TNETV2666FIBZWT datasheet(160/249 Pages) TI1

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MXI/CLKIN

MXO

Crystal

MXVSS

15−30 MHz

1.8 V

MXVDD

1C2

) C2)

TMS320C6424

SPRS347D – MARCH 2007 – REVISED DECEMBER 2009

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6.6

External Clock Input From MXI/CLKIN Pin

The C6424 device includes two options to provide an external clock input:

•

Use an on-chip oscillator with external crystal.

•

Use an external 1.8-V LVCMOS-compatible clock input.

The optimal external clock input frequency is 15–30 MHz. Section 6.6.1 provides more details on Option 1,

using an on-chip oscillator with external crystal. Section 6.6.2 provides details on Option 2, using an

external 1.8-V LVCMOS-compatible clock input.

6.6.1

Clock Input Option 1- Crystal

In this option, a crystal is used as the external clock input to the C6424.

The 15–30-MHz oscillator provides the reference clock for all C6424 subsystems and peripherals. The

on-chip oscillator requires an external 15–30-MHz crystal connected across the MXI and MXO pins, along

with two load capacitors, as shown in Figure 6-9. The external crystal load capacitors must be connected

only to the 15–30-MHz oscillator ground pin (MXVSS). Do not connect to board ground (VSS). The MXVDD

pin can be connected to the same 1.8 V power supply as DVDDR2.

Figure 6-9. 15–30-MHz System Oscillator

The load capacitors, C1 and C2, should be chosen such that the equation is satisfied (typical values are

C1 = C2 = 10 pF). CL in the equation is the load specified by the crystal manufacturer. All discrete

components used to implement the oscillator circuit should be placed as close as possible to the

associated oscillator pins (MXI and MXO) and to the MXVSS pin.

Table 6-13. Input Requirements for Crystal (1)

PARAMETER

MIN

TYP

MAX

UNIT

Start-up time (from power up until oscillating at stable frequency of 30

MHz)

Oscillation frequency

MHz

ESR

Ω

(1)

For audio applications, stability of the input clock is very important. The user should select crystals with low enough ppm to ensure good

audio quality for the specific application.

6.6.2

Clock Input Option 2—1.8-V LVCMOS-Compatible Clock Input

In this option, a 1.8-V LVCMOS-Compatible Clock Input is used as the external clock input to the C6424.

The external connections are shown in Figure 6-10. The MXI/CLKIN pin is connected to the 1.8-V

LVCMOS-Compatible clock source. The MXO pin is left unconnected. The MXVSS pin is connected to

board ground (VSS). The MXVDD pin can be connected to the same 1.8-V power supply as DVDDR2.

160

Peripheral Information and Electrical Specifications

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