TMS320VC5506 datasheet(73/120 Pages) TI

73 / 120 page

Electrical Specifications

October 2006 − Revised January 2008

SPRS375C

5.6

Clock Options

The frequency of the reference clock provided at the X2/CLKIN pin can be divided by a factor of two or four

or multiplied by one of several values to generate the internal machine cycle.

5.6.1 Internal System Oscillator With External Crystal

The internal oscillator is always enabled following a device reset. The oscillator requires an external crystal

connected across the X1 and X2/CLKIN pins. If the internal oscillator is not used, an external clock source

must be applied to the X2/CLKIN pin and the X1 pin should be left unconnected. Since the internal oscillator

can be used as a clock source to the PLLs, the crystal oscillation frequency can be multiplied to generate the

CPU clock and USB clock, if desired.

The crystal should be in fundamental-mode operation, and parallel resonant, with a maximum effective series

resistance (ESR) specified in Table 5−1. The connection of the required circuit is shown in Figure 5−2. Under

some conditions, all the components shown are not required. The capacitors, C1 and C2, should be chosen

such that the equation below is satisfied. CL in the equation is the load specified for the crystal that is also

specified in Table 5−1.

1C2

) C

X2/CLKIN

Crystal

Figure 5−2. Internal System Oscillator With External Crystal

Table 5−1. Recommended Crystal Parameters

FREQUENCY RANGE (MHz)

MAX ESR (

Ω)

TYP CLOAD (pF)

MAX CSHUNT (pF)

RS (Ω)

20−15

15−12

12−10

100

10−8

470

8−6

1.5k

6−5

2.2k

Although the recommended ESR presented in Table 5−1 is maximum, theoretically a crystal with a lower

maximum ESR might seem to meet the requirement. It is recommended that crystals which meet the

maximum ESR specification in Table 5−1 are used.

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