PIC16F684T-E Datasheet, PDF (23/192 Pages) Microchip Technology – 14-Pin, Flash-Based 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F684
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.
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”).
TABLE 3-1: OSCILLATOR DELAY EXAMPLES
Switch From
Switch To
Frequency
Sleep/POR
Sleep/POR
LFINTOSC (31 kHz)
Sleep/POR
LFINTOSC (31 kHz)
LFINTOSC
HFINTOSC
EC, RC
EC, RC
LP, XT, HS
HFINTOSC
31 kHz
125 kHz to 8 MHz
DC – 20 MHz
DC – 20 MHz
32 kHz to 20 MHz
125 kHz to 8 MHz
Oscillator Delay
Oscillator Warm-Up Delay (TWARM)
2 cycles
1 cycle of each
1024 Clock Cycles (OST)
1 μs (approx.)
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.
FIGURE 3-2:
EXTERNAL CLOCK (EC)
MODE OPERATION
Clock from
Ext. System
I/O
OSC1/CLKIN
PIC^® MCU
OSC2/CLKOUT⁽¹⁾
Note 1: Alternate pin functions are listed in the
Section 1.0 “Device Overview”.
© 2007 Microchip Technology Inc.
DS41202F-page 21