SLAU367P October 2012 – April 2020 MSP430FR5041 , MSP430FR5043 , MSP430FR50431 , MSP430FR5847 , MSP430FR58471 , MSP430FR5848 , MSP430FR5849 , MSP430FR5857 , MSP430FR5858 , MSP430FR5859 , MSP430FR5867 , MSP430FR58671 , MSP430FR5868 , MSP430FR5869 , MSP430FR5870 , MSP430FR5872 , MSP430FR58721 , MSP430FR5887 , MSP430FR5888 , MSP430FR5889 , MSP430FR58891 , MSP430FR5922 , MSP430FR59221 , MSP430FR5947 , MSP430FR59471 , MSP430FR5948 , MSP430FR5949 , MSP430FR5957 , MSP430FR5958 , MSP430FR5959 , MSP430FR5962 , MSP430FR5964 , MSP430FR5967 , MSP430FR5968 , MSP430FR5969 , MSP430FR5969-SP , MSP430FR59691 , MSP430FR5970 , MSP430FR5972 , MSP430FR59721 , MSP430FR5986 , MSP430FR5987 , MSP430FR5988 , MSP430FR5989 , MSP430FR5989-EP , MSP430FR59891 , MSP430FR5992 , MSP430FR5994 , MSP430FR59941 , MSP430FR6005 , MSP430FR6007 , MSP430FR6035 , MSP430FR6037 , MSP430FR60371 , MSP430FR6041 , MSP430FR6043 , MSP430FR60431 , MSP430FR6045 , MSP430FR6047 , MSP430FR60471 , MSP430FR6820 , MSP430FR6822 , MSP430FR68221 , MSP430FR6870 , MSP430FR6872 , MSP430FR68721 , MSP430FR6877 , MSP430FR6879 , MSP430FR68791 , MSP430FR6887 , MSP430FR6888 , MSP430FR6889 , MSP430FR68891 , MSP430FR6920 , MSP430FR6922 , MSP430FR69221 , MSP430FR6927 , MSP430FR69271 , MSP430FR6928 , MSP430FR6970 , MSP430FR6972 , MSP430FR69721 , MSP430FR6977 , MSP430FR6979 , MSP430FR69791 , MSP430FR6987 , MSP430FR6988 , MSP430FR6989 , MSP430FR69891
Multiplying two fractional numbers using the default multiplication mode with MPYFRAC = 0 and MPYSAT = 0 gives a result with two sign bits. For example, if two 16-bit Q15 numbers are multiplied, a 32-bit result in Q30 format is obtained. To convert the result into Q15 format manually, the first 15 trailing bits and the extended sign bit must be removed. However, when the fractional mode of the multiplier is used, the redundant sign bit is automatically removed, yielding a result in Q31 format for the multiplication of two 16-bit Q15 numbers. Reading the result register RES1 gives the result as 16-bit Q15 number. The 32-bit Q31 result of a multiplication of two 32-bit Q31 numbers is accessed by reading registers RES2 and RES3.
The fractional mode is enabled with MPYFRAC = 1 in register MPY32CTL0. The actual content of the result registers is not modified when MPYFRAC = 1. When the result is accessed using software, the value is left shifted one bit, resulting in the final Q formatted result. This allows user software to switch between reading both the shifted (fractional) and the unshifted result. The fractional mode should only be enabled when required and disabled after use.
In fractional mode, the SUMEXT register contains the sign extended bits 32 and 33 of the shifted result for 16×16-bit operations and bits 64 and 65 for 32×32-bit operations – not only bits 32 or 64, respectively.
The MPYC bit is not affected by the fractional mode. It always reads the carry of the nonfractional result.
; Example using
; Fractional 16x16 multiplication
BIS #MPYFRAC,&MPY32CTL0 ; Turn on fractional mode
MOV &FRACT1,&MPYS ; Load 1st operand as Q15
MOV &FRACT2,&OP2 ; Load 2nd operand as Q15
MOV &RES1,&PROD ; Save result as Q15
BIC #MPYFRAC,&MPY32CTL0 ; Back to normal mode
| Operation
(OP1 × OP2) |
Result Ready in MCLK Cycles | After | ||||
|---|---|---|---|---|---|---|
| RES0 | RES1 | RES2 | RES3 | MPYC Bit | ||
| 8/16 × 8/16 | 3 | 3 | 4 | 4 | 3 | OP2 written |
| 24/32 × 8/16 | 3 | 5 | 6 | 7 | 7 | OP2 written |
| 8/16 × 24/32 | 3 | 5 | 6 | 7 | 7 | OP2L written |
| N/A | 3 | 4 | 4 | 4 | OP2H written | |
| 24/32 × 24/32 | 3 | 8 | 10 | 11 | 11 | OP2L written |
| N/A | 3 | 5 | 6 | 6 | OP2H written | |