TMS320VC5504 datasheet(64/128 Pages) TI

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6.3.4

LDO Input Decoupling

6.3.5

LDO Output Decoupling

6.4 External Clock Input From RTC_XI, CLKIN, and USB_MXI Pins

6.4.1

Real-Time Clock (RTC) On-Chip Oscillator With External Crystal

TMS320VC5504

Fixed-Point Digital Signal Processor

SPRS609A – JUNE 2009 – REVISED JULY 2009

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As with the selection of any component, verification of capacitor availability over the product's production

lifetime should be considered.

On the VC5504 the recommended decoupling capacitance for the DSP core supplies should be 1

µF in

parallel with 0.01-

µF capacitor per supply pin.

The LDO inputs should follow the same decoupling guidelines as other power-supply pins above.

The LDO circuits implement a voltage feedback control system which has been designed to optimize gain

and stability tradeoffs. As such, there are design assumptions for the amount of capacitance on the LDO

outputs. For proper device operation, the following external decoupling capacitors should be used:

• ANA_LDOO– 1µF

• DSP_LDOO – 1µF

• USB_LDOO – none required

The VC5504 DSP includes two options to provide an external clock input to the system clock generator:

• Use the on-chip real-time clock (RTC) oscillator with an external 32.768-kHz crystal connected to the

RTC_XI and RTC_XO pins.

• Use an external 11.2896-, 12.0-, or 12.288-MHz LVCMOS clock input fed into the CLKIN pin that

operates at the same voltage as the DVDDIO supply (1.8-, 2.5-, 2.8-, or 3.3-V).

The CLK_SEL pin determines which input is used as the clock source for the system clock generator, For

more details, see Section 4.5.1, Device and Peripheral Configurations at Device Reset. The crystal for the

RTC oscillator is not required if CLKIN is used as the system reference clock; however, the RTC must still

be powered. The RTC registers starting at I/O address 1900h will not be accessible without an RTC clock.

This includes the RTC Power Management Register which provides control to the on-chip LDOs and

WAKEUP and RTC_CLKOUT pins. Section 6.4.1, Real-Time Clock (RTC) On-Chip Oscillator With

External Crystal provides more details on using the RTC on-chip oscillator with an external crystal.

Section 6.4.2, CLKIN Pin With LVCMOS-Compatible Clock Input provides details on using an external

LVCMOS-compatible clock input fed into the CLKIN pin.

Additionally, the USB requires a reference clock generated using a dedicated on-chip oscillator with a

12-MHz external crystal connected to the USB_MXI and USB_MXO pins. The USB reference clock is not

required if the USB peripheral is not being used. Section 6.4.3, USB On-Chip Oscillator With External

Crystal provides details on using the USB on-chip oscillator with an external crystal.

The on-chip oscillator requires an external 32.768-kHz crystal connected across the RTC_XI and RTC_XO

pins, along with two load capacitors, as shown in Figure 6-3. The external crystal load capacitors must be

connected only to the RTC oscillator ground pin (VSSRTC). Do not connect to board ground (VSS). Position

the VSS lead on the board between RTC_XI and RTC_XO as a shield to reduce direct capacitance

between RTC_XI and RTC_XO leads on the board. The CVDDRTC pin can be connected to the same

power supply as CVDD , or may be connected to a different supply that meets the recommended operating

conditions (see Section 5.2), if desired.

Peripheral Information and Electrical Specifications

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