16-Bit Core 17x17 Multiplier

The dsPIC provides a 17-bit by 17-bit multiplier which executes all multiplication operations in one instruction cycle. On the dsPIC, this multiplier is shared between the DSP and MCU instructions. The 17-bit by 17-bit multiplier is capable of performing mixed-sign multiplication 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, while signed operands are sign-extended into the 17th bit of the multiplier input value. The results of the multiplication can be either a 32-bit fractional number in 1.31 format, or a 32-bit signed or unsigned integer. The output of the multiplier and scaler is fed into a sign-extension block, which produces a 40-bit sign-extended result.

Even though both inputs to the multiplier are 16-bit data, a 17-bit multiplier is used in order to correctly handle the fixed-point computation of (-1.0 * -1.0) or (0x8000 * 0x8000). Before the multiplication is performed, both 16-bit operands are sign-extended to 17-bits.

Multiplier.png
  • The 17x17 Multiplier is part of ALL 16-Bit devices.
  • dsPIC® DSC
    • Part of DSP engine, feeds MAC
    • Fed by X and Y data spaces
  • PIC24
    • Adapted from DSP engine
    • Considered part of ALU
    • Fed by W registers only
  • Used for both MCU and DSP multiplication instructions
  • Single cycle operation
  • Performs 16x16 multiplication
    • Operands can be
      • 16-bit integer with 32-bit integer product
      • 1.15 fractional with 1.31 fractional product
  • A 17x17 multiplier for 16-bit data?
    • Simplifies signed-unsigned MCU multiplication
      • Before multiplication is performed, sign is preserved in Bit 17
    • Correctly performs (-1.0) * (-1.0) operation

Signed Fractional Multiplies

  • Signed Fractional Multiplying
    • Multiplying two 16-bit signed fractional numbers (1.15 format) result in one 32-bit product with two sign bits (2.30 format)
    • The two resulting sign bits will always be identical EXCEPT for the case -1 x -1
      • Sign bits are preserved in bit 17 of the multiplier
      • Multiplier restores bit 17 when done to account for this case
    • Multiplier will shift result one to the left to align result back to 1.31 expected by the accumulators
    • Zero is shifted into LSB
SignedFracMult.png

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