TNETV2501INPGF datasheet(140/197 Pages) TI1

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Electrical Specifications

140

December 2002 − Revised November 2008

SPRS206K

5.6

Clock Options

This section provides the timing requirements and switching characteristics for the various clock options

available on the 5501.

5.6.1 Internal System Oscillator With External Crystal

The 5501 includes an internal oscillator which can be used in conjunction with an external crystal to generate

the input clock to the DSP. 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 PLL,

the crystal oscillation frequency can be multiplied to generate the input clock to the different clock groups of

the DSP.

GPIO4 is sampled on the rising edge of the reset signal to set the state of the CLKMD0 bit of the Clock Mode

Control Register (CLKMD), which in turns, determines the clock source for the DSP. The CLKMD0 bit selects

either the internal oscillator output (OSCOUT) or the X2/CLKIN pin as the input clock source for the DSP. If

GPIO4 is low at reset, the CLKMD0 bit will be set to ‘0’ and the internal oscillator and the external crystal

generate the input clock for the DSP. If GPIO4 is high, the CLKMD0 bit will be set to ‘1’ and the input clock

will be taken directly from the X2/CLKIN pin.

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

resistance (ESR) as 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)

MAXIMUM ESR

SPECIFICATIONS (

Ω)

CLOAD (pF)

MAXIMUM

CSHUNT (pF)

RS (kΩ)

20−15

15−12

12−10

2.8

10−8

2.2

8−6

8.8

6−5

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