PIC16F946 Datasheet, PDF (73/274 Pages) Microchip Technology – 64-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver and nanoWatt Technology

PIC16F946
4.0 CLOCK SOURCES
4.1 Overview
The PIC16F946 has a wide variety of clock sources
and selection features to allow it to be used in a wide
range of applications while maximizing performance
and minimizing power consumption. Figure 4-1
illustrates a block diagram of the PIC16F946 clock
sources.
Clock sources can be configured from external oscillators,
quartz crystal resonators, ceramic resonators, and
Resistor-Capacitor (RC) circuits. In addition, the system
clock source can be configured from one of two internal
oscillators, with a choice of speeds selectable via
software. Additional clock features include:
• Selectable system clock source between external
or internal via software.
• Two-Speed Clock Start-up mode, which
minimizes latency between external oscillator
start-up and code execution.
• Fail-Safe Clock Monitor (FSCM) designed to
detect a failure of the external clock source (LP,
XT, HS, EC or RC modes) and switch to the
Internal Oscillator.
The PIC16F946 can be configured in one of eight clock
modes.
1. EC – External clock with I/O on RA6.
2. LP – Low-gain Crystal or Ceramic Resonator
Oscillator mode.
3. XT – Medium-gain Crystal or Ceramic Resonator
Oscillator mode.
4. HS – High-gain Crystal or Ceramic Resonator
mode.
5. RC – External Resistor-Capacitor (RC) with
FOSC/4 output on RA6.
6. RCIO – External Resistor-Capacitor with I/O on
RA6.
7. INTOSC – Internal oscillator with FOSC/4 output
on RA6 and I/O on RA7.
8. INTOSCIO – Internal oscillator with I/O on RA6
and RA7.
Clock source modes are configured by the FOSC<2:0>
bits in the Configuration Word register (see
Section 16.0 “Special Features of the CPU”). The
internal clock can be generated by two oscillators. The
HFINTOSC is a high-frequency calibrated oscillator.
The LFINTOSC is a low-frequency uncalibrated
oscillator.
FIGURE 4-1:
PIC16F946 SYSTEM CLOCK BLOCK DIAGRAM
OSC2
OSC1
External Oscillator
Sleep
Internal Oscillator
HFINTOSC
8 MHz
LFINTOSC
31 kHz
FOSC<2:0>
(Configuration Word)
SCS
(OSCCON<0>)
LP, XT, HS, RC, RCIO, EC
IRCF<2:0>
(OSCCON<6:4>)
8 MHz
111
4 MHz
110
2 MHz
101
1 MHz
100
500 kHz
011
250 kHz
010
125 kHz ₀₀₁
31 kHz ₀₀₀
System Clock
(CPU and Peripherals)
LCD Module
Power-up Timer (PWRT)
Watchdog Timer (WDT)
Fail-Safe Clock Monitor (FSCM)
© 2005 Microchip Technology Inc.
Preliminary
DS41265A-page 71