mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 8

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

The MCU is designed to use a 32.768 kHz AT-cut crystal to produce an 8.389 MHz CLKOUT signal. The
frequency of the internal clock can be increased or decreased by writing to the SYNCR register. Figure 3
shows clock circuitry for a Daishinku DMX-38 32.768 kHz crystal, but the circuit will work for most other
32.768 kHz crystals also. To use other crystal values (the allowable range is 20 kHz–50 kHz), consult the
crystal vendor for analysis of the crystal components needed.
For the most accurate oscillator frequency, use the Pierce version of the crystal (rather than the series res-
onant version) with C1 and C2 values to match the specified load capacitance of the crystal. As a side note,
start-up time is inversely proportional to frequency. A 32 kHz crystal may take several hundred milliseconds
to start up. Be aware: all crystals are not created equal nor are they close. In fact, there are several different
types of tuning elements that can be used for low frequency oscillators in the range of 32.768 kHz. While
there are many characteristics of the various tuning elements that can be precisely measured, there are oth-
er characteristics that are extremely difficult to measure and express in a useful way.
Maximum power dissipation of a crystal is generally specified by the manufacturer of the device. Crystal
power dissipation is a function of the reactance of the combined input and output capacitance of the internal
amplifier of the microcontroller and of the external circuit components including the crystal itself. The man-
ufacturer specifies this value and also specifies a circuit, usually one like that of Figure 3 , along with the
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RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-KHZ CRYSTAL.
SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.
RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-KHZ CRYSTAL.
SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.
Some older versions of the MC68331 and MC68332 require different components.
The MC68331 mask set that requires different components is OC47T. The
MC68332 mask sets that require different components are 1C17P, 1C32J, OC53T,
and 1C53T. See Figure 4 for an illustration.
Figure 3 System Clock with a 32.768 kHz Reference Crystal
Figure 4 Oscillator Circuit for Some Older Mask Sets
Freescale Semiconductor, Inc.
For More Information On This Product,
C1
22 PF*
C2
22 PF*
V
C1
22 PF*
C2
22 PF*
V
Go to: www.freescale.com
SSI
SSI
NOTE
RS
330K
RS
100
R
10M
R
10M
f
f
EXTAL
EXTAL
XTAL
XTAL
M68331/332TUT/D
MC68331/332
332TUT XTAL CONN 1
332TUT XTAL CONN 2