Datasheet

© 2007-2011 Microchip Technology Inc. DS70264E-page 25
dsPIC33FJ12GP201/202
3.5 Arithmetic Logic Unit (ALU)
The dsPIC33FJ12GP201/202 ALU is 16 bits wide and
is capable of addition, subtraction, bit shifts and logic
operations. Unless otherwise mentioned, arithmetic
operations are 2’s complement in nature. Depending
on the operation, the ALU can affect the values of the
Carry (C), Zero (Z), Negative (N), Overflow (OV) and
Digit Carry (DC) Status bits in the SR register. The C
and DC Status bits operate as Borrow
and Digit Borrow
bits, respectively, for subtraction operations.
The ALU can perform 8-bit or 16-bit operations,
depending on the mode of the instruction that is used.
Data for the ALU operation can come from the W
register array or data memory, depending on the
addressing mode of the instruction. Likewise, output
data from the ALU can be written to the W register array
or a data memory location.
The dsPIC33FJ12GP201/202 CPU incorporates hard-
ware support for both multiplication and division. This
includes a dedicated hardware multiplier and support
hardware for 16-bit-divisor division.
Refer to the “16-bit MCU and DSC Programmer’s Ref-
erence Manual” (DS70157) for information on the SR
bits affected by each instruction.
3.5.1 MULTIPLIER
Using the high-speed 17-bit x 17-bit multiplier of the DSP
engine, the ALU supports unsigned, signed or mixed-sign
operation in several MCU multiplication modes:
• 16-bit x 16-bit signed
• 16-bit x 16-bit unsigned
• 16-bit signed x 5-bit (literal) unsigned
• 16-bit unsigned x 16-bit unsigned
• 16-bit unsigned x 5-bit (literal) unsigned
• 16-bit unsigned x 16-bit signed
• 8-bit unsigned x 8-bit unsigned
3.5.2 DIVIDER
The divide block supports 32-bit/16-bit and 16-bit/16-bit
signed and unsigned integer divide operations with the
following data sizes:
• 32-bit signed/16-bit signed divide
• 32-bit unsigned/16-bit unsigned divide
• 16-bit signed/16-bit signed divide
• 16-bit unsigned/16-bit unsigned divide
The quotient for all divide instructions ends up in W0
and the remainder in W1. 16-bit signed and unsigned
DIV instructions can specify any W register for both the
16-bit divisor (Wn) and any W register (aligned) pair
(W(m+1):Wm) for the 32-bit dividend. The divide
algorithm takes one cycle per bit of divisor, so both
32-bit/16-bit and 16-bit/16-bit instructions take the
same number of cycles to execute.
3.6 DSP Engine
The DSP engine consists of a high-speed 17-bit x
17-bit multiplier, a barrel shifter and a 40-bit
adder/subtracter (with two target accumulators, round
and saturation logic).
The dsPIC33FJ12GP201/202 is a single-cycle instruc-
tion flow architecture; therefore, concurrent operation of
the DSP engine with MCU instruction flow is not possible.
However, some MCU ALU and DSP engine resources
can be used concurrently by the same instruction
(e.g., ED, EDAC).
The DSP engine can also perform accumula-
tor-to-accumulator operations that require no additional
data. These instructions are ADD, SUB and NEG.
The DSP engine has options selected through bits in
the CPU Core Control register (CORCON), as listed
below:
• Fractional or integer DSP multiply (IF)
• Signed or unsigned DSP multiply (US)
• Conventional or convergent rounding (RND)
• Automatic saturation on/off for ACCA (SATA),
ACCB (SATB) and writes to data memory
(SATDW)
• Accumulator Saturation mode selection (ACC-
SAT)
A block diagram of the DSP engine is shown in
Figure 3-3.
TABLE 3-1: DSP INSTRUCTIONS
SUMMARY
Instruction
Algebraic
Operation
ACC Write
Back
CLR
A = 0
Yes
ED A = (x – y)
2
No
EDAC A = A + (x – y)
2
No
MAC A = A + (x * y) Yes
MAC A = A + x
2
No
MOVSAC No change in A Yes
MPY A = x * y No
MPY A = x
2
No
MPY.N A = – x * y No
MSC A = A – x * y Yes