PIC16C5X clock oscillator-EEWORLD

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The PIC16C5X series has four oscillator options: standard crystal/ceramic oscillator (XT), low frequency low power oscillator (LP), high speed oscillator (HS) and resistor-capacitor oscillator (RC). In EPROM-type microcontrollers and microcontrollers with the suffix RC, the oscillator circuit can be selected by programming the configuration bit EPROM. The oscillator in the QTP microcontroller is set at the factory and can be tested by specific methods.

[1]. Quartz crystal oscillator.

In the XT, HS or LP mode of the PIC16C5X microcontroller, OSC1 and OSC2 require an external quartz crystal or ceramic oscillator, as shown in the figure below. The HS oscillator requires a series resistor Rs, and the value of Rs ranges from 100 ohms to 1k ohms.

When the microcontroller uses a quartz crystal oscillator or a ceramic oscillator, the capacitor used and the frequency generated are different. A large capacitor value is conducive to oscillation stability, but it prolongs the start-up time.

When using a ceramic oscillator, the selected capacitor capacity and the generated oscillation frequency are shown in Table 1. When using a quartz oscillator, the selected capacitor capacity and the generated oscillation frequency are shown in Table 2.

Table 1 Ceramic oscillators and matching capacitors
Oscillator Type	Ceramic oscillator frequency	Capacitance (C1=C2)/pF
XT	455kHz	150-330
	2.0MHz	20-330
	4.0MHz	20-330
HS	8.0MHz	20-330

[2]. RC oscillator.

For applications where timing accuracy is not required, RC oscillators can significantly reduce costs. The frequency of the RC oscillator is related to the power supply voltage VDD, the external resistor REXT, the external capacitor CEXT, and the operating temperature. Due to inconsistencies in parameters during the manufacturing process, the frequency of the oscillator may be different for each microcontroller. In addition, the different lead distribution capacitances of different packaging methods will also affect the oscillation frequency, which is particularly obvious when the value of the external capacitor CEXT is small. Users also need to consider the errors of the external resistors REXT and CEXT used.

Table 2 Quartz crystal oscillator and matching capacitor
Oscillator Type	Oscillator frequency	Capacitance C1/pF	Capacitance C2/pF
LP	32kHz	15-30	15
XT	100kHz	15-30	200-200
	200kHz	15-30	100-200
	455kHz	15-30	15-100
	1MHz	15-30	15-30
	2MHz	15	15
	4MHz	15	15
HS	4MHz	15	15
	8MHz	15	15
	20MHz	15	15

Figure 2 is the wiring diagram of the RC oscillation circuit. When the value of the external resistor REXT is less than 2.2kΩ, the oscillator may not work stably or even fail to oscillate. When the resistance REXT is too large (such as greater than 1MΩ), the oscillator is easily affected by noise, humidity and leakage interference and becomes unstable. We recommend that the value range of REXT be 5-100kΩ.

The oscillator can work without an external capacitor, but in order to prevent interference and improve stability, it is recommended to use a capacitor of more than 20pF. When using a small capacitor, the oscillation frequency will change due to the influence of the distributed capacitance of the printed board and the package leads. Table 3 shows the oscillation frequency at different resistance and capacitance values. When RC oscillates, OSC2 is output as CLKOUT (CLKOUT=fOSC/4).

Table 3 Oscillation frequency of RC oscillator with different resistance and capacitance
CEXT/pF	REXT/kΩ	Average oscillation frequency fOSC@5V,25℃
20	3.3	4.973MHz	±27%
	5	3.82MHz	±21%
	10	2.22MHz	±21%
	100	262.15kHz	±31%
100	3.3	1.63MHz	±13%
	5	1.19MHz	±13%
	100	684.64kHz	±18%
	100	71.56kHz	±25%
300	3.3	660kHz	±10%
	5	484.1kHz	±14%
	100	267.63kHz	±15%
	100	29.44kHz	±19%

[3]. External oscillation.

In the PIC16C5X microcontroller, an external oscillator source can also be used to provide a clock source. The external clock is input into the microcontroller from the OSC1 terminal, and OSC2 is left empty. The external clock is only used for microcontrollers with HS, XT and LP oscillators. See Figure 3.

Keywords：PIC16C5X Reference address：PIC16C5X clock oscillator

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Recommended ReadingLatest update time:2024-11-16 13:23

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