Yeah, but if you have a 'transmission line' you would avoid needing to
vary the track width or doing other things to create impedance bumps.
And using I2C to go 40-50 feet is taking it way beyond what it was
designed for. Going those sort of distances one uses other techniques,
or runs the I2C much slower.
toggle quoted message
Show quoted text
On Fri, 12 Jan 2024 at 00:12, Gene Heskett <gheskett@...> wrote:
On 1/11/24 13:52, Alan Pearce via groups.io wrote:
I wouldn't have thought you would need to treat I2C lines as
transmission lines, the data rate is quite slow (even at high speed
I2C you are still only at 4MHz). It is not as though you are laying
out a memory array where a heap of parallel address and data lines
need to be synchronised to a double data rate clock, and so edge
timings are highly critical. The I2C transfer clocking is designed to
deal with a small amount of ringing on the clock and data lines.
Considering it was originally designed to be run in a wire harness in
a TV set, it should be quite robust.
But as to track width on a PCB, as you are not constrained by
requiring impedance controlled tracks, make them wide enough that they
will be robust in manufacturing of the PCB (i.e. don't create
under-etching problems, corner peel problems, etc) so that the PCB
house minimises rejection rates, and then during assembly &
integration the tracks are robust enough to withstand knocks and rough
handling without causing micro-cracks in the tracks. Personally I
would go for something like 0.5mm, only thinning when needed to
squeeze through gaps between pins and pads.
And every time you do that, there's a huge bump in the impedance at that
point. If you don't have a T.D.R., make it or get one. I've made one a
few times, and made it work well enough to tell a tower crew looking for
a burnout in a transmission line which joint to take apart to find the
top or bottom of a line fire. Saves at least a day getting the fire
damage repaired and the tv station back on the air.
I2c is the most ticklish transmission they ever threw at us, designed to
work at ttl voltage levels, the drivers have so little surplus power
that it can't be properly terminated to make it a real transmission
line. That limits it to under 40 feet. src terminated, maybe 50 feet.
With more modern fasrer circuitry.
On Thu, 11 Jan 2024 at 16:39, John Woodgate <jmw@...> wrote:
0.2 mm sounds OK, but it's only eight thousandths of an inch. Any defect, even a change of hardness/annealing, in the copper creates a stress-concentration joint, leading to a crack. I think you should make the tracks as wide as possible, for good reliability.
On 2024-01-11 16:01, Robert via groups.io wrote:
The lines concerned are I2C clock and data, and yes, I will be
considering them as transmission lines. However, there's no point
laying something down that is electrically just fine if the board cannot
be manufactured reliably or the tracks break during assembly (it will
get bolted down, but before that happens someone will have to pick it up
and move it into position).
Chinese PCB houses can routinely achieve 0.15 mm on normal sized boards.
So am I worrying about nothing, because they will in fact have no
trouble with a track (say) 0.2 mm wide and 500 mm long, if that's what I
would choose from an electrical viewpoint?
Regards,
Robert.
* Plain text email - safe, readable, inclusive. *
.
Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
