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PIC12F683 Datasheet(PDF) 22 Page - Microchip Technology |
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PIC12F683 Datasheet(HTML) 22 Page - Microchip Technology |
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22 / 148 page  PIC12F683 DS41211B-page 20 Preliminary 2004 Microchip Technology Inc. 3.2 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 modules (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 PIC12F683. The PIC12F683 has two internal oscillators: the 8 MHz High-Frequency Internal Oscillator (HFINTOSC) and 31 kHz Low-Frequency Internal Oscillator (LFINTOSC). The system clock can be selected between external or internal clock sources via the System Clock Selection (SCS) bit (see Section 3.5 “Clock Switching”). 3.3 External Clock Modes 3.3.1 OSCILLATOR START-UP TIMER (OST) If the PIC12F683 is configured for LP, XT or HS modes, the Oscillator Start-up Timer (OST) counts 1024 oscil- lations from the OSC1 pin, following a Power-on Reset (POR) and 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 PIC12F683. 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 3.6 “Two-Speed Clock Start-up Mode”). TABLE 3-1: OSCILLATOR DELAY EXAMPLES 3.3.2 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 pin and the GP5 pin 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 PIC12F683 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 Switch From Switch To Frequency Oscillator Delay Sleep/POR LFINTOSC HFINTOSC 31 kHz 125 kHz-8 MHz 5 µs–10 µs (approx.) CPU Start-up(1) Sleep/POR EC, RC DC – 20 MHz LFINTOSC (31 kHz) EC, RC DC – 20 MHz Sleep/POR LP, XT, HS 31 kHz-20 MHz 1024 Clock Cycles (OST) LFINTOSC (31 kHz) HFINTOSC 125 kHz-8 MHz 1 µs (approx.) Note 1: The 5 µs–10 µs start-up delay is based on a 1 MHz system clock. OSC1/CLKIN I/O (OSC2) GP4 Clock from Ext. System PIC12F683 |
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