TMS320DM6431 datasheet(149/227 Pages) TI1

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6.6 External Clock Input From MXI/CLKIN Pin

6.6.1

Clock Input Option 1—Crystal

MXI/CLKIN

MXO

Crystal

27MHz

MXV

1.8V

MXV

1C2

) C2)

6.6.2

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

TMS320DM6431

Digital Media Processor

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SPRS342C – NOVEMBER 2006 – REVISED JUNE 2008

The DM6431 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 27 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.

In this option, a crystal is used as the external clock input to the DM6431.

The 27-MHz oscillator provides the reference clock for all DM6431 subsystems and peripherals. The

on-chip oscillator requires an external 27-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 27-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. 27-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

PARAMETER

MIN

TYP

MAX

UNIT

Start-up time (from power up until oscillating at stable frequency of 27

MHz)

Oscillaton frequency

MHz

ESR

Ω

In this option, a 1.8-V LVCMOS-Compatible Clock Input is used as the external clock input to the DM6431.

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.

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