PIC17C44 Datasheet, PDF (99/241 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM/ROM Microcontroller

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
PM2 ⁽¹⁾
bit15-7
U-x
—
U-x
—
U-x
—
U-x
—
U-x
—
U-x
—
U-x
—
bit0
U-x
—
bit15-7
R/P - 1
PM1
U-x
—
R/P - 1 R/P - 1 R/P - 1 R/P - 1 R/P - 1
PM0 WDTPS1 WDTPS0 FOSC1 FOSC0
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
R = Readable bit
P = Programmable bit
U = Unimplemented
- n = Value for Erased Device
(x = unknown)
Note 1: This bit does not exist on the PIC17C42. Reading this bit will return an unknown value (x).
© 1996 Microchip Technology Inc.
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