3.3.12.3.2 Saturation And Overflow Modes

The dsPIC33A CPU supports three Saturation and Overflow modes.

• Accumulator 71-Bit Saturation

  In this mode, the saturation logic loads the maximally positive 9.63 value (0x7F_FFFF_FFFF_FFFF_FFFF) or maximally negative 9.63 value (0x80_0000_0000_0000_0000) into the target accumulator. The SA or SB bit is set and remains set until cleared by the user application. This Saturation mode is useful for extending the dynamic range of the accumulator.

  To configure for this mode of saturation, set the Accumulator Saturation Mode Select (ACCSAT) bit (CORCON[4]). Additionally, set the ACCA Saturation Enable (SATA) bit (CORCON[7] and/or the ACCB Saturation Enable (SATB) bit (CORCON[6]) to enable accumulator saturation.

• Accumulator 63-Bit Saturation

  In this mode, the saturation logic loads the maximally positive 1.63 value (0x00_7FFF_FFFF_FFFF_FFFF) or maximally negative 1.63 value (0xFF_8000_0000_0000_0000) into the target accumulator. The SA or SB bit is set and remains set until cleared by the user. When this Saturation mode is in effect, the guard bits, 64 through 71, are not used except for sign extension of the accumulator value. Consequently, the OA, OB or OAB bits in SR are never set.

  To configure for this mode of overflow and saturation, the ACCSAT (CORCON[4]) bit must be cleared. Additionally, the SATA (CORCON[7]) and/or SATB (CORCON[6]) bits must be set to enable accumulator saturation.

• Accumulator Catastrophic Overflow

  If the SATA (CORCON[7]) and/or SATB (CORCON[6]) bits are not set, then no saturation operation is performed on the accumulator, and the accumulator is allowed to overflow all the way up to bit 71 (destroying its sign). If the Catastrophic Overflow Trap Enable (COVTE) bit (INTCON4[8] in the interrupt controller) is set, a catastrophic overflow initiates an arithmetic error trap.

Accumulator saturation and overflow detection can only result from the execution of a DSP instruction that modifies one of the two accumulators via the 72-bit DSP ALU. Saturation and overflow detection do not take place when the accumulators are accessed via the MCU class of instructions. Furthermore, the Accumulator Status bits shown in Table 3-6 are not modified. However, the MCU Status bits (Z, N, C, OV, DC) will be modified, depending on the MCU instruction that accesses the accumulator.