SLLA500 December 2020 TUSB2E22

4 Matched Trace Length and Skew for eUSB2

In eUSB2, intra-pair skew can result in distortion to a single-ended 1 (SE1) line state that can result in false bus conditions seen at the eUSB2 repeater. While the eUSB2 repeater implements mechanisms to help filter these conditions, eUSB2 differential traces lengths should be matched to help minimize this skew. The transmit differential skew of the eUSB2 data lines should not exceed 500 ps.

Assuming a maximum budget of 270 ps skew for the driver and a guard band of 160 ps, that leaves a budget of up to 70 ps of interconnect skew. Referencing the AC specifications from the eUSB2 Physical Layer Supplement in Table 4-1 and using typical propagation delay numbers for a microstrip in FR4 of 150 ps/in (a stripline of 180 ps/in), it is recommend to keep skew to less than one tenth of the fastest possible rise time, so ideally the maximum routing skew would be 10 ps or 66 mils (55 mils). TI recommends that eUSB2 traces be matched within 50 mils using serpentine routing if necessary.

Table 4-1 eUSB2 AC Specifications

Parameter	Min	Max	Units
High-speed transmit rise and fall time (20%-80%)⁽¹⁾ ⁽³⁾	100		ps
Full-speed / low-speed transmit rise and fall time (10%-90%)⁽⁴⁾	2	6	ns
Transmit rise/fall mismatch ⁽¹⁾ ⁽²⁾		25	%

(1) Defined under an ideal 80-Ω RX differential termination with maximum supply voltage variation.

(2) Rise/fall mismatch = absolute delta of (rise-fall time) / (average of rise and fall time).

(3) This parameter is informative, not normative.

(4) Measured with 2.5-pF test load at eUSB2 device or eUSB2 repeater end and assuming 10-inch channel trace in between eUSB2 Host and eUSB2 repeater or eUSB device.