect to the

weighting of the original input words. Hence, if the contents of

the GWR are 00010, it indicates that the largest word currently

being processed has its binary point two bits to the right of the

original data at the start of the BFP calculations.

The contents of this register are updated at the end of each

pass, according to the largest value of WTOUT occuring during

that pass. For example, if WTOUT = 11, then GWR will be

increased by 2 (see Table 6). The GWR is presented in two’s

complement format. In normal mode, GWR4:0 are not used and

should be left unconnected.

Start of BFP. This input should be held low for the first cycle

of the first pass of the BFP calculations (see Fig.7). It serves to

reset the internal registers associated with BFP control. When

operating in normal mode this input should be tied low.

End of pass. This input should be held low for the last cycle

of each pass and for the lay time between passes. It instructs

the control logic to update the value of the global weighting reg-

ister and prepare the BFP circuitry for the next pass. When op-

erating in normal mode this input should be tied low.

AR15:13 (BFP Mode Only)

Three MSBs of the real part of the A-word. These are used

in the FFT butterfly application (see Fig. 4) to determine the

magnitude of the real part of the A-word and, hence, to deter-

mine if there will be any change of word growth in the

PDSP16318 Complex Accumulator. When operating in normal

mode, these inputs are not used and may be tied low.

AI15:13 (BFP Mode Only)

Three MSBs of the imaginary part of the A-word. Used in the

same fashion as AR15:13.

SFTR2:0 (BFP Mode Only) Accumulator result shift control.

These pins should be linked directly to the S2:0 pins on the

PDSP16318 Complex Accumulator. They control the

accumulator’s barrel shifter (see Table 5). The purpose of this

shift is to minimise sign extension in the multiplier or accumulator

ALUs. In normal mode, SFTR2:0 are not used and should be

left unconnected.

Table 5 Accumulator shifts (BFP mode)

SFTR2:0

000

001

010

011

100

101

110

111

Reserved

Reserved

Reserved

Shift right by one

No shift

Shift left by one

Shift left by two

Reserved

Function

SOBFP (BFP Mode Only)

EOPSS (BFP Mode Only)