DSPIC30F Datasheet, PDF (33/248 Pages) Microchip Technology – dsPIC High-Performance 16-bit Digital Signal Controller Family Overview

The same multiplier is used to support the MCU multi-
ply instructions which include integer 16-bit signed,
unsigned and mixed sign multiplies. Additional data
paths are provided to allow these instructions to write
the result back into the W array and X data bus (via the
W array). These paths are placed prior to the data
scaler. The IF bit in the CORCON register, therefore,
only affects the result of the MAC class of DSP instruc-
tions. All other multiply operations are assumed to be
integer operations. If the user executes a MAC instruc-
tion on fractional data without clearing the IF bit, the
result must be explicitly shifted left by the user program
after multiplication in order to obtain the correct result.
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.
2.5.2
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 via the
barrel shifter, prior to accumulation.
2.5.2.1
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 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.
dsPIC30F
Six STATUS register bits have been provided to
support saturation and overflow; they are:
1. OA:
AccA overflowed into guard bits
2. OB:
AccB overflowed into guard bits
3. SA:
AccA saturated (bit 31 overflow and saturation)
or
AccA overflowed into guard bits and saturated
(bit 39 overflow and saturation)
4. SB:
AccB saturated (bit 31 overflow and saturation)
or
AccB overflowed into guard bits and saturated
(bit 39 overflow and saturation)
5. OAB:
Logical OR of OA and OB
6. 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 (OVATEN, OVBTEN) in
the INTCON1 register (refer to Section 5.0) is set. This
allows the user to take immediate action, for example,
to correct system gain.
The SA and SB bits are modified each time data
passes through the adder/subtracter 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 satu-
ration is not enabled, SA and SB default to bit 39 over-
flow 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.
 2004 Microchip Technology Inc.
Preliminary
DS70083G-page 31