DSPIC33FJ32MC202 Datasheet, PDF (24/292 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

dsPIC33FJ32MC202/204 and dsPIC33FJ16MC304
2.6.1 MULTIPLIER
The 17-bit x 17-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
multiplier input value. The output of the 17-bit x 17-bit
multiplier/scaler is a 33-bit value that is sign-extended
to 40 bits. Integer data is inherently represented as a
signed 2’s complement value, where the Most Signifi-
cant bit (MSb) is defined as a sign bit. The range of an
N-bit 2’s complement integer is -2^N-1 to 2^N-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 is -
2,147,483,648 (0x8000 0000) to 2,147,483,647
(0x7FFF FFFF).
When the multiplier is configured for fractional multipli-
cation, the data is represented as a 2’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 2’s complement fraction
with this implied radix point is -1.0 to (1 – 2^1-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, the 16 x 16 multi-
ply operation generates a 1.31 product that has a pre-
cision of 4.65661 x 10^-10.
The same multiplier is used to support the MCU multi-
ply instructions, which include integer 16-bit signed,
unsigned and mixed sign multiply operations.
The MUL instruction can 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.
2.6.2
DATA ACCUMULATORS AND
ADDER/SUBTRACTER
The data accumulator consists of a 40-bit adder/
subtracter 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 using
the barrel shifter prior to accumulation.
2.6.2.1
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).
• 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 that
controls accumulator data saturation, if selected. It
uses the result of the adder, the Overflow Status bits
described previously and the SAT<A:B>
(CORCON<7:6>) and ACCSAT (CORCON<4>) mode
control bits to determine when and to what value to
saturate.
Six STATUS register bits support saturation and
overflow:
• OA: ACCA overflowed into guard bits
• OB: ACCB overflowed into guard bits
• SA: ACCA saturated (bit 31 overflow and
saturation)
or
ACCA overflowed into guard bits and saturated
(bit 39 overflow and saturation)
• SB: ACCB saturated (bit 31 overflow and
saturation)
or
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 bits (OVATE, OVBTE) in
the INTCON1 register are set (refer to Section 6.0
“Interrupt Controller”). This allows the user applica-
tion to take immediate action, for example, to correct
system gain.
DS70283B-page 22
Preliminary
© 2007 Microchip Technology Inc.