TMS320DM647 datasheet(47/166 Pages) TI

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3.3 Debugging Considerations

3.4 Pullup/Pulldown Resistors

TMS320DM647/TMS320DM648

Digital Media Processor

SPRS372 – MAY 2007

Figure 3-12. MAC_ADDR_RW1 Register

CRC

Reserved

R/W-CRC for the MAC_ID

R/W-00000000

MAC_ID

R-MAC ADDRESS[47:32]

LEGEND: R/W = Read/Write; R = Read only; -n = value after reset

Table 3-11. MAC_ADDR_RW1 Register Field Descriptions

Bit

Field

Value

Description

31:24

CRC

CRC of the

This field will hold the CRC of the MAC address of that particular device.

MAC ID

23:16

Reserved

0x00

Reserved

15:0

MAC_ID

Mac

Bit 0 of MAC_ID is Bit 32 of MAC Address

Address[47:32]

of the device

Proper board design should specify that input pins to the device always be at a valid logic level and not

floating. This may be achieved via pullup/pulldown resistors. The DM64x features internal pullup (IPU) and

internal pulldown (IPD) resistors on most pins to eliminate the need, unless otherwise noted, for external

pullup/pulldown resistors.

An external pullup/pulldown resistor must be used in the following situations:

• Boot and Configuration Pins: If the pin is both routed out and in high-impedance mode, an external

pullup/pulldown resistor must be used, even if the IPU/IPD matches the desired value/state.

• Other Input Pins: If the IPU/IPD does not match the desired value/state, use an external

pullup/pulldown resistor to pull the signal to the opposite rail.

If the boot and configuration pins are both routed out and in high-impedance mode, it is recommended

that an external pullup/pulldown resistor be used. Although internal pullup/pulldown resistors exist on

these pins and they may match the desired configuration value, providing external connectivity can help

specify that valid logic levels are latched on these important boot configuration pins. In addition, applying

external pullup/pulldown resistors on the boot and configuration pins adds convenience to the user in

debugging and flexibility in switching operating modes.

Tips for choosing an external pullup/pulldown resistor:

• Select a resistor with the largest possible resistance

• Calculate the worst-case leakage current that flows through this external resistor. Worst-case leakage

current can be calculated by adding up all the leakage current at the pin—e.g., the input current (II)

from DM64x, and leakage current from the other device(s) to which this pin is connected.

• Specify that the voltage at the pin stays well within the low-/high-level input voltages (V

IL or VIH) when

worst-case leakage current is flowing through this external resistor.

–

To oppose an IPU and pull the signal to a logic low, the voltage at the pin must stay well below VIL.

–

To oppose an IPD and pull the signal to a logic high, the voltage at the pin must stay well above

VIH.

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