MOTOROLA
MC68341 USER’S MANUAL
4- 9
4.2.2.6.2 Using the Periodic Timer as a Real-Time Clock. The periodic interrupt timer
can be used as a real-time clock interrupt by setting it up to generate an interrupt with a
one-second period. Rearranging the periodic timer period equation to solve for the desired
count value:
PITR count value
=
(PIT period) (EXTAL (or EXTCLK) frequency)
(Prescaler value) (22)
PITR count value
=
(1) (32768)
(512) (22)
PITR count value
=
16 (decimal)
Therefore, when using a 32.768-kHz crystal, the PITR should be loaded with a value of
$10 with the prescaler enabled to generate interrupts at a one-second rate.
4.2.2.7 SIMULTANEOUS INTERRUPTS BY SOURCES IN THE SIM41. If multiple
interrupt sources at the same interrupt level are simultaneously asserted in the SIM41, it
will prioritize and service the interrupts in the following order: 1) software watchdog, 2)
periodic interrupt timer, 3) RTC alarm, and 4) external interrupts.
4.2.3 Clock Synthesizer Operation
The clock synthesizer can operate with either an external crystal or an external oscillator
for reference, using the internal phase-locked loop (PLL) and voltage-controlled oscillator
(VCO), or an external clock can directly drive the external clock input (EXTCLK) at the
operating frequency. For RTC operation, 32.768 kHz crystal operation must be used. The
four modes of clock operation are listed in Table 4-1.
Table 4-1. Clock Operating Modes
Mode
Description
MODCK
Reset
Value
VCCSYN
Operating
Value
Crystal Mode
External crystal used with the on-chip PLL and VCO to generate
a system clock and CLKOUT of programmable rates.
1
VCC
External Clock
Mode without PLL
The desired operating frequency is driven into EXTCLK
resulting in a system clock and CLKOUT of the same frequency,
not tightly coupled.
0
0 V
External Clock
Mode with PLL
The desired operating frequency is driven into EXTCLK,
resulting in a system clock and CLKOUT of the same frequency,
with a tight skew between input and output signals.
0
VCC
Limp Mode
Upon input signal loss for either clock mode using the PLL,
operation continues at approximately one-half operating speed
(affected by the value of the X-bit in the SYNCR).
X
VCC
4.2.3.1 CRYSTAL MODE. In crystal mode (see Figure 4-4), the clock synthesizer can
operate from the on-chip PLL and VCO, using a parallel resonant crystal connected
between the EXTAL and XTAL pins. An external oscillator can also be connected to
EXTCLK as a reference frequency source, as shown in Figure 4-5. A 32.768-kHz watch
crystal provides an inexpensive reference, but the reference crystal or external oscillator
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Freescale Semiconductor, Inc.
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Go to: www.freescale.com
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