[Arm-netbook] HDMI High-Frequency Layout: Timing

Luke Kenneth Casson Leighton lkcl at lkcl.net
Tue Aug 1 11:29:24 BST 2017

ok reading in full, cutting extraneous, answering only with confirmation.

On Tue, Aug 1, 2017 at 10:57 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:

> So, while we will try to match the impedance of our traces
> (transmission lines) as carefully as possible to the characteristic
> impedance specified for the cable, until we get to 4.3 inches from the
> signal source the line driver should be able to squash most
> reflections on the leading edges (first quarter wavelength).

 actual distance is 55mm, under half the 4.3in.

> Inter-pair skew:  (Requirements for HDMI v1.4)
>     clock-data skew:  Δt <150ps[3]
>      => Δl < v * Δt = 22mm ~= 870mil
>     T(Pixel) = 1/(pixel clock) = 2.94ns
>     Skew(Inter-Pair) < 0.20 * T(Pixel)[4][5] = 588ps
>      => Δl < v * Δt = 88.2mm ~= 3470mil
>     Chrontel suggests matching between any two pairs be within 100mil.[5]
>     => Δl < 100mil => Δt < Δl / v = 2540um / (150um/ps) = 17ps

 actual difference between CK and Tx2 is 55 - 48mm, or 7mm. so... 275
mil.  whoops.

 between CK and Tx2 is 55 - 52 = 3mm, so... 118 mil.  again whoops.

> Of these design parameters Chrontel's 100mil recommendation seems to
> be the most restrictive, but still not out of the realm of possibility
> and probably a good precautionary limit.  With only 17ps of inter-pair
> skew we meet even much tighter skew timings.  Having non-vanishing
> inter-pair skew seems to actually be beneficial for reducing
> Electro-Magnetic Interference (EMI) by avoiding simultaneous
> transitions on multiple lines.  Indeed the standard seems to be
> designed to recover up to 5 bits of worst-case inter-pair skew.[6]
> (Half of the 10-bit pixel time.)  A Texas Instruments (TI) employee
> specifically suggested to keep the clock pair longer than the data
> pairs.[7]

 sounds like a good idea... and has to happen anyway: the clock lines
have slightly further to go.

> Intra-pair skew:  (Requirements for HDMI v1.4)
>     Toradex:  Δt < 5ps[3]
>      => Δl < v * Δt = 0.75mm ~= 30 mil
>     Chrontel, Texas Instruments:  T(bit) = 0.1 * T(Pixel) = 294ps
>     Skew(Intra-Pair) = 0.15 * T(bit)[4][5] = 44.1ps
>      => Δl = v * Δt =  6.62mm ~= 261 mil
>     (Chrontel suggests, without saying why, that matching between
> signals should be within 5mil.[5]  Given the context and calculations
> above suggesting 261 mil for total, this should probably be 5mil skew
> per segment.)

 i try to meet that - 5 didn't know it was as little as 5mil though.
that's absolutely tiny!

> Chamferred Corners (or Trace Bend Geometry)
> I'm glad to see you are already using 45 degree bends instead of 90
> degree corners.  This helps the corners maintain the proper impedance.
> When serpentine traces (meanders) are needed to attain certain lengths
> of a single-ended trace, make sure individual segment lengths are at
> least 1.5x the width of the trace.  Also, the spacing between parallel
> segments of the same trace should be at least 4x the width of the
> trace.[9]

 that's going to be very very hard to achieve: there is an extremely
limited amount of space.

> Length Matching
> Differential pair signals should not propagate asynchronously over a
> distance greater than 15mm[10] = 590mil.  Thus the compensation for
> length mis-matches should be placed as close to the mismatch as
> possible.  Differential traces can be segmented by a connector, pad
> (component or IC), or via.[10][5]  "Each segment of a differential
> pair connection needs to be matched individually."[10]

 yeah i saw that in the toradex recommendations, otherwise there's
skew between traces.

> Ideal serpentine trace geometry for equalizing differential traces
> consists of the following proportions:  the spacing between traces in
> the meanders should not exceed twice the normal spacing, and the
> length of more widely spaced traces should not exceed three times the
> normal trace width.[10, see Figure 23]

 there's a lot of other stuff in here which is really good, such as
making sure that lengths on each *layer* are matched, and that even
when turning corners the lengths are matched.  and matching just after
VIAs *not* before... damn

 so thank you - much to correct and think about.  really appreciated
you finding all this stuff richard.


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