DSPIC30F MICROCHIP [Microchip Technology], DSPIC30F Datasheet - Page 34

• Manufacturer Part Number: DSPIC30F
• Description: General Purpose and Sensor Families High-Performance Digital Signal Controllers
• Manufacturer: MICROCHIP [Microchip Technology]

dsPIC30F
The device supports three saturation and overflow
modes:
1.
2.
3.
2.5.2.2
The MAC class of instructions (with the exception of
MPY, MPY.N, ED and EDAC) can optionally write a
rounded version of the high word (bits 31 through 16)
of the accumulator that is not targeted by the instruction
into data space memory. The write is performed across
the X bus into combined X and Y address space. The
following Addressing modes are supported:
1.
2.
DS70083G-page 32
Bit 39 Overflow and Saturation:
When bit 39 overflow and saturation occurs, the
saturation logic loads the maximally positive 9.31
(0x7FFFFFFFFF), or maximally negative 9.31
value (0x8000000000) into the target accumula-
tor. The SA or SB bit is set and remains set until
cleared by the user. This is referred to as ‘super
saturation’ and provides protection against erro-
neous data, or unexpected algorithm problems
(e.g., gain calculations).
Bit 31 Overflow and Saturation:
When bit 31 overflow and saturation occurs, the
saturation logic then loads the maximally posi-
tive 1.31 value (0x007FFFFFFF), or maximally
negative 1.31 value (0x0080000000) into the
target accumulator. The SA or SB bit is set and
remains set until cleared by the user. When this
Saturation mode is in effect, the guard bits are
not used (so the OA, OB or OAB bits are never
set).
Bit 39 Catastrophic Overflow:
The bit 39 overflow status bit from the adder is
used to set the SA or SB bit which remain set
until cleared by the user. No saturation operation
is performed and the accumulator is allowed to
overflow (destroying its sign). If the COVTE bit in
the INTCON1 register is set, a catastrophic
overflow can initiate a trap exception.
W13, Register Direct:
The rounded contents of the non-target
accumulator are written into W13 as a 1.15
fraction.
[W13]+=2, Register Indirect with Post-Increment:
The rounded contents of the non-target accumu-
lator are written into the address pointed to by
W13 as a 1.15 fraction. W13 is then
incremented by 2 (for a word write).
Accumulator ‘Write Back’
Preliminary
2.5.2.3
The round logic is a combinational block which per-
forms a conventional (biased) or convergent (unbi-
ased) round function during an accumulator write
(store). The Round mode is determined by the state of
the RND bit in the CORCON register. It generates a 16-
bit, 1.15 data value which is passed to the data space
write saturation logic. If rounding is not indicated by the
instruction, a truncated 1.15 data value is stored and
the LS Word is simply discarded.
The two Rounding modes are shown in Figure 2-10.
Conventional rounding takes bit 15 of the accumulator,
zero-extends it and adds it to the ACCxH word (bits 16
through 31 of the accumulator). If the ACCxL word
(bits 0 through 15 of the accumulator) is between
0x8000 and 0xFFFF (0x8000 included), ACCxH is
incremented. If ACCxL is between 0x0000 and
0x7FFF, ACCxH is left unchanged. A consequence of
this algorithm is that over a succession of random
rounding operations, the value will tend to be biased
slightly positive.
Convergent (or unbiased) rounding operates in the
same manner as conventional rounding, except when
ACCxL equals 0x8000. If this is the case, the LS bit
(bit 16 of the accumulator) of ACCxH is examined. If it
is ‘1’, ACCxH is incremented. If it is ‘0’, ACCxH is not
modified. Assuming that bit 16 is effectively random in
nature, this scheme will remove any rounding bias that
may accumulate.
The SAC and SAC.R instructions store either a trun-
cated (SAC) or rounded (SAC.R) version of the contents
of the target accumulator to data memory via the X bus
(subject to data saturation, see Section 2.5.2.4). Note
that for the MAC class of instructions, the accumulator
write back operation will function in the same manner,
addressing combined MCU (X and Y) data space
though the X bus. For this class of instructions, the data
is always subject to rounding.
Round Logic
 2004 Microchip Technology Inc.