PIC17C7XX_13 Datasheet, PDF (17/306 Pages) Microchip Technology – High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D

4.0 ON-CHIP OSCILLATOR
CIRCUIT
The internal oscillator circuit is used to generate the
device clock. Four device clock periods generate an
internal instruction clock (TCY).
There are four modes that the oscillator can operate in.
They are selected by the device configuration bits dur-
ing device programming. These modes are:
• LF
• XT
• EC
• RC
Low Frequency (FOSC  2 MHz)
Standard Crystal/Resonator Frequency
(2 MHz  FOSC  33 MHz)
External Clock Input
(Default oscillator configuration)
External Resistor/Capacitor
(FOSC  4 MHz)
There are two timers that offer necessary delays on
power-up. One is the Oscillator 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 (nomi-
nal) on POR and BOR. The PWRT is designed to keep
the part in RESET while the power supply stabilizes.
With these two timers on-chip, most applications need
no external RESET circuitry.
SLEEP mode is designed to offer a very low current
power-down mode. The user can wake from SLEEP
through external RESET, Watchdog Timer Reset, or
through an interrupt.
Several oscillator options are made available to allow
the part to better fit the application. The RC oscillator
option saves system cost while the LF crystal option
saves power. Configuration bits are used to select var-
ious options.
4.1 Oscillator Configurations
4.1.1
OSCILLATOR TYPES
The PIC17CXXX can be operated in four different oscil-
lator modes. The user can program two configuration
bits (FOSC1:FOSC0) to select one of these four
modes:
• LF
• XT
• EC
• RC
Low Power Crystal
Crystal/Resonator
External Clock Input
Resistor/Capacitor
The main difference between the LF and XT modes is
the gain of the internal inverter of the oscillator circuit,
which allows the different frequency ranges.
For more details on the device configuration bits, see
Section 17.0.
PIC17C7XX
4.1.2
CRYSTAL OSCILLATOR/CERAMIC
RESONATORS
In XT or LF modes, a crystal or ceramic resonator is con-
nected to the OSC1/CLKIN and OSC2/CLKOUT pins to
establish oscillation (Figure 4-2). The PIC17CXXX oscil-
lator design requires the use of a parallel cut crystal. Use
of a series cut crystal may give a frequency out of the
crystal manufacturers specifications.
For frequencies above 24 MHz, it is common for the
crystal to be an overtone mode crystal. Use of overtone
mode crystals require a tank circuit to attenuate the
gain at the fundamental frequency. Figure 4-3 shows
an example circuit.
4.1.3
OSCILLATOR/RESONATOR
START-UP
As the device voltage increases from Vss, the oscillator
will start its oscillations. The time required for the oscil-
lator to start oscillating depends on many factors.
These include:
• Crystal/resonator frequency
• Capacitor values used (C1 and C2)
• Device VDD rise time
• System temperature
• Series resistor value (and type) if used
• Oscillator mode selection of device (which selects
the gain of the internal oscillator inverter)
Figure 4-1 shows an example of a typical oscillator/
resonator start-up. The peak-to-peak voltage of the
oscillator waveform can be quite low (less than 50% of
device VDD) when the waveform is centered at VDD/2
(refer to parameter #D033 and parameter #D043 in the
electrical specification section).
FIGURE 4-1:
OSCILLATOR/
RESONATOR START-UP
CHARACTERISTICS
Crystal Start-up Time
Time
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DS30289C-page 17