Motorola MPC860 PowerQUICC User Manual page 398

Part IV. Hardware Interface
can be selected as OSCCLK, while the crystal oscillator circuit supplies a separate
low-frequency reference.
A typical conÞguration will use a canned oscillator (e.g 4 MHz or 50 MHz) with the
EXTCLK input selected as OSCCLK, and will use a 32.768 KHz or 38.4 KHz crystal at
EXTAL and XTAL to provide PITRTCLK.
15.2.1.1 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 deÞned by:
1. The maximum operating frequency of the MPC860
2. The default SPLL multiplying factor (deÞned in Section 15.2.2.1, ÒSPLL Reset
ConÞgurationÓ) and
3. The minimum operating frequency of the SPLL, which is 15 MHz.
15.2.1.2 Crystal Oscillator Support (EXTAL and XTAL)
The MPC860 provides support for crystal oscillator circuits with the oscillator module
(OSCM). The OSCM has two different modes, supporting two different ranges of
frequencies: 30Ð50 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 conÞguration; refer to
Section 15.2.2.1, ÒSPLL Reset ConÞguration.
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 ampliÞer, an external
parallel resonant crystal, two capacitors, and two resistors, as shown in Figure 15-3.
EXTAL is the ampliÞer input for the crystal circuit; XTAL is the ampliÞer output.
Example values for the passive components of the crystal circuits are provided in
Figure 15-3. However, because this is a sensitive analog circuit, these values cannot be
guaranteed. These components may have to be tuned due to design-speciÞc 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 Þnd an acceptable range of component values,
with the Þnal design value being selected in the middle of this range.
Lastly, it should also be noted that future changes in the device technology (e.g. shrinks)
may change the characteristics of the input and output impedance of the on-chip ampliÞer.
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
15-4 MPC860 PowerQUICC UserÕs Manual MOTOROLA