PIC17C44 datasheet(99/241 Pages) MICROCHIP

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DS30412C-page 99

PIC17C4X

14.0

SPECIAL FEATURES OF THE

CPU

What sets a microcontroller apart from other proces-

sors are special circuits to deal with the needs of real

time applications. The PIC17CXX family has a host of

such features intended to maximize system reliability,

minimize cost through elimination of external compo-

nents, provide power saving operating modes and offer

code protection. These are:

• OSC selection

• Reset

- Power-on Reset (POR)

- Power-up Timer (PWRT)

- Oscillator Start-up Timer (OST)

• Interrupts

• Watchdog Timer (WDT)

• SLEEP

• Code protection

The PIC17CXX has a Watchdog Timer which can be

shut off only through EPROM bits. It runs off its own RC

oscillator for added reliability. There are two timers that

offer necessary delays on power-up. One is the Oscil-

lator Start-up Timer (OST), intended to keep the chip in

RESET until the crystal oscillator is stable. The other is

the Power-up Timer (PWRT), which provides a ﬁxed

delay of 96 ms (nominal) on power-up only, designed to

keep the part in RESET while the power supply stabi-

lizes. With these two timers on-chip, most applications

need no external reset circuitry.

The SLEEP mode is designed to offer a very low cur-

rent power-down mode. The user can wake from

SLEEP through external reset, Watchdog Timer Reset

or through an interrupt. Several oscillator options are

also made available to allow the part to ﬁt the applica-

tion. The RC oscillator option saves system cost while

the LF crystal option saves power. Conﬁguration bits

are used to select various options. This conﬁguration

word has the format shown in Figure 14-1.

FIGURE 14-1: CONFIGURATION WORD

R/P - 1

U - x

PM2 (1)

—

bit15-7

bit0

U - x

R/P - 1

U - x

R/P - 1

—

PM1

—

PM0

WDTPS1 WDTPS0

FOSC1

FOSC0

R = Readable bit

P = Programmable bit

U = Unimplemented

- n = Value for Erased Device

(x = unknown)

bit15-7

bit0

bit 15-9: Unimplemented: Read as a '1'

bit 15,6,4:PM2, PM1, PM0, Processor Mode Select bits

111 = Microprocessor Mode

110 = Microcontroller mode

101 = Extended microcontroller mode

000 = Code protected microcontroller mode

bit 7, 5: Unimplemented: Read as a '0'

bit 3-2:

WDTPS1:WDTPS0, WDT Postscaler Select bits

11 = WDT enabled, postscaler = 1

10 = WDT enabled, postscaler = 256

01 = WDT enabled, postscaler = 64

00 = WDT disabled, 16-bit overﬂow timer

bit 1-0:

FOSC1:FOSC0, Oscillator Select bits

11 = EC oscillator

10 = XT oscillator

01 = RC oscillator

00 = LF oscillator

Note 1: This bit does not exist on the PIC17C42. Reading this bit will return an unknown value (x).

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