datasheet (3).pdf - EEWeb

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dsPIC33FJ32MC302/304, dsPIC33FJ64MCX02/X04 AND dsPIC33FJ128MCX02/X04 FIGURE 3-3: DSP ENGINE BLOCK DIAGRAM Y Data Bus 40 Carry/Borrow Out Carry/Borrow In 40-bit Accumulator A 40-bit Accumulator B Saturate 40 Adder Sign-Extend 16 33 Negate 40 To/From W Array 40 17-bit Multiplier/Scaler 16 Barrel Shifter Round Logic Zero Backfill DS70291E-page 34 © 2011 Microchip Technology Inc. 32 32 40 16 40 S a t u r a t e 16 16 X Data Bus
dsPIC33FJ32MC302/304, dsPIC33FJ64MCX02/X04 AND dsPIC33FJ128MCX02/X04 3.7.1 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 two’s complement value, where the Most Significant bit (MSb) is defined as a sign bit. The range of an N-bit two’s complement integer is -2 N-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 is -2,147,483,648 (0x8000 0000) to 2,147,483,647 (0x7FFF FFFF) When the multiplier is configured for fractional multiplication, 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 – 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 multiply operation generates a 1.31 product that has a precision of 4.65661 x 10 -10 . The same multiplier is used to support the MCU multiply 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 direct a 16-bit result, and word operands direct a 32-bit result to the specified registers in the W array. 3.7.2 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 destination. For the ADD and LAC instructions, the data to be accumulated or loaded can be optionally scaled using the barrel shifter prior to accumulation. 3.7.2.1 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 (CORCON) and ACCSAT (CORCON) 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 corresponding Overflow Trap Flag Enable bits (OVATE, OVBTE) in the INTCON1 register are set (refer to Section 7.0 “Interrupt Controller”). This allows the user application 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 application. 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 is 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, the SA and SB bits generate an arithmetic warning trap when saturation is disabled. © 2011 Microchip Technology Inc. DS70291E-page 35
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