DSP56720 datasheet(13/54 Pages) FREESCALE

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SymphonyTM DSP56720 / DSP56721 Multi-Core Audio Processors, Rev.1

Freescale Semiconductor

2.1.7

External Clock Operation

The DSP56720/DSP56721 system clock is derived from the on-chip oscillator or is externally supplied. To use the on-chip

oscillator, connect a crystal and associated resistor/capacitor components to EXTAL and XTAL; see the example in Figure 9.

Figure 9. Using the On-Chip Oscillator

If the DSP56720/DSP56721 system clock is an externally supplied square wave voltage source, it is connected to EXTAL

(Figure 10). When the external square wave source is connected to EXTAL, the XTAL pin is not used.

Figure 10. External Clock Timing

Table 8. Clock Operation

No.

Characteristics

Symbol

Min

Max

Units

EXTAL input high 1

(40% to 60% duty cycle)

• Crystal oscillator

• Square wave input

Eth

16.67

2.5

100

inf

XTAL

EXTAL

Suggested component values:

Fosc = 24.576 MHz

R = 1 M ±10%

C (EXTAL)= 18 pF

Calculations are for a 5 – 30 MHz crystal with the following parameters:

• Shunt capacitance (C0) of 10 pF – 12 pF

• Series resistance 40 Ohm

C (XTAL) = 18 pF

• Drive level of 10

μW

XTAL1

EXTAL

VIL

VIH

Midpoint

Note:

The midpoint is 0.5 (VIH + VIL).

ETH

ETL

ETC

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