MC68340 datasheet(106/441 Pages) FREESCALE

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MOTOROLA

MC68340 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 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 SIM40. If multiple

interrupt sources at the same interrupt level are simultaneously asserted in the SIM40, it

will prioritize and service the interrupts in the following order: 1) software watchdog, 2)

periodic interrupt timer, and 3) 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 clock signal at the operating frequency.

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 or oscillator used with the on-chip PLL and VCO to

generate a system clock and CLKOUT of programmable rates.

5 V

External Clock

Mode without PLL

The desired operating frequency is driven into EXTAL resulting in a

system clock and CLKOUT of the same frequency, not tightly coupled.

0 V

External Clock

Mode with PLL

The desired operating frequency is driven into EXTAL, resulting in a

system clock and CLKOUT of the same frequency, with a tight skew

between input and output signals.

0 V

5 V

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).

5 V

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,

or an external oscillator connected to EXTAL as a reference frequency source. The

oscillator circuit is shown in Figure 4-5. A 32.768-kHz watch crystal provides an

inexpensive reference, but the reference crystal or external oscillator frequency can be

any frequency in the range specified in Section 11 Electrical Characteristics. When

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