Skew within the Byte signal net class
directly reduces the setup and hold margin for the DQ and DM nets. As described with
the ADDR_CTRL signal net class and associated CK0 clock net class, this skew must be
controlled. The data byte skew must be managed through controlling the lengths of
the routed tracks within a defined group of signals. The only way to practically
match skews on a PCB is to lengthen the shorter traces up to the length of the
longest net in the net class and the associated clock. Consider Z-axis delays (VIAs)
with accurate stackup information during analysis.
The DDR PHY includes a per-bit deskew
feature, enabled by default. This capability allows signal routing with looser delay
matching tolerance as specified in Table 2-7. If this feature is disabled, then skews must be tightly matched. Measure the
propagation delay of each signal from the SoC die to the DRAM device pin. The
designer is free to length match using smaller tolerance than values shown in the
table. Refer to Additional Information: SOC Package
Delays during the initial PCB design phase. Perform a simulation and generate a delay
report to confirm skews are within the specified tolerance.
Note: This is not required nor
recommended to match the lengths across all byte lanes. Length matching is only
required within each byte.
Table 2-7 contains the routing specifications for the Byte0, Byte1, Byte2, and Byte3
routing groups. Each signal net class and the associated clock net class is routed
and matched independently. These parameters are recommendations only, intended to
get the design close to success prior to simulation. To make sure the PCB design
meets all requirements, the design is required to be simulated and those results
compared with the simulation results defined in Section 3.
Table 2-7 Data Group Routing
Specifications
Number |
Parameter |
MIN |
TYP |
MAX |
UNIT |
LP4_DRS1 |
Propagation delay of net class DQSx
(RSD1) |
|
|
250 (1) |
ps |
LP4_DRS2 |
Propagation delay of net class BYTEx (RSD2) |
|
|
250 (1) |
ps |
LP4_DRS3 |
Difference in propagation delays of CK0 pair and each DQS pair.
(RSAC1 + RSAC2 - RSD1)(2) |
0 (3)(4) |
|
3(3)(4) |
tCK |
LP4_DRS4 |
Skew within net class DQSx Skew of
DQSx to DQSx_n (RSD1) |
|
|
1.5 (4)(5) |
ps |
LP4_DRS5 |
Skew across DQSx and BYTEx net classes. (Skew of RSD1 and RSD2) (6) |
|
|
150 (3)(4) |
ps |
LP4_DRS6 |
Difference in propagation delays of shortest DQ/DM bit in BYTEx
and respective DQSx. (RSD2 -
RSD1) |
-50 (3)(4) |
|
|
ps |
LP4_DRS7 |
VIAs Per Trace |
|
|
2 (1) |
VIAs |
LP4_DRS8 |
VIA Stub Length |
|
40 (7) |
|
Mils |
LP4_DRS9 |
VIA Count Difference |
|
|
0 (8) |
VIAs |
LP4_DRS10 |
RSD1 center-to-center spacing (between clock net class) |
5w (9) |
|
|
|
LP4_DRS11 |
RSD1 center-to-center spacing (within clock net class) (10) |
See note below |
|
LP4_DRS12 |
RSD2 center-to-center spacing (between signal net class) |
5w (9) |
|
|
|
LP4_DRS13 |
RSD2 center-to-center spacing (to self or within signal net
class) |
3w (9) |
|
|
|
(1) Max value is based upon conservative signal integrity approach.
FR4 material assumed with Dk equals approx. 3.7 - 3.9 and Df equals approx.
0.002. This value can be extended only if detailed signal integrity analysis of
rise time and fall time confirms desired operation.
(2) Propagation delay of CK0 pair
must be greater than propagation delay of each DQS pair. Consider one leg of any
T-branch trace segments when delay matching.
(3) Simulation
(11) must be performed and the delay report analyzed to make sure delays are
within the limit. Delay reports from PCB layout tools use a simplified
calculation based on a constant propagation velocity factor. TI recommends
initially delay matching in PCB layout tool to a target less than 20% of the
limit.
(4) Consider the delays from SOC die
pad to the DRAM pin (that is, delays of SOC package and delays of PCB up to the
DRAM pin. DRAM package delays are omitted). Refer to
Additional Information: SOC Package
Delays.
(5) Recommendation for PCB layout tool design. Required to be
verified by simulation
(11), confirm JEDEC defined Vix_DQS_ratio (20%) and Vix_CK_ratio (25%) are
satisfied, also need to have good eye margins. Refer to
Section 3.5.3.1.
(6) Skew matching is only done within
a byte including DQS. Skew matching across bytes is neither required nor
recommended.
(1) Recommended skew control on T-branch trace segments
(Balanced-T) is intended to optimize signal integrity (waveform reflections).
This is not required nor recommended to match skew across all T-branch trace
segments, just for each branch of a specific signal.
(1) Recommendation for PCB layout tool design. Required to be
verified in simulation
(11). Simulating worst PVT corner, verify a required minimum of CK - 75ps and
maximum of CK + 75ps is satisfied. Recommend routing net classes CK0 and
ADDR_CTRL on same signal layer for better skew control.
(7) VIA stub control (micro VIA or backdrilling) is required if
operating LPDDR4 above 3200Mbps depending on simulation
(11) results.
(8) VIA count difference can increase by 1 only if accurate 3-D
modeling of the signal flight times; including accurately modeled signal
propagation through VIAs and has been applied to make sure skew maximums are not
exceeded.
(9) Center-to-center spacing is allowed to fall to minimum 2w for
up to 500 mils of routed length (only near endpoints). Spacing minimums can be
relaxed if simulations
(11) accurately capture crosstalk between neighboring victim and aggressor traces
and show good margin. Consider also VIA spacing. Signals with adjacent VIAs near
SOC must not also have adjacent VIAs near the DRAM.
(10) P to N spacing set to make sure proper differential impedance.
The designer must control the impedance so that inadvertent impedance mismatches
are not created. Generally speaking, center-to center spacing must be either 2w
or slightly larger than 2w to achieve a differential impedance equal to twice
the single-ended impedance, Zo, on that layer. Refer to impedance targets in
Table 1-1.
(11) Simulation refers to a
power-aware IBIS Signal Integrity (SI) simulation. Simulate across process,
voltage, and temperature (PVT). Refer to
Section 3.