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5.6

Clock Options

5.6.1

Internal System Oscillator With External Crystal

1C2

) C2)

X2/CLKIN

Crystal

TMS320VC5502

Fixed-Point Digital Signal Processor

SPRS166H – APRIL 2001 – REVISED NOVEMBER 2004

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

on the 5502.

The 5502 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.

Figure 5-2. Internal System Oscillator With External Crystal

Table 5-1. Recommended Crystal Parameters

MAXIMUM ESR

MAXIMUM

FREQUENCY RANGE (MHz)

CLOAD (pF)

RS (kΩ)

SPECIFICATIONS (

Ω)

CSHUNT (pF)

20-15

15–12

12–10

2.8

10–8

2.2

8–6

8.8

6–5

Specifications

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