Datasheet

PIC16C745/765
DS41124D-page 100 Preliminary  1999-2013 Microchip Technology Inc.
13.2 Oscillator Configurations
13.2.1 OSCILLATOR TYPES
The PIC16C745/765 can be operated in four different
oscillator modes. The user can program a configuration
bit (FOSC0) to select one of these four modes:
• EC External Clock
• E4 External Clock with internal PLL enabled
• HS High Speed Crystal/Resonator
• H4 High Speed Crystal/Resonator with
internal PLL enabled
13.2.2 CRYSTAL OSCILLATOR/CERAMIC
RESONATORS
In HS mode, a crystal or ceramic resonator is con-
nected to the OSC1/CLKIN and OSC2/CLKOUT pins to
establish oscillation (Figure 13-1). The PIC16C745/
765 oscillator 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. When in
HS mode, the device can have an external clock
source to drive the OSC1/CLKIN pin (Figure 13-2). In
this mode, the oscillator start-up timer is active for a
period of 1024*T
OSC. See the PIC Mid-Range MCU
Reference Manual (DS33023) for details on building an
external oscillator.
FIGURE 13-1: CRYSTAL/CERAMIC
RESONATOR OPERATION
(HS OSC CONFIGURATION)
TABLE 13-1: CERAMIC RESONATORS
TABLE 13-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
13.2.3 H4 MODE
In H4 mode, a PLL module is switched on in-line with
the clock provided across OSC1 and OCS2. The output
of the PLL drives F
INT.
13.2.4 PLL
An on-board 4x PLL provides a cheap means of gener-
ating a stable 24 MHz F
INT, using an external 6 MHz
resonator. After power-up, a PLL settling time of less
than TPLLRT is required.
C1
C2
XTAL
OSC2
Note1
OSC1
Rf
SLEEP
To internal
logic
PIC16C745/765
Rs
Note 1: A series resistor may be required for AT strip cut
crystals.
Ranges Tested:
Mode Freq OSC1 OSC2
HS 6.0 MHz 10 - 68 pF 10 - 68 pF
These values are for design guidance only. See notes at
bottom of page.
Osc Type Crystal
Freq
Cap. Range
C1
Cap. Range
C2
HS 6.0 MHz
15 - 33 pF 15 - 33 pF
These values are for design guidance only. See notes at
bottom of page.
Note 1: Higher capacitance increases the stability
of the oscillator, but also increases the start-
up time.
2: Since each resonator/crystal has its own
characteristics, the user should consult the
resonator/crystal manufacturer for appropri-
ate values of external components.
3: Rs may be required in HS mode to avoid
overdriving crystals with low drive level
specification.
4: When migrating from other PIC devices,
oscillator performance should be verified.
5: Users should consult the USB Specification
1.1 to ensure their resonator/crystal oscilla-
tor meets the required jitter limits for USB
operation.