mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 7

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

Three socket manufacturers are:
3M — [phone], AMP — [phone], and Yamaichi — [phone].
2.5 Clock Circuitry
The designer must decide whether to use the internal frequency synthesizer circuit or an external clock to
produce the system clock signal. Both options are discussed in the following paragraphs.
2.5.1 Using the Internal Frequency Synthesizer Circuit
The MCU uses a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) to generate an internal
high speed clock. This arrangement permits low power operation using only the low frequency oscillator.
Low frequency in CMOS technology translates into low power because power consumption is proportional
to frequency.
The internal frequency synthesizer circuit is enabled when the MODLCK pin is pulled high during reset. The
synthesizer requires a reference frequency in order to operate. There are two reference frequency options:
a crystal oscillator circuit or an external clock reference, such as a canned oscillator circuit (a single package
which contains the crystal and buffer circuit) as the input.
2.5.2 Using a Crystal Oscillator Circuit
2.5.2.1 Oscillator Components
The crystal oscillator used is a Pierce oscillator, also known as a parallel resonant crystal oscillator. It is
shown in Figure 3 . Its components consist of a series resistor, a feedback resistor, a crystal, an inverter,
and two capacitors:
MC68331/332
M68331/332TUT/D
Rs — Series resistor Rs must be large enough to appropriately limit current to the crystal and yet small
enough to provide enough current to start oscillation quickly. The smaller Rs, the faster the oscillator
will start. However, if Rs is too small, the crystal will start up in unpredictable modes or dissipate too
much power. This can cause heating problems. In extreme cases, the crystal may even be damaged
and not work properly again. If Rs is too large, the oscillator will start very slowly or not at all. The best
way to minimize start-up time is to minimize the size of Rs within the guidelines of the maximum power
dissipation.
The crystal manufacturer generally recommends a range of values to use. To ensure that Rs is large
enough to prevent the crystal from being overdriven, observe the output frequency as a function of V
on the CLKOUT pin. If the crystal is overdriven at start-up, i.e., the first 500 ms or so after the power is
turned on, the frequency will be very unstable.
Rf — Feedback resistor Rf is used to bias the inverter between EXTAL and XTAL inside the MCU. Rf
affects the loop gain; lower values reduce gain, while higher values increase gain.
C1 and C2 — The series combination of C1 and C2 provides the parallel load for the crystal. Their val-
ues may be varied to trim frequency. In high frequency applications, C1 and C2 are usually equal. How-
ever, in low frequency applications, C1 can be smaller than C2 (about 5 pF) to provide a higher voltage
at the EXTAL input. A wider voltage swing at this input will result in lower power-supply current. Usually,
the actual capacitances will be smaller than the intended capacitances since circuit and layout capaci-
tances add to the values of C1 and C2.
Inverter — The inverter is inside the MCU. It provides the 180 degree phase shift necessary for oscil-
lation.
Crystal — The crystal is made of piezoelectric quartz. It must be a good quality crystal that is capable
of suppressing harmonics and overtones and quickly locking onto the fundamental frequency. If a par-
ticular crystal type or brand is prone to starting with overtones or harmonics, don’t use it. No amount of
circuit design can compensate for a bad or poor quality crystal.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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