ok reading in full, cutting extraneous, answering only with confirmation.
On Tue, Aug 1, 2017 at 10:57 AM, Richard Wilbur richard.wilbur@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.
l.