Datasheet

PIC16F72X/PIC16LF72X
DS41341E-page 94 © 2009 Microchip Technology Inc.
FIGURE 7-4: CERAMIC RESONATOR
OPERATION
(XT OR HS MODE)
7.6.4 EXTERNAL RC MODES
The external Resistor-Capacitor (RC) modes support
the use of an external RC circuit. This allows the
designer maximum flexibility in frequency choice while
keeping costs to a minimum when clock accuracy is not
required. There are two modes: RC and RCIO.
In RC mode, the RC circuit connects to OSC1.
OSC2/CLKOUT outputs the RC oscillator frequency
divided by 4. This signal may be used to provide a clock
for external circuitry, synchronization, calibration, test
or other application requirements. Figure 7-5 shows
the external RC mode connections.
FIGURE 7-5: EXTERNAL RC MODES
In RCIO mode, the RC circuit is connected to OSC1.
OSC2 becomes an additional general purpose I/O pin.
The RC oscillator frequency is a function of the supply
voltage, the resistor (REXT) and capacitor (CEXT) values
and the operating temperature. Other factors affecting
the oscillator frequency are:
• threshold voltage variation
• component tolerances
• packaging variations in capacitance
The user also needs to take into account variation due
to tolerance of external RC components used.
TABLE 7-1: SUMMARY OF REGISTERS ASSOCIATED WITH CLOCK SOURCES
Note 1: A series resistor (RS) may be required for
ceramic resonators with low drive level.
2: The value of R
F varies with the Oscillator mode
selected.
3: An additional parallel feedback resistor (R
P)
may be required for proper ceramic resonator
operation.
C1
C2
Ceramic
R
S
(1)
OSC1/CLKIN
RF
(2)
Sleep
To Internal
Logic
PIC
®
MCU
RP
(3)
Resonator
OSC2/CLKOUT
OSC2/CLKOUT
(1)
CEXT
REXT
PIC
®
MCU
OSC1/CLKIN
F
OSC/4 or
Internal
Clock
VDD
VSS
Recommended values: 10 kΩ ≤ REXT ≤ 100 kΩ, <3V
3 kΩ ≤ R
EXT ≤ 100 kΩ, 3-5V
C
EXT > 20 pF, 2-5V
Note 1: Alternate pin functions are described in
Section 6.1 “Alternate Pin Function”.
2: Output depends upon RC or RCIO clock mode.
I/O
(2)
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
POR, BOR
Value on
all other
Resets
(1)
CONFIG1
(1)
— CP MCLRE PWRTE WDTE FOSC2 FOSC1 FOSC0 — —
OSCCON
— — IRCF1 IRCF0 ICSL ICSS — — --10 qq-- --10 qq--
OSCTUNE
— — TUN5 TUN4 TUN3 TUN2 TUN1 TUN0 --00 0000 --uu uuuu
Legend: x = unknown, u = unchanged, – = unimplemented locations read as ‘0’. Shaded cells are not used by oscillators.
Note 1: See Configuration Word 1 (Register 8-1) for operation of all bits.