DSPIC30F datasheet(34/248 Pages) MICROCHIP

34 / 248 page

dsPIC30F

DS70083G-page 32

Preliminary

 2004 Microchip Technology Inc.

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 accumula-

tor. The SA or SB bit is set and remains set until

cleared by the user. This is referred to as ‘super

saturation’ and provides protection against erro-

neous data, or unexpected algorithm problems

(e.g., gain calculations).

Bit 31 Overflow and Saturation:

When bit 31 overflow and saturation occurs, the

saturation logic then loads the maximally posi-

tive 1.31 value (0x007FFFFFFF), or maximally

negative 1.31 value (0x0080000000) 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 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 remain set

until cleared by the user. 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.

2.5.2.2

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

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

1.15

fraction.

W13

then

incremented by 2 (for a word write).

2.5.2.3

Round Logic

The round logic is a combinational block which per-

forms a conventional (biased) or convergent (unbi-

ased) 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 which 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 LS Word is simply discarded.

The two Rounding modes are shown in Figure 2-10.

Conventional rounding takes bit 15 of the accumulator,

zero-extends it and adds it to the ACCxH word (bits 16

through 31 of the accumulator). If the ACCxL word

(bits 0 through 15 of the accumulator) 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 succession of random

rounding operations, the value will tend to be biased

slightly positive.

Convergent (or unbiased) rounding operates in the

same manner as conventional rounding, except when

ACCxL equals 0x8000. If this is the case, the LS 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 will remove any rounding bias that

may accumulate.

The SAC and SAC.R instructions store either a trun-

cated (SAC) or rounded (SAC.R) version of the contents

of the target accumulator to data memory via the X bus

(subject to data saturation, see Section 2.5.2.4). Note

that for the MAC class of instructions, the accumulator

write back operation will function 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.

DSPIC30F датащи(PDF) 34 Page - Microchip Technology

DSPIC30F датащи(HTML) 34 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL