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PIC16C781/782
DS41171A-page 120
Preliminary
2001 Microchip Technology Inc.
FIGURE 14-2:
EC OSC
CONFIGURATION
14.2.4
RC MODE
For timing insensitive applications, the RC device
option offers additional cost savings. The RC oscillator
frequency is a function of:
supply voltage
resistor (REXT) and capacitor (CEXT) values
operating temperature
In addition, the oscillator frequency varies from unit to
unit due to normal process variation. The difference in
lead frame capacitance between package types also
affects the oscillation frequency, especially for low
CEXT values. The user should allow for variations due
to tolerance of external R and C components used.
nected to the PIC16C781/782. For REXT values below
2.2 k
, the oscillator operation may become unstable
or stop completely. For very high REXT values (e.g., 1
M
or greater), the oscillator becomes sensitive to:
noise
humidity
leakage
Microchip recommends keeping REXT between 3 k
and 100 k
.
Although the oscillator will operate with no external
capacitor (CEXT = 0 pF), we recommend using values
above 20 pF for noise and stability reasons. With no or
small external capacitance, the oscillation frequency
can vary dramatically due to changes in external
capacitances, such as board trace capacitance or
package lead frame capacitance.
to part due to normal process variation. The variation is
greater for large values of R (since leakage current
variations affect RC frequency more for large R) and for
small values of C (since variations of input capacitance
affect RC frequency more).
due to VDD for given REXT and CEXT values (or for fre-
quency variation due to operating temperature for
given R, C, and VDD values).
FIGURE 14-3:
RC OSCILLATOR MODE
14.2.5
INTRC MODE
The internal RC oscillator provides a fixed 4 MHz/37
kHz (nominal) system clock at VDD = 5V and 25°C. See
and temperature ranges. The INTRC oscillator does
not run during RESET.
14.2.6
DUAL SPEED OPERATION FOR
INTRC MODE
A software programmable slow speed mode is avail-
able with the INTRC oscillator. This feature allows the
firmware to dynamically toggle the oscillator speed
between normal and slow frequencies.
The nominal
slow frequency is 37 kHz. Applications that require low
current power savings, but cannot tolerate putting the
part into SLEEP, may use this mode.
The OSCF bit (PCON<3>) is used to control dual
details.
When changing the INTRC internal oscillator speed,
there is a brief period of time when the processor is
inactive. When the speed changes from fast to slow,
the processor inactive period is in the range of 100
S
to 300
S. For a speed change from slow to fast, the
processor is inactive between 1.25
S and 3.25 S,
nominal.
14.2.7
CLKOUT
In the INTRC and RC modes, the PIC16C781/782 can
be configured to provide a clock out signal by program-
ming the configuration word. The oscillator frequency,
divided by 4, can be used for test purposes or to syn-
chronize other logic.
In the INTRC and RC modes, if the CLKOUT output is
enabled, CLKOUT is held low during RESET.
RA7/OSC1/CLKIN
RA6/OSC2/CLKOUT/
I/O
Clock from
ext. system
PIC16C781/782
RA7
T1CKI
RA6/OSC2/CLKOUT/I1CKI
CEXT
VDD
REXT
VSS
PIC16C781/782
RA7/OSC1/
FOSC/4
Internal
Clock
CLKIN