PIC18LC658-ESLASHCL datasheet(71/366 Pages) MICROCHIP

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 2000 Microchip Technology Inc.

Advanced Information

DS30475A-page 71

PIC18CXX8

6.0

8 X 8 HARDWARE MULTIPLIER

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

the PIC18CXX8 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 into the 16-bit product regis-

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

affect any flags in the STATUS 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 algo-

rithms

The performance increase allows the device to be used

in applications previously reserved for Digital Signal

Processors.

Table 6-1 shows a performance comparison between

enhanced devices using the single cycle hardware mul-

tiply, and performing the same function without the

hardware multiply.

TABLE 6-1:

PERFORMANCE COMPARISON

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

µs

16 x 16 unsigned

Without hardware multiply

242

24.2

µs96.8 µs

242

µs

Hardware multiply

2.4

µs

9.6

µs

16 x 16 signed

Without hardware multiply

254

25.4

µs102.6 µs

254

µs

Hardware multiply

3.6

µs

14.4

µs

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