GSW141

[sethp]

An (editable) placeholder to collect some discussion about the part in the title.

[sethp]

Eval Kit

There's a $300 eval board for the GSW145

(through the highly advanced technique of "flipping back and forth really fast" I've compared the GSW145 to the GSW141; I think the main/only difference is that the former is capable of 2.5G speeds, so I suspect they're actually the same design that's just been binned differently)

The best reference I've found so far for it is the photograph by way of: https://www.mouser.com/new/maxlinear/maxlinear-ethernet-eval-boards

In roughly left-right, top-bottom order, I see:

1. A row of ports that "open" along the top, some labels
   • Leftmost is an SFP port labeled "2.5G/SGMII" (the silkscreen says "CON2")
   • Then, what appears to be 4x RJ-45 ports labeled with:

     GSW140 Ref. Board
     A31	Default Assembly	V1.2.2
     1746	SNR 003	P00

   • Then, a single RJ-45 port with a QR code that decodes to "EASY7085(GSW140)_Ref_v122_A31_V1.3_TTT_1746SNR003"
   • Rightmost is an SFP port with what appears to be a quality control sticker on it (the silkscreen says "CON3")
2. A row of magnetics (transformers)
   • 2x of what seem like the same two-port part but with different labels (1650 vs 1651) but the same part number? (HN4843CG)
   • A single-port-sized transformer labeled "1651 / HN2443CG" (where / indicates a line break)
3. An IC with what looks like the hand-written initials "SC" on it (no part number visible) with the silkscreen "U1" (no leads are visible)
4. An IC with part number PEF7072 (a single-port PHY transceiver)
   • There's an inductor L3 nearby that's labeled 4R7 (4.7µH?)
5. A USB-B port of some kind on the rightmost edge silk-screened CON1
6. A barrel jack connector with a red sticker that says "+5V" and is silkscreened DCIN +5V
7. Two crystals (corroboration)
   • One silkscreened Y1 next to "U1", no visible part number
   • One silkscreened Y2 next to "U8", reads (roughly) "TXC o/ 25.0Uc3G" (where / indicates a line break)
8. What looks like a power regulation circuit driven by IC "U2" (no part number visible) with some inductors (L1/L2), capacitors, a pair of (0Ω?) resistors
9. lots and lots of very tiny misc. resistors and capacitors
   • though, almost all of them look like they're unpopulated? especially the ones nearby "U8"
10. Three (unpopulated) headers
    • J3 looks like JTAG for "U1"
    • J4 looks like JTAG for "U8"
    • J15 breaks out 8 pins from roughly the "southwest" corner of "U1"
11. A totally unfamiliar part I don't recognize labeled "U3" (right next to "J3")
12. There's lots of "stippling" or "raised dots" or "braille" on the left "half" of the board, especially underneath/around a the big "TTT" sticker
13. Around "U1" there's also lots of raised solder bumps in between pairs of traces

[sethp]

Oh, and one more:

• How do we get the xMII signals "out" for development purposes?

Seems like SGMII would be a lot easier to wrangle, being something like 2-6 pins as opposed to RGMII's, like 10-15 or whatever. The cost there being a higher clock rate, though, which means we'd need some care in the connector: maybe an edge connector? Or some other very short board->board trace?

[sethp]

re:

What is the "stippling" from 12 called? What does it do (presumably, some sort of filtering/analog noise reduction)?

Per @dougli1sqrd that's probably a bunch of vias that tie the ground planes together, a technique known as "via stitching"

[dougli1sqrd]

The fact that the solder (or whatever it is) is in between the signal pairs could indicate ... something? But if it's acting as a "barrier" between different pairs of signals that's what that ground pour between the signals should be for anyway. I tried googling around but didn't really find anything. I have like medium confidence that those aren't "needed"? I feel like it must be more than just solder, otherwise you wouldn't do it like that. If you wanted to put solder on the board you have to expose copper there on the pcb. But why expose copper just to cover it with solder? The solder doesn't seem like it would do anything electrically more than just having copper there. So I can only guess that it's soldering something there. But what I have no idea.

[dougli1sqrd]

And yeah I'm thinking edge connector, and also test points to probe what's going out of the xMII interfaces

[sethp]

The solder doesn't seem like it would do anything electrically more than just having copper there. So I can only guess that it's soldering something there. But what I have no idea.

Maybe it's connected to the "shield" bits, rather than ground? Or, more conductive material implies more inductive "inertia", right? And perhaps it's the 3D-ness of it is what makes it superior to a "flat" copper trace in terms of, like, "attracting" the noisy emissions from the differential pair? (isn't one of the benefits of differential pairs that the noise emissions kinda cancel each other out tho?)

I appreciate you looking around, either way. It's definitely very strange!

[dougli1sqrd]

In my preliminary PCB routing playing around, I've been using the https://www.mouser.com/ProductDetail/ABRACON/ARJM11C7-502-KB-EW2?qs=YCa%2FAAYMW00En29mj%252BAmyw%3D%3D as the ethernet RJ45 jacks as they have internal magnetics and cheap and available.

I am curious though if this is the part, or if I have the pinouts correct? Like, It's kinda fine, but it seems like each rj45 jack and each diff pair within each jack are all swapped so that everything has to be routed around everything else.

It'd be much easier if the individual diff pairs and the order of the diff pairs were flipped the lines could come directly out of the RJ45 pins into the Switch chip more like in this image

Not sure if a different part would have a different pinout that would be easier to route?

[sethp]

re: external LEDs & magetnics with grouped ports, looks like it's possible to get down to ~$1/port on mouser with something like this 1x4 configuration with no LEDs and magnetics, and then maybe one of these little $0.78 buggers per port (via "signal transformers" mouser search but I didn't put too much effort into it [there's probably parts that bake in multiple channels, and are therefore cheaper in aggregate]).

Looks like both parts have footprints available for download, too, so that's nice.

[dougli1sqrd]

if we make modular jacks

you can make modular jacks?? I guess it makes sense they would call it modular

Yeah I searched around previously for external transformers before and didn't know what I was looking for so I opted to stay with mag jacks. But maybe it could end up being cheaper separately!

[sethp]

Yeah, I'm not entirely sure what we're looking at still either: the turn ratio being 1:1 means we're not actually doing any "transforming", I think, just filtering ("choking") and DC isolation.

[sethp]

oh, I wanted to throw out there that for "modular jacks" what I meant was like, a common edge connector that we could plug different jack configurations into. Like, given some abstract controller that has 5Gb of aggregate bidirectional bandwidth, make it possible to plug in either 2x2.5GbE ports or 5x1GbE ports.

Sort of similar the framework model of having a common "bus" (for them, USB3.1; for us.... PCIe?) and a bunch of "port configurations" that hang off that bus. Given how spendy and tradeoff-laden the PHY layer seems to be, I wonder if adding more flexibility there would make sense.

I'm not thinking this is a "next" thing, or even a "next next," just something I'm curious about in general.

[dougli1sqrd]

ah yeah confusingly digikey calls the category that RJ45 jacks are in "modular/ethernet connectors"

[dougli1sqrd]

Different Arrangements of the GSW141 Breakout PCB?

RJ45 Jacks

• Internal Magnetics | External Magnetics
• LEDs | No LEDs
• 1000 (or higher) Base-T
  • I think 1G capable means that there are 4 twisted pairs
• "Tab Up" | "Tab Down"
  • This means the tab on the cable. When tab is down, the cat-5 pins all in a row point "up",
    and vise versa when tab is up.
  • The GSW141 chip routes traces best with "tab down"
  • "Tab up" + on the reverse side of the board is equivalent to "tab down"
• How many ports per part

Internal Magnetics

Use ARJM11C7-502-KB-EW2 on mouser

• Tab up, so would have to be placed on the reverse side of the PCB
• $3.09 @ 10

4x 1 unit = $12.36

---

Use TMJG26945AENL on mouser, 1x2 port

• Tab down
• $11.34 per unit

2x 1 unit = $22.68

---

Use TMJG46945AENL on mouser, 1x4 port

• Tab down
• $18.84 per unit

External Magnetics

• Mouser 1149848 4 port RJ45 w/ LEDs, $4.34 each
  • Tab up, so would have to be placed on PCB reverse side
• Mouser LAN2VSOD48351C2 dual port magnetics chip, ~$2 each

1 @ 4.34 + 2 @ $2 = $8.34

---

• Mouser 1149854 2 port RJ45 w/LEDs, $2.78 each
  • Tab up, reverse PCB side
• Mouser LAN2VSOD48351C2 dual port magnetics chip, ~$2 each

2 @ 2.78 + 2 @ 2 = $9.56