PIC16F688

Manufacturer Part NumberPIC16F688
ManufacturerMicrochip Technology Inc.
PIC16F688 datasheet
 


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Page 25/202:

MODE OPERATION

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3.3
Clock Source Modes
Clock Source modes can be classified as external or
internal.
• 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.
TABLE 3-1:
OSCILLATOR DELAY EXAMPLES
Switch From
Switch To
LFINTOSC
Sleep/POR
HFINTOSC
Sleep/POR
EC, RC
LFINTOSC (31 kHz)
EC, RC
Sleep/POR
LP, XT, HS
LFINTOSC (31 kHz)
HFINTOSC
3.4.2
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 input and the OSC2 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 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.
© 2007 Microchip Technology Inc.
PIC16F688
3.4
External Clock Modes
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.
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”).
Frequency
Oscillator Delay
31 kHz
Oscillator Warm-Up Delay (T
125 kHz to 8 MHz
DC – 20 MHz
2 instruction cycles
DC – 20 MHz
1 cycle of each
32 kHz to 20 MHz
1024 Clock Cycles (OST)
1 μs (approx.)
125 kHz to 8 MHz
FIGURE 3-2:
EXTERNAL CLOCK (EC)

MODE OPERATION

OSC1/CLKIN
Clock from
Ext. System
OSC2/CLKOUT
I/O
Note 1:
Alternate pin functions are listed in
Section 1.0 “Device Overview”.
)
WARM
®
PIC
MCU
(1)
DS41203D-page 23