DSPIC33FJ32GP204TESLASHML datasheet(35/403 Pages) MICROCHIP

35 / 403 page

 2009 Microchip Technology Inc.

Preliminary

DS70292D-page 35

dsPIC33FJ32GP302/304, dsPIC33FJ64GPX02/X04, AND dsPIC33FJ128GPX02/X04

The

Overflow

and

Saturation

Status

bits

can

optionally be viewed in the STATUS Register (SR) as

the logical OR of OA and OB (in bit OAB) and the

logical OR of SA and SB (in bit SAB). Programmers

can check one bit in the STATUS register to

determine if either accumulator has overflowed, or

one bit to determine if either accumulator has

saturated.

This

useful

for

complex

number

arithmetic, which typically uses both accumulators.

The device supports three Saturation and Overflow

modes:

• Bit 39 Overflow and Saturation:

When bit 39 overflow and saturation occurs, the

saturation logic loads the maximally positive 9.31

(0x7FFFFFFFFF) or maximally negative 9.31 value

(0x8000000000) into the target accumulator. The

SA or SB bit is set and remains set until cleared by

the user application. This condition is referred to as

‘super saturation’ and provides protection against

erroneous data or unexpected algorithm problems

(such as gain calculations).

• Bit 31 Overflow and Saturation:

When bit 31 overflow and saturation occurs, the

saturation logic then loads the maximally positive

1.31 value (0x007FFFFFFF) or maximally nega-

tive 1.31 value (0x0080000000) into the target

accumulator. The SA or SB bit is set and remains

set until cleared by the user application. When

this Saturation mode is in effect, the guard bits are

not used, so the OA, OB or OAB bits are never

set.

• Bit 39 Catastrophic Overflow:

The bit 39 Overflow Status bit from the adder is

used to set the SA or SB bit, which remains set

until cleared by the user application. No saturation

operation is performed, and the accumulator is

allowed to overflow, destroying its sign. If the

COVTE bit in the INTCON1 register is set, a

catastrophic overflow can initiate a trap exception.

3.7.3

ACCUMULATOR ‘WRITE BACK’

The MAC class of instructions (with the exception of

MPY,

MPY.N,

and EDAC) can optionally write a

rounded version of the high word (bits 31 through 16)

of the accumulator that is not targeted by the instruction

into data space memory. The write is performed across

the X bus into combined X and Y address space. The

following addressing modes are supported:

• W13, Register Direct:

The rounded contents of the non-target

accumulator are written into W13 as a

1.15 fraction.

• [W13] + = 2, Register Indirect with Post-Increment:

The rounded contents of the non-target accumu-

lator are written into the address pointed to by

W13 as a 1.15 fraction. W13 is then incremented

by 2 (for a word write).

3.7.3.1

Round Logic

The round logic is a combinational block that performs

a conventional (biased) or convergent (unbiased)

round function during an accumulator write (store). The

Round mode is determined by the state of the RND bit

in the CORCON register. It generates a 16-bit, 1.15

data value that is passed to the data space write

saturation logic. If rounding is not indicated by the

instruction, a truncated 1.15 data value is stored and

the least significant word is simply discarded.

Conventional rounding zero-extends bit 15 of the accu-

mulator and adds it to the ACCxH word (bits 16 through

31 of the accumulator).

• If the ACCxL word (bits 0 through 15 of the accu-

mulator) is between 0x8000 and 0xFFFF (0x8000

included), ACCxH is incremented.

• If ACCxL is between 0x0000 and 0x7FFF, ACCxH

is left unchanged.

A consequence of this algorithm is that over a succes-

sion of random rounding operations, the value tends to

be biased slightly positive.

Convergent (or unbiased) rounding operates in the

same manner as conventional rounding, except when

ACCxL equals 0x8000. In this case, the Least

Significant bit (bit 16 of the accumulator) of ACCxH is

examined:

• If it is ‘1’, ACCxH is incremented.

• If it is ‘0’, ACCxH is not modified.

Assuming that bit 16 is effectively random in nature,

this scheme removes any rounding bias that may

accumulate.

The SAC and SAC.R instructions store either a

truncated (SAC), or rounded (SAC.R) version of the

contents of the target accumulator to data memory via

the

bus,

subject

data

saturation

(see

Section 3.7.3.2 “Data Space Write Saturation”). For

the MAC class of instructions, the accumulator write-

back operation functions in the same manner,

addressing combined MCU (X and Y) data space

though the X bus. For this class of instructions, the data

is always subject to rounding.

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