TMS320VC5507 datasheet(66/108 Pages) TI1

66 / 108 page

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X2/CLKIN

Crystal

SM320VC5507-EP

SPRS613 – SEPTEMBER 2009

www.ti.com

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 Equation 2 below is satisfied. CL in Equation 2 is the load specified for the crystal that is also specified in

Table 5-1.

(2)

Figure 5-2. Internal System Oscillator With External Crystal

Table 5-1. Recommended Crystal Parameters

FREQUENCY RANGE

MAX ESR (

Ω)

TYP CLOAD (pF)

MAX CSHUNT (pF)

RS (Ω)

(MHz)

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.

5.6.2

Layout Considerations

Since parasitic capacitance, inductance and resistance can be significant in any circuit, good PC board layout

Electrical Specifications

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