Intel 8XC251SP, 8XC251SA, 8XC251SQ, 8XC251SB Clock Sources, On-chip Oscillator Crystal, XTAL1 XTAL2

MINIMUM HARDWARE SETUP

11.3 CLOCK SOURCES

The 8XC251Sx can obtain the system clock signal from an external clock source (Figure 11-3) or it can generate the clock signal using the on-chip oscillator amplifier and external capacitors and resonator (Figure 11-2).

11.3.1 On-chip Oscillator (Crystal)

This clock source uses an external quartz crystal connected from XTAL1 to XTAL2 as the fre- quency-determining element (Figure 11-2). The crystal operates in its fundamental mode as an inductive reactance in parallel resonance with capacitance external to the crystal. Oscillator de- sign considerations include crystal specifications, operating temperature range, and parasitic board capacitance. Consult the crystal manufacturer’s data sheet for parameter values. With high quality components, C1 = C2 = 30 pF is adequate for this application.

Pins XTAL1 and XTAL2 are protected by on-chip electrostatic discharge (ESD) devices, D1 and D2, which are diodes parasitic to the RF FETs. They serve as clamps to VCC and VSS. Feedback resistor RF in the inverter circuit, formed from paralleled n- and p- channel FETs, permits the PD bit in the PCON register (Figure 12-1 on page 12-2) to disable the clock during powerdown.

Noise spikes at XTAL1 and XTAL2 can disrupt microcontroller timing. To minimize coupling between other digital circuits and the oscillator, locate the crystal and the capacitors near the chip and connect to XTAL1, XTAL2, and VSS with short, direct traces. To further reduce the effects of noise, place guard rings around the oscillator circuitry and ground the metal crystal case.

Quartz Crystal

or Ceramic Resonator

C1

C2

8XC251Sx

XTAL1

XTAL2

To Internal

Timing Circuit

VCC

D1 PD#

D2

RF

A4143-02

Figure 11-2. CHMOS On-chip Oscillator

11-3