PDSP16116
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.
SOBFP (BFP Mode Only)
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
′
and B
′
words as it will form the input word tags, WTA and
WTB, for each complex word during the next pass. In normal
mode, WTOUT1:0 are not used and should be left unconnected.
WTOUT1:0
00
01
10
11
Weighting of the output relative to
the current global weighting register
One less
The same
One more
Two more
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.
EOPSS (BFP Mode Only)
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 6 Word tag weightings
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.
WTB1:0
Function
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
SFTR2:0
000
001
010
011
100
101
110
111
Function
Reserved
Reserved
Reserved
Shift right by one
No shift
Shift left by one
Shift left by two
Reserved
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.
SFTA1:0
00
01
10
11
Function
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
Table 5 Accumulator shifts (BFP mode)
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.
Table 8 External A-word shift control
8
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