
2005 Microchip Technology Inc.
Preliminary
DS41232B-page 31
PIC12F635/PIC16F636/639
3.3.2
EC MODE
The External Clock (EC) mode allows an externally
generated logic level as the system clock source.
When operating in this mode, an external clock source
is connected to the OSC1 pin and the RA5 pin is
available for general purpose I/O.
Figure 3-2 shows the
pin connections for EC mode.
The Oscillator Start-up Timer (OST) is disabled when
EC mode is selected. Therefore, there is no delay in
operation after a Power-on Reset (POR) or wake-up
from Sleep. Because the PIC12F635/PIC16F636/639
design is fully static, stopping the external clock input
will have the effect of halting the device while leaving all
data intact. Upon restarting the external clock, the
device will resume operation as if no time had elapsed.
FIGURE 3-2:
EXTERNAL CLOCK (EC)
MODE OPERATION
3.3.3
LP, XT, HS MODES
The LP, XT and HS modes support the use of quartz
crystal resonators or ceramic resonators connected to
selects a low, medium or high gain setting of the
internal inverter-amplifier to support various resonator
types and speed.
LP Oscillator mode selects the lowest gain setting of
the
internal
inverter-amplifier.
LP
mode
current
consumption is the least of the three modes. This mode
is best suited to drive resonators with a low drive level
specification, for example, tuning fork type crystals.
XT Oscillator mode selects the intermediate gain
setting of the internal inverter-amplifier. XT mode
current consumption is the medium of the three modes.
This mode is better suited to drive resonators with a
medium drive level specification, for example, low-
frequency AT-cut quartz crystal resonators.
HS Oscillator mode selects the highest gain setting of
the
internal
inverter-amplifier.
HS
mode
current
consumption is the highest of the three modes. This
mode is better suited for resonators that require a high
drive setting, for example, high-frequency AT-cut
quartz crystal resonators or ceramic resonators.
quartz crystal and ceramic resonators, respectively.
FIGURE 3-3:
QUARTZ CRYSTAL
OPERATION (LP, XT OR
HS MODE)
FIGURE 3-4:
CERAMIC RESONATOR
OPERATION
(XT OR HS MODE)
OSC1/CLKIN
I/O (OSC2)
RA4
Clock from
Ext. System
PIC12F635/PIC16F636/639
Note 1: Quartz
crystal
characteristics
vary
according
to
type,
package
and
manufacturer. The user should consult the
manufacturer data sheets for specifications
and recommended application.
2: Always verify oscillator performance over
the VDD and temperature range that is
expected for the application.
Note 1:
A series resistor (RS) may be required for
quartz crystals with low drive level.
2:
The value of RF varies with the Oscillator
mode selected (typically between 2 M
Ω to
10 M
Ω).
C1
C2
Quartz
OSC2
RS(1)
OSC1
RF(2)
Sleep
PIC12F635/PIC16F636/639
Crystal
To Internal
Logic
Note
1: A series resistor (RS) may be required for
ceramic resonators with low drive level.
2: The value of RF varies with the Oscillator
mode selected (typically between 2 M
Ω to
10 M
Ω).
3: An additional parallel feedback resistor (RP)
may be required for proper ceramic resonator
operation (typical value 1 M
Ω).
C1
C2 Ceramic
OSC2
RS(1)
OSC1
RP(3)
Resonator
RF(2)
Sleep
PIC12F635/PIC16F636/639
To Internal
Logic