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MXI/CLKIN

MXO

Crystal

MXVSS

27 MHz

1.8 V

MXVDD

RBIAS

(optional)

1C2

) C2)

TMS320DM6446

SPRS283H – DECEMBER 2005 – REVISED SEPTEMBER 2010

www.ti.com

6.5

External Clock Input From MXI/CLKIN Pin

The DM6446 device has two input pins for an external clock source, MXI/CLKIN and M24XI. The

MXI/CLKIN pin provides the clock source for PLL1 and PLL2 whose optimal frequency is 27 MHz. The

M24XI pin provides the clock source for the USB PLL whose optimal frequency is 24 MHz.

The DM6446 device includes two options to provide an external clock input:

1. Use an on-chip oscillator with external crystal or ceramic resonator circuit (only supporting

parallel-resonant mode; it does not provide overtone support). For more details, see Section 6.5.1,

Clock Input Option 1 – Crystal.

2. Use an external 1.8-V LVCMOS-compatible clock input. For more details, see Section 6.5.2, Clock

Input Option 2 – 1.8-V LVCMOS-Compatible Clock Input.

6.5.1

Clock Input Option 1 – Crystal

6.5.1.1

27-MHz Crystal for System Oscillator

In this option, a crystal is used as the external clock input to the DM6446 PLL1 and PLL2.

The 27-MHz oscillator provides the reference clock for all DM6446 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 6-10. 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 DVDD18.

Figure 6-10. 27-MHz System Oscillator

The RBIAS resistor is optional. If the RBIAS resistor is used, it should equal 1 M

Ω ±5%. 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.

100

Peripheral and Electrical Specifications

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