PIC18F8720 datasheet(87/380 Pages) MICROCHIP

87 / 380 page

 2004 Microchip Technology Inc.

DS39609B-page 85

PIC18F6520/8520/6620/8620/6720/8720

8.0

8 X 8 HARDWARE MULTIPLIER

8.1

Introduction

An 8 x 8 hardware multiplier is included in the ALU of

the PIC18FXX20 devices. By making the multiply a

hardware operation, it completes in a single instruction

cycle. This is an unsigned multiply that gives a 16-bit

result. The result is stored in the 16-bit product register

pair (PRODH:PRODL). The multiplier does not affect

any flags in the ALUSTA register.

Making the 8 x 8 multiplier execute in a single cycle

gives the following advantages:

• Higher computational throughput

• Reduces code size requirements for multiply

algorithms

The performance increase allows the device to be used

in applications previously reserved for Digital Signal

Processors.

Table 8-1 shows a performance comparison between

enhanced devices using the single-cycle hardware

multiply and performing the same function without the

hardware multiply.

8.2

Operation

Example 8-1 shows the sequence to do an 8 x 8

unsigned multiply. Only one instruction is required

when one argument of the multiply is already loaded in

the WREG register.

Example 8-2 shows the sequence to do an 8 x 8 signed

multiply. To account for the sign bits of the arguments,

each argument’s Most Significant bit (MSb) is tested

and the appropriate subtractions are done.

EXAMPLE 8-1:

8 x 8 UNSIGNED

MULTIPLY ROUTINE

EXAMPLE 8-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

TABLE 8-1:

PERFORMANCE COMPARISON

MOVF

ARG1, W

;

MULWF

ARG2

; ARG1 * ARG2 ->

; PRODH:PRODL

MOVF

ARG1, W

;

MULWF

ARG2

; ARG1 * ARG2 ->

; PRODH:PRODL

BTFSC

ARG2, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG1

MOVF

ARG2, W

;

BTFSC

ARG1, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG2

Routine

Multiply Method

Program

Memory

(Words)

Cycles

(Max)

Time

@ 40 MHz

@ 10 MHz

@ 4 MHz

8 x 8 unsigned

Without hardware multiply

6.9

µs27.6 µs69 µs

Hardware multiply

100 ns

400 ns

µs

8 x 8 signed

Without hardware multiply

9.1

µs36.4 µs91 µs

Hardware multiply

600 ns

2.4

µs6 µs

16 x 16 unsigned

Without hardware multiply

242

24.2

µs96.8 µs242 µs

Hardware multiply

2.8

µs

11.2

µs28 µs

16 x 16 signed

Without hardware multiply

254

25.4

µs

102.6

µs254 µs

Hardware multiply

4.0

µs16.0 µs40 µs

PIC18F8720 Folha de dados(PDF) 87 Page - Microchip Technology

PIC18F8720 Folha de dados(HTML) 87 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

Folha de dados Download

Ligação URL