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.

  • 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
© 2017 Microchip Technology, Inc.
Notice: ARM and Cortex are the registered trademarks of ARM Limited in the EU and other countries.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.