VSC7440 datasheet(441/444 Pages) MICROSEMI

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Design Guidelines

VMDS-10492 VSC7440 Datasheet Revision 4.2

424

When routing the DDR interface, attention must be paid to the skew, primary concern is skew within the

byte lane between the differential strobe and the single-ended signals. Skew recommendations for the

DDR interface are listed in the following table.

Power supply recommendations:

•

Use a shared voltage reference between the VSC7440’s device’s DDR_Vref supply and the DDR

device’s reference voltage.

•

Generate the DDR_Vref from the VDD_IODDR supply using a resistor divider with value of 1 kΩ and

an accuracy of 1% or better.

•

Use a decoupling capacitance of at least 0.1 µF on the supply in a manner similar to VDD_IODDR and

VSS to ensure tracking of supply variations; however, the time constant of the resistor divider and

decoupling capacitance should not exceed the nRESET assertion time after power on.

•VDD_IODDR pins must not share vias. Use at least one via for each VDD_IODDR pin. The extra

inductance from sharing vias may cause bit errors in the DDR interface.

Routing recommendations:

•

DDR_CK/DDR_CKn must be routed as a differential pair with a 100

Ω differential characteristic

impedance.

•

DDR_xDQS/DDR_xDQSn must be routed as a differential pair with a 100

Ω differential

characteristic impedance.

•

To minimize crosstalk, the characteristic impedance of the single-ended signals should be

determined predominantly by the distance to the reference plane and not the distance to the

neighboring traces.

•

The crosstalk should be below –20 dB.

•

If the DDR interface is not used, connect VDD_IODDR and DDR_VREF to ground. Leave all other DDR

signals unconnected (floating). VDD_IODDR can also be left floating; however, DDR_VREF must also

be left floating.

9.3.7

Thermal Diode External Connection

The internal on-die thermal diode can be used with an external temperature monitor to easily and

accurately measure the junction temperature of this device.

The thermal diode is extremely sensitive to noise. To minimize the temperature measurement errors,

follow these guidelines:

•

Route the THERMDC and THERMDA signals as a differential pair with a differential impedance less

than 100

Ω.

Table 339 •

Recommended Skew Budget

Description

Signal

Maximum Skew

Skew within byte lane 0

DDR_LDQS/DDR_LDQSn

50 ps

Skew within byte lane 1

DDR_UDQS/DDR_UDQSn

50 ps

Skew within address, command, and control

bus

DDR_CK/DDR_CKn

DDR_nRAS

DDR_CKE

DDR_ODT[1:0]

DDR_nCAS

DDR_nWE

DDR_BA[2:0]

DDR_A[15:0]

100 ps

Skew between control bus clock and byte lane

0 clock

DDR_CK/DDR_CKn

DDR_LDQS/DDR_LDQSn

1250 ps

Skew between control bus clock and byte lane

1 clock

DDR_CK/DDR_CKn

DDR_UDQS/DDR_UDQSn

1250 ps

Control bus differential clock intra-pair skew

DDR_CK/DDR_CKn

5 ps

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