DSPIC30F6010_06 datasheet(18/222 Pages) MICROCHIP

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dsPIC30F6010

DS70119E-page 16

2.4.1

MULTIPLIER

The 17x17-bit multiplier is capable of signed or

unsigned operation and can multiplex its output using a

scaler to support either 1.31 fractional (Q31) or 32-bit

integer results. Unsigned operands are zero-extended

into the 17th bit of the multiplier input value. Signed

operands are sign-extended into the 17th bit of the mul-

tiplier input value. The output of the 17x17-bit multiplier/

scaler is a 33-bit value, which is sign-extended to 40

bits. Integer data is inherently represented as a signed

two’s complement value, where the MSB is defined as

a sign bit. Generally speaking, the range of an N-bit

two’s complement integer is -2N-1 to 2N-1 – 1. For a

16-bit integer, the data range is -32768 (0x8000) to

32767 (0x7FFF), including 0. For a 32-bit integer, the

data

range

-2,147,483,648

(0x8000 0000)

2,147,483,645 (0x7FFF FFFF).

When the multiplier is configured for fractional multipli-

cation, the data is represented as a two’s complement

fraction, where the MSB is defined as a sign bit and the

radix point is implied to lie just after the sign bit

(QX format). The range of an N-bit two’s complement

fraction with this implied radix point is -1.0 to (1-21-N).

For a 16-bit fraction, the Q15 data range is -1.0

(0x8000) to 0.999969482 (0x7FFF), including 0 and

has a precision of 3.01518x10-5. In Fractional mode, a

16x16 multiply operation generates a 1.31 product,

which has a precision of 4.65661x10-10.

The same multiplier is used to support the MCU multi-

ply instructions, which include integer 16-bit signed,

unsigned and mixed sign multiplies.

The MUL instruction may be directed to use byte or

word sized operands. Byte operands will direct a 16-bit

result, and word operands will direct a 32-bit result to

the specified register(s) in the W array.

2.4.2

DATA ACCUMULATORS AND

ADDER/SUBTRACTER

The data accumulator consists of a 40-bit adder/sub-

tracter with automatic sign extension logic. It can select

one of two accumulators (A or B) as its pre-

accumulation source and post-accumulation destina-

tion. For the ADD and LAC instructions, the data to be

accumulated or loaded can be optionally scaled via the

barrel shifter, prior to accumulation.

2.4.2.1

Adder/Subtracter, Overflow and

Saturation

The adder/subtracter is a 40-bit adder with an optional

zero input into one side and either true or complement

data into the other input. In the case of addition, the

carry/borrow input is active high and the other input is

true data (not complemented), whereas in the case of

subtraction, the carry/borrow input is active low and the

other input is complemented. The adder/subtracter

generates overflow status bits SA/SB and OA/OB,

which are latched and reflected in the status register.

• Overflow from bit 39: this is a catastrophic

overflow in which the sign of the accumulator is

destroyed.

• Overflow into guard bits 32 through 39: this is a

recoverable overflow. This bit is set whenever all

the guard bits are not identical to each other.

The adder has an additional saturation block which

controls accumulator data saturation, if selected. It

uses the result of the adder, the overflow status bits

described above, and the SATA/B (CORCON<7:6>)

and ACCSAT (CORCON<4>) mode control bits to

determine when and to what value to saturate.

Six status register bits have been provided to support

saturation and overflow; they are:

OA:

ACCA overflowed into guard bits

OB:

ACCB overflowed into guard bits

SA:

ACCA saturated (bit 31 overflow and saturation)

ACCA overflowed into guard bits and saturated

(bit 39 overflow and saturation)

SB:

ACCB saturated (bit 31 overflow and saturation)

ACCB overflowed into guard bits and saturated

(bit 39 overflow and saturation)

OAB:

Logical OR of OA and OB

SAB:

Logical OR of SA and SB

The OA and OB bits are modified each time data

passes through the adder/subtracter. When set, they

indicate that the most recent operation has overflowed

into the accumulator guard bits (bits 32 through 39).

The OA and OB bits can also optionally generate an

arithmetic warning trap when set and the correspond-

ing overflow trap flag enable bit (OVATE, OVBTE) in

the INTCON1 register (refer to Section 5.0 “Inter-

rupts”) is set. This allows the user to take immediate

action, for example, to correct system gain.

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