2009 Microchip Technology Inc.
DS40044G-page 101
PIC16F627A/628A/648A
14.2.6
RC OSCILLATOR
For applications where precise timing is not a
requirement, the RC oscillator option is available. The
operation and functionality of the RC oscillator is
dependent upon a number of variables. The RC oscillator
frequency is a function of:
Supply voltage
Resistor (REXT) and capacitor (CEXT) values
Operating temperature
The oscillator frequency will vary from unit-to-unit due
to normal process parameter variation. The difference
in lead frame capacitance between package types will
also affect the oscillation frequency, especially for low
CEXT values. The user also needs to account for the
tolerance of the external R and C components.
connected.
FIGURE 14-5:
RC OSCILLATOR MODE
The RC Oscillator mode has two options that control
the unused OSC2 pin. The first allows it to be used as
a general purpose I/O port. The other configures the
pin as an output providing the FOSC signal (internal
clock divided by 4) for test or external synchronization
purposes.
14.2.7
CLKOUT
The PIC16F627A/628A/648A can be configured to
provide a clock out signal by programming the
Configuration Word. The oscillator frequency, divided
by 4 can be used for test purposes or to synchronize
other logic.
14.2.8
SPECIAL FEATURE: DUAL-SPEED
OSCILLATOR MODES
A software programmable dual-speed oscillator mode
is provided when the PIC16F627A/628A/648A is
configured in the INTOSC oscillator mode. This feature
allows users to dynamically toggle the oscillator speed
between 4 MHz and 48 kHz nominal in the INTOSC
mode. Applications that require low-current power
savings, but cannot tolerate putting the part into Sleep,
may use this mode.
There is a time delay associated with the transition
between fast and slow oscillator speeds. This oscillator
speed transition delay consists of two existing clock
pulses and eight new speed clock pulses. During this
clock speed transition delay, the System Clock is halted
causing the processor to be frozen in time. During this
delay, the program counter and the CLKOUT stop.
The OSCF bit in the PCON register is used to control
14.3
Reset
The PIC16F627A/628A/648A differentiates between
various kinds of Reset:
a) Power-on Reset (POR)
b) MCLR Reset during normal operation
c)
MCLR Reset during Sleep
d) WDT Reset (normal operation)
e) WDT wake-up (Sleep)
f)
Brown-out Reset (BOR)
Some registers are not affected in any Reset condition;
their status is unknown on POR and unchanged in any
other Reset. Most other registers are reset to a “Reset
state” on Power-on Reset, Brown-out Reset, MCLR
Reset, WDT Reset and MCLR Reset during Sleep.
They are not affected by a WDT wake-up, since this is
viewed as the resumption of normal operation. TO and
PD bits are set or cleared differently in different Reset
used in software to determine the nature of the Reset.
See
Table 14-7 for a full description of Reset states of
all registers.
A simplified block diagram of the on-chip Reset circuit
The MCLR Reset path has a noise filter to detect and
ignore small pulses. See
Table 17-7 for pulse width
specification.
CEXT
VDD
REXT
VSS
PIC16F627A/628A/648A
FOSC/4
Internal
Clock
CLKIN
RA7/OSC1/
RA6/OSC2/CLKOUT
Recommended Values: 3 k
Ω ≤ REXT ≤ 100 kΩ (VDD ≥ 3.0V)
10 k
Ω ≤REXT ≤100 kΩ(VDD< 3.0V)
CEXT
> 20 pF