Texas Instruments MSP430x1xx - Manual (Page 99)

Hardware Multiplier Software Restrictions
6-11
Hardware Multiplier
6.5.2 Hardware Multiplier Software Restrictions—Interrupt Routines
The entire multiplication routine requires only three steps:
1) Move operand OP1 to the hardware multiplier; this defines the type of
multiplication.
2) Move operand OP2 to the hardware multiplier; the multiplication starts.
3) Process the result of the multiplication in the RESLO, RESHI, and
SUMEXT registers.
The following considerations describe the main routines that use hardware
multiplication. If no hardware multiplication is used in the main routine,
multiplication in an interrupt routine is protected from further interrupts,
because the GIE bit is reset after entering the interrupt service routine.
Typically, a multiplication operation that uses the entire data process occurs
outside an interrupt routine and the interrupt routines are as short as possible.
A multiplication operation in an interrupt routine has some feedback to the
multiplication operation in the main routine.
6.5.2.1 Interrupt Following an OP1 Transfer
The two LSBs of the first operand address define the type of multiplication
operation. This information cannot be recovered by any later operation.
Therefore an interrupt must not be accepted between the first two steps: move
operand OP1 and OP2 to the multiplier.
6.5.2.2 Interrupt Following an OP2 Transfer
After the first two steps, the multiplication result is in the corresponding
registers RESLO, RESHI, and SUMEXT. It can be saved on the stack (using
the PUSH instruction) and can be restored after completing another
multiplication operation (using the POP instruction). However, this operation
takes additional code and cycles in the interrupt routine. You can avoid this,
by making an entire multiplication routine uninterruptible, by disabling any
interrupt (DINT) before entering the multiplication routine, and by enabling
interrupts (EINT) after the multiplication routine is completed. The negative
aspect of this method is that the critical interrupt latency is increased drastically
for events that occur during this period.
6.5.2.3 General Recommendation
In general, one should avoid a hardware multiplication operation within an
interrupt routine when a hardware multiplication is already used in the main
program. (This will depend upon the application-specific software, applied
libraries, and other included software.) The methods previously discussed
have some negative implications; therefore, the best practice is to keep
interrupt routines as short as possible.