PIC16F684
3.2
Clock Source Modes
3.3
External Clock Modes
Clock source modes can be classified as external or
internal.
3.3.1 OSCILLATOR START-UP TIMER (OST)
If the PIC16F684 is configured for LP, XT or HS modes,
the Oscillator Start-up Timer (OST) counts 1024 oscil-
lations from the OSC1 pin, following a Power-on Reset
(POR) and 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 PIC16F684. 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 modules
(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 PIC16F684. The PIC16F684 has two
internal oscillators, the 8 MHz High-Frequency
Internal Oscillator (HFINTOSC) and 31 kHz
Low-Frequency Internal Oscillator (LFINTOSC).
The system clock can be selected between external or
internal clock sources via the System Clock Selection
(SCS) bit (see Section 3.5 “Clock Switching”).
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 3.6
“Two-Speed Clock Start-up Mode”).
TABLE 3-1:
OSCILLATOR DELAY EXAMPLES
Switch From
Switch To
Frequency
Oscillator Delay
LFINTOSC
HFINTOSC
31 kHz
125 kHz to 8 MHz
Sleep/POR
5 µs-10 µs (approx.) CPU Start-up(1)
Sleep/POR
LFINTOSC (31 kHz)
Sleep/POR
EC, RC
EC, RC
DC – 20 MHz
DC – 20 MHz
LP, XT, HS
HFINTOSC
31 kHz to 20 MHz
125 kHz to 8 MHz
1024 Clock Cycles (OST)
LFINTOSC (31 kHz)
1 µs (approx.)
Note 1: The 5 µs to 10 µs start-up delay is based on a 1 MHz system clock.
3.3.2
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 pin and the RA5 pin is
available for general purpose I/O. Figure 3-2 shows the
pin connections for EC mode.
OSC1/CLKIN
Clock from
Ext. System
PIC16F684
I/O (OSC2)
RA4
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 PIC16F684 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.
DS41202C-page 20
Preliminary
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
