DSPIC30F0010AT-30ISLASHPF datasheet(21/232 Pages) MICROCHIP

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DS70150D-page 21

dsPIC30F6010A/6015

The SA and SB bits are modified each time data passes

through the adder/subtractor, but can only be cleared by

the user. When set, they indicate that the accumulator

has overflowed its maximum range (bit 31 for 32-bit

saturation, or bit 39 for 40-bit saturation) and will be

saturated if saturation is enabled. When saturation is not

enabled, SA and SB default to bit 39 overflow and thus

indicate that a catastrophic overflow has occurred. If the

COVTE bit in the INTCON1 register is set, SA and SB

bits will generate an arithmetic warning trap when

saturation is disabled.

The Overflow and Saturation Status bits can optionally

be viewed in the STATUS register (SR) as the logical

OR of OA and OB (in bit OAB) and the logical OR of SA

and SB (in bit SAB). This allows programmers to check

one bit in the STATUS register to determine if either

accumulator has overflowed, or one bit to determine if

either accumulator has saturated. This would be useful

for complex number arithmetic which typically uses

both the accumulators.

The device supports three Saturation and Overflow

modes.

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

accumulator. 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 erroneous 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

positive

1.31

value

(0x007FFFFFFF)

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.

2.4.2.2

Accumulator ‘Write-Back’

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:

W13, Register Direct:

The

rounded

contents

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

accumulator are written into the address pointed

to by W13 as a 1.15 fraction. W13 is then

incremented by 2 (for a word write).

2.4.2.3

Round Logic

The round logic is a combinational block, which

performs a conventional (biased) or convergent

(unbiased) 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 least significant word is simply discarded.

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

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 LSb

(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

truncated (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 2.4.2.4 “Data Space Write Saturation”).

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.

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