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The Clock Module
A typical configuration uses a canned oscillator (4 MHz or 50 MHz) with the EXTCLK
input selected as OSCCLK, and a 32.768 kHz or 38.4 kHz crystal at EXTAL and XTAL to
provide PITRTCLK.
14.2.1.1 Off-Chip Oscillator Input (EXTCLK)
The external clock input EXTCLK is generated from an external source, which is typically
a canned oscillator. The acceptable frequency range of this input source is defined by:
1. The maximum operating frequency of the MPC850
2. The default SPLL multiplying factor (defined in Section 14.2.2.1, "SPLL Reset
Configuration")
3. The minimum operating frequency of the SPLL, which is 15 MHz
14.2.1.2 Crystal Oscillator Support (EXTAL and XTAL)
The MPC850 provides support for crystal oscillator circuits with the oscillator module
(OSCM). The OSCM has two different modes, supporting two different ranges of
frequencies: 30–50 kHz (referred to as 32 kHz mode) or 3-5 MHz (referred to as 4 MHz
mode). The mode of OSCM is selected simultaneously with SPLL configuration; refer to
Section 14.2.2.1, "SPLL Reset Configuration.
The clock source of OSCM can be provided by a crystal circuit or an external oscillator. If
an external oscillator is used, it should be connected to EXTAL, and XTAL should be left
unconnected. If a crystal circuit is used, it should be connected between EXTAL and
XTAL. The crystal circuit is composed of an on-chip inverting amplifier, an external
parallel resonant crystal, two capacitors, and two resistors, as shown in Figure 14-3.
EXTAL is the amplifier input for the crystal circuit; XTAL is the amplifier output.
Example values for the passive components of the crystal circuits are provided in
Figure 14-3 However, because this is a sensitive analog circuit, these values cannot be
guaranteed. These components may have to be tuned due to design-specific parasitic
capacitance variation due, for example, to layout and board composition. Careful
consideration must be given to component placement and layout, keeping components as
near as possible to the chip and keeping all trace lengths to a minimum. It should be noted
that the sensitivity of crystal circuits to external component values is so great that even
probing the circuit will change its behavior to the point that it may fail to resonate. In
practice, experimentation will be required to find an acceptable range of component values,
with the final design value being selected in the middle of this range.
Lastly, it should also be noted that future changes in the device technology (shrinks) may
change the characteristics of the input and output impedance of the on-chip amplifier.
Motorola reserves the right to perform these changes, and designers should be prepared to
modify their crystal circuits appropriately should these changes cause their crystal circuit
designs to fail. This risk should be taken into account when the design is performed; if
potential manufacturing downtime due to redesign of crystal circuits is unacceptable, a
canned oscillator circuit should be used instead.
MPC850 Family User's Manual
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