PIC18F45K20-I/MV【PDF 167/234 页】MCU 32KB FLASH 1536B RAM 40-UQFN

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Sheet目录3873

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dsPIC33F

DS70165E-page 36

Preliminary

2007 Microchip Technology Inc.

The SA and SB bits are modified each time data

passes through the adder/subtracter, but can only be

cleared by the user. When set, they indicate that the

accumulator has overflowed its maximum range (bit 31

for 32-bit saturation or bit 39 for 40-bit saturation) and

will be saturated (if saturation is enabled). When

saturation is not enabled, SA and SB default to bit 39

overflow and, thus, indicate that a catastrophic over-

flow has occurred. If the COVTE bit in the INTCON1

arithmetic warning trap when saturation is disabled.

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). This allows programmers to 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 would be useful

for complex number arithmetic which typically uses

both the 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 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 remains 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.

2.6.2.2

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

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).

2.6.2.3

Round Logic

The round logic is a combinational block which

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 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 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 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 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 Signifi-

cant 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 2.6.2.4 “Data Space Write Saturation”). 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.

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