PIC16F631/677/685/687/689/690
3.3
Clock Source Modes
3.4
External Clock Modes
Clock Source modes can be classified as external or
internal.
3.4.1 OSCILLATOR START-UP TIMER (OST)
If the Oscillator module is configured for LP, XT or HS
modes, the Oscillator Start-up Timer (OST) counts
1024 oscillations from OSC1. This occurs following a
Power-on Reset (POR) and when the Power-up Timer
(PWRT) has expired (if configured), or a wake-up from
Sleep. During this time, the program counter does not
increment and program execution is suspended. The
OST ensures that the oscillator circuit, using a quartz
crystal resonator or ceramic resonator, has started and
is providing a stable system clock to the Oscillator
module. When switching between clock sources, a
delay is required to allow the new clock to stabilize.
These oscillator delays are shown in Table 3-1.
• External Clock modes rely on external circuitry for
the clock source. Examples are: Oscillator mod-
ules (EC mode), quartz crystal resonators or
ceramic resonators (LP, XT and HS modes) and
Resistor-Capacitor (RC) mode circuits.
• Internal clock sources are contained internally
within the Oscillator module. The Oscillator
module has two internal oscillators: the 8 MHz
High-Frequency Internal Oscillator (HFINTOSC)
and the 31 kHz Low-Frequency Internal Oscillator
(LFINTOSC).
The system clock can be selected between external or
internal clock sources via the System Clock Select
(SCS) bit of the OSCCON register. See Section 3.6
“Clock Switching” for additional information.
In order to minimize latency between external oscillator
start-up and code execution, the Two-Speed Clock
Start-up mode can be selected (see Section 3.7 “Two-
Speed Clock Start-up Mode”).
TABLE 3-1:
Switch From
OSCILLATOR DELAY EXAMPLES
Switch To
Frequency
Oscillator Delay
LFINTOSC
HFINTOSC
31 kHz
125 kHz to 8 MHz
DC – 20 MHz
Sleep/POR
Oscillator Warm-up Delay (TWARM)
Sleep/POR
EC, RC
2 cycles
LFINTOSC (31 kHz)
Sleep/POR
EC, RC
DC – 20 MHz
1 cycle of each
1024 Clock Cycles (OST)
1 μs (approx.)
LP, XT, HS
HFINTOSC
32 kHz to 20 MHz
125 kHz to 8 MHz
LFINTOSC (31 kHz)
3.4.2
EC MODE
FIGURE 3-2:
EXTERNAL CLOCK (EC)
MODE OPERATION
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 input and the OSC2 is available
for general purpose I/O. Figure 3-2 shows the pin
connections for EC mode.
OSC1/CLKIN
Clock from
Ext. System
PIC® MCU
(1)
I/O
OSC2/CLKOUT
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 PIC® MCU 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.
Note 1: Alternate pin functions are listed in the
Section 1.0 “Device Overview”.
© 2007 Microchip Technology Inc.
DS41262D-page 49
MICROCHIP [ MICROCHIP ]
