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Oscillator Options for the PIC18F452: Modes and Configuration

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quinlan-kennedy
19 diapositives · 327 Views · Nov 12, 2014

The PIC18F452 microcontroller offers eight distinct oscillator modes, enabling versatile applications. Users can program configuration bits (FOSC2, FOSC1, FOSC0) to select from Low Power (LP), Crystal (XT), High Speed (HS), or PLL-enhanced modes. The RC option provides cost-effective solutions, though with limited stability. Advanced features include external clock inputs and Phase Locked Loop (PLL) capabilities for frequency multiplication. This guide covers selecting oscillators, managing power-up delays, and switching oscillator sources efficiently.

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Oscillator Options for the PIC18F452: Modes and Configuration
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  1. ECT 357 D:2 Oscillator Options

  2. Today’s Quote: The secret of achievement is to not let what you’re doing get to you before you get to it. Commit thy works unto the Lord, and thy thoughts shall be established. Proverbs 16:3

  3. Oscillator Options • The PIC18F452 can be operated in eight different oscillator modes. The user can program three configuration bits (FOSC2, FOSC1, and FOSC0) to select one of these eight modes:

  4. Oscillator Modes • LP Low Power Crystal • XT Crystal/Resonator • HS High Speed Crystal/Resonator • HS + PLL High Speed Crystal/Resonator with PLL enabled • RC External Resistor/Capacitor • RCIO External Resistor/Capacitor with I/O pin enabled • EC External Clock • ECIO External Clock with I/O pin enabled

  5. Crystal Oscillator/Ceramic Resonators • In XT, LP, HS or HS+PLL Oscillator modes, a crystal or ceramic resonator is connected to the OSC1 and OSC2 pins to establish oscillation.

  6. External Clock Input (HS,XT,LP) • An external clock source may also be connected to the OSC1 pin in the HS, XT and LP modes.

  7. RC Option • The “RC” and “RCIO” device options are cheap, however are not relatively stable. The RC oscillator frequency is a function of the supply voltage, the resistor (REXT) and capacitor (CEXT) values and the operating temperature. • In the RC Oscillator mode, the oscillator frequency divided by 4 is available on the OSC2 pin. This signal may be used for test purposes or to synchronize other logic.

  8. RC Mode Diagram

  9. External Clock Input (EC and ECIO) • The EC and ECIO Oscillator modes require an external clock source to be connected to the OSC1 pin. The feedback device between OSC1 and OSC2 is turned off in these modes to save current. There is no oscillator start-up time required after a Power-on Reset or after a recovery from SLEEP mode. • In the EC Oscillator mode, the oscillator frequency divided by 4 is available on the OSC2 pin.

  10. HS/PLL • A Phase Locked Loop circuit is provided as a programmable option for users that want to multiply the frequency of the incoming crystal oscillator signal by 4. For an input clock frequency of 10 MHz, the internal clock frequency will be multiplied to 40 MHz. This is useful for customers who are concerned with EMI due to high frequency crystals. The PLL can only be enabled when the oscillator configuration bits are programmed for HS mode.

  11. HS/PLL Diagram

  12. Oscillator Switching Feature • The system clock can be switched from the main oscillator to an alternate low frequency clock source (Timer 1). If a low frequency crystal (32 kHz, for example) has been attached to the Timer1 oscillator pins and the Timer1 oscillator has been enabled, the device can switch to a Low Power Execution system clock sources.

  13. Device Clock Sources

  14. System Clock Switch Bit • The system clock source switching is performed under software control. The system clock switch bit, SCS(OSCCON<0>) controls the clock switching. When the SCS bit is ’0’, the system clock source comes from the main oscillator that is selected by the FOSC configuration bits in Configuration Register1H. When the SCS bit is set, the system clock source will come from the Timer1 oscillator. The SCS bit is cleared on all forms of RESET.

  15. Oscillator Configuration Register

  16. Oscillator Transitions • The PIC18F452 device contains circuitry to prevent “glitches” when switching between oscillator sources. Essentially, the circuitry waits for eight rising edges of the clock source that the processor is switching to. This ensures that the new clock source is stable and that its pulse width will not be less than the shortest pulse width of the two clock sources.

  17. SLEEP Mode • When the device executes a SLEEP instruction, the on-chip clocks and oscillator are turned off and the device is held at the beginning of an instruction cycle (Q1 state). With the oscillator off, the OSC1 and OSC2 signals will stop oscillating.

  18. Power Up Delays • Power up delays are controlled by two timers. The delays ensure that the device is kept in RESET, until the device power supply and clock are stable. The first timer is the Power-up Timer (PWRT), which optionally provides a fixed delay of 72 ms (nominal) on power-up only (POR and BOR). The second timer is the Oscillator Start-up Timer (OST), intended to keep the chip in RESET until the crystal oscillator is stable.