DSPIC33FJ12MC202TESS datasheet(29/304 Pages) MICROCHIP

29 / 304 page

DS70265E-page 29

dsPIC33FJ12MC201/202

3.6.3.1

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 (lsw) is simply discarded.

Conventional rounding will zero-extend bit 15 of the

accumulator and will add 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 (LSb), 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 X bus, subject

data

saturation

(see

Section 3.6.3.2 “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.

3.6.3.2

Data Space Write Saturation

In addition to adder/subtracter saturation, writes to data

space can also be saturated, but without affecting the

contents of the source accumulator. The data space

write saturation logic block accepts a 16-bit, 1.15

fractional value from the round logic block as its input,

together with overflow status from the original source

(accumulator) and the 16-bit round adder. These inputs

are combined and used to select the appropriate 1.15

fractional value as output to write to data space

memory.

If the SATDW bit in the CORCON register is set, data

(after rounding or truncation) is tested for overflow and

adjusted accordingly:

• For input data greater than 0x007FFF, data

written to memory is forced to the maximum

positive 1.15 value, 0x7FFF.

• For input data less than 0xFF8000, data written to

memory is forced to the maximum negative 1.15

value, 0x8000.

The MSb of the source (bit 39) is used to determine the

sign of the operand being tested.

If the SATDW bit in the CORCON register is not set, the

input data is always passed through unmodified under

all conditions.

3.6.4

BARREL SHIFTER

The barrel shifter can perform up to 16-bit arithmetic or

logic right shifts, or up to 16-bit left shifts, in a single

cycle. The source can be either of the two DSP

accumulators or the X bus (to support multi-bit shifts of

The shifter requires a signed binary value to determine

both the magnitude (number of bits) and direction of the

shift operation. A positive value shifts the operand right.

A negative value shifts the operand left. A value of ‘0’

does not modify the operand.

The barrel shifter is 40 bits wide, thereby obtaining a

40-bit result for DSP shift operations and a 16-bit result

for MCU shift operations. Data from the X bus is

presented to the barrel shifter between bit positions 16

and 31 for right shifts, and between bit positions 0 and

16 for left shifts.

DSPIC33FJ12MC202TESS Datasheet(PDF) 29 Page - Microchip Technology

DSPIC33FJ12MC202TESS Datasheet(HTML) 29 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

Datasheet Download

Link URL