So at this point I've done at least preliminary "hardware isn't catastrophically busticated" bringup on just about everything other than the QSGMII PHY, which was unresponsive on MDIO during a 30 second first attempt but I didn't do any more extensive debug.

Power: voltages all in range, but no ripple measurements yet

Supervisor: working fine, firmware basically done except for watchdog/warm reboot functionality

Main MCU: no issues

Thermal: everything working fine with fans at max RPM. Haven't tried to PWM them yet but they're so quiet at max RPM I may not even bother

10G SFP+: links up and can read EEPROM and DOM

RGMII PHY: fully functional and passing traffic

SGMII PHYs: SGMII links up with e-11 BER and seems to work fine. MDIO alive. BaseT side seems fine on g12, but g13 only links up at 100mbit speeds. Suspecting solder issue but have not investigated yet.

QDR-II+ SRAM: fully functional

Took out the debug prints and fixed the IRQ issue, as well as a hardware crypto engine issue.

Now I can ping it with 250us RTT (through a router and two or three switch hops).

And I get further when trying to SSH to it. Now the device kicks me off after the SSH2_MSG_NEWKEYS message. Wonder why?

Hmmm. I'm seeing AES-GCM decryption fail, but when I dump out the calculated keys I get the same keys that OpenSSH is trying to use clientside.

Which makes me think it's some subtle difference in behavior between the STM32F7 crypto engine (which I originally wrote this code for) and the STM32H7 crypto engine (which I'm now using). This is going to be fuuuun.

Hexdumping more stuff it seems I'm getting valid decryption of the message now (a SSH_MSG_SERVICE_REQUEST with service of "ssh-userauth").

But the GCM tag is way off.

This is the frustrating thing about debugging crypto. If you get any of the magic incantations even slightly wrong, you get random gibberish with no clue about what went wrong.

It lives!

Somehow I was sending replies to SSH_MSG_CHANNEL_REQUEST packets by writing *into the incoming packet buffer*, not the reply.

And by dumb luck I guess whatever uninitialized garbage was in the reply buffer happened to resemble a valid SSH_MSG_CHANNEL_SUCCESS message before, but not now? Lol.

Anyway, this is a great success. Kid is up from her nap so that's it for a while, tonight after bedtime I'll hook up the Curve25519 accelerator on the FPGA to speed session creation a bit, then work on a bunch of CLI commands to dump PHY information and such.

Bumped the optimization level on my firmware up from -O0 to -O2 because creating a SSH session was too slow.

But the FPGA curve25519 accelerator is still over 48x faster than the software implementation. Pretty happy with that :)

Ephemeral ECDH key generation and shared secret calculation now use the FPGA accelerator and SSH session creation now feels about as fast as it does when logging into a regular PC.

I can probably extend the same accelerator block (with some minimal tweaks) to also support the public key side of signing, but for now crypto_sign() is still being done entirely in software and only the two crypto_scalarmult() calls in the SSH session creation are accelerated.

Still a massive improvement in responsiveness, it cut about 400ms of latency off session creation.

Added some code to poll PHY MDIO register state (not using irq pins yet) and the SGMII PHYs seem less happy. One refuses to report link up over MDIO (even if the LEDs are on and the link partner says it's up), the other reports link up but flaps.

Just when I thought that was working...

So, here's the basic setup.

Blue and green wires go to the MDIO bus, which is slow enough (2.5 Mbps with very low drive strength) that I'm not worried about reflections off a few inches of flying wire. Standard 10x passive probes clip to the other end of each.

The two black probes are Teledyne LeCroy QuickLink solder in probe tips. One is going to a D420-A and the other to a D1330; both have way more bandwidth than I need to see SGMII clearly.

I'd use my own AKL-PT5 probes for this measurement (well within their capabilities) except that I'd need to AC couple the measurement and somehow I only have one SMA DC block on the shelf right now. That will be rectified by the end of the week.

Initial observations: The SGMII RX waveform looks decent enough and passes the eye mask required for the FPGA to decode it. I've seen better, but I'm in no hurry to rework the board because of this.

Swing and drive strength on the TX seem a bit excessive, I should probably turn it down. The eye is wide open but the PHY could probably hear the FPGA from the next room over!

Valid MDIO traffic is present. This particular waveform has two packets at the start, then four, then two more.

The MCU reads four registers per polling cycle: basic control and status of PHY 0, then of PHY 1. After polling each PHY, it checks for a link state change and logs a message to the UART.

The long delay after the last packet suggests this is being detected as a link state change.

The SGMII link appears to be up the whole time, and at no point does it fall back into the negotiation state. So the link is probably *not* actually flapping; this smells like a bug on the microcontroller side.

We expect g13 (phy address 1) to be down, and g12 (phy address 0) to be up at 1 Gbps.

Looking at the actual MDIO bus traffic in this capture, we have:

* PHY 1 ctl: 1G/full
* PHY 1 stat: Down
* PHY 0 ctl: 1G/full
* PHY 0 stat: Up
* PHY 1 ctl: 1G/full
* PHY 1 stat: Down
* PHY 0 ctl: 1g/full
* PHY 0 stat: Up

Nothing seems obviously wrong here.

OK, this is starting to smell like an FPGA issue.

We know the actual MDIO traffic on the wire is fine, but sometimes we're reading 0x7949 for the basic control register on port 12.

Interestingly, this is the same value we just read from the basic status register on port 13.

And then at 22.420, we read the basic control register for port 13 as 0x116d. 0x6d is the value we just read from the basic status register on port 12.

So I think there's some kind of bug in the FPGA MDIO-to-QSPI bridge where sometimes it will return a previous value instead of what was actually read.

New FPGA bitstream with some additional debug logic, as well as changing the FPGA output buffer from DIFF_HSTL_I_DCI_18 to LVDS.

TX data eye measured at the PHY side is still reasonably open, but way lower amplitude than before. I'll double check the spec later but this should be plenty open enough.

And here's the bug, caught red handed.

We start with the MDIO transceiver being busy with a read of address 0x00. The read data register is still 0x7949, the previous value, because the read is still in progress.

At T=7862 the MCU begins a 4-word burst read of REG_DP_MDIO (0x004c). This is a 32-bit little endian register with the read value in the low 16 bits, a bunch of write-only configuration, and a busy flag in the MSB.

By T=7887 when we read SPI address 0x004f (where the busy flag is) the read has just finished.

So the MCU thinks it's successfully read the whole register.

The fix is pretty simple: latch the busy flag when address 0x4c is read (the entire 32-bit register has to be read in one go, byte access is not supported). The MCU will then read {busy, 0x7949} just like it did on the previous poll, then read the correct value on the subsequent polling cycle.

Yay, no more flapping!

Tomorrow's problem: while g12 links up fine at gigabit speed, last time I tried g13 would struggle a bit then come up at 100 Mbps (verified by link partner).

That's probably a hardware issue of some sort since g12 and g13 are supposed to be identically configured. The RJ45 pinout is mirrored because of the tab-up vs tab-down jacks, but that should (famous last words) be fine because the DP83867 has a register to enable ABCD -> DCBA mirroring, which I think I've set correctly.

I made a quick pass over the schematic and nothing seemed otherwise different, it was largely copy-pasted other than the PHYADDR strap pins.

Yet another command that I wish "real" switches had.

There will of course be fancy commands that include nice detailed decodes of port state. But sometimes there's no substitute for getting close to the metal.

Well, that was a slightly larger yak than I originally expected but it's been thoroughly shaved.

SSH clients on the switch can now see log messages. For now this is enabled by default, although long term I might have this controlled by a per-unit configuration setting or off by default with a Cisco-style "terminal monitor" command to start seeing log messages.

During development I want ALL the logs so I'll leave it like this for now.

Next step will be to implement some of the commands I copied over (commented out) from the Ethernet tap board, and make any tweaks needed to support the additional PHY chipsets on the board.

In particular, I want to be able to send test patterns out both DP83867's to check for soldering issues before I debug the 100mbit-only link issue further.

Ok, I should sleep...

But on the plus side, I have the code to send test patterns working (including the three special test patterns that the DP83867 specifies in addition to the IEE-defined ones).

Won't be able to actually debug the g13 100mbit issue until tomorrow after work but I should have all the groundwork laid now.

Oh I'm sorry you wanted *less* cable spaghetti? i swear you said you wanted *more*. I even bought a new roll of ESD tape to wrangle it all.

Got the baseT test fixture cabled up so I can troubleshoot g13's link issues after work, but didn't have time to collect any data yet.

If you haven't seen it before, this is a handy dandy little gizmo consisting of two RJ45 jacks connected back to back by dual directional couplers.

This gives me 16 SMA outputs with 10 dB attenuated views of each of the 8 wires in the twisted pair cable, seen from both directions. I'm using an 8 channel PicoScope 6824E to look at all 8 lines coming out of the DP83867, ignoring the inbound data from the other side.

Hmmm.

Set up a test pattern on g13 and expected to see it coming out all pairs of the link, but only seeing it on pair A.

Thought this pointed to a soldering issue, except I'm seeing it on g12 as well (which links up just fine).

So I guess I need to read the datasheet and see if there's a test pattern mux register or something I'm missing...

Yep, there is. MMD 0x1f register 0x25, TMCH_CTRL, defaults to only sending the test pattern out pair A.

With that fixed, on g12 I'm seeing the test pattern on all pairs. So we know our register config is correct there.

And now here's what we see on g13. One of these is not like the other.

Probably a solder defect but I'll need to pull the board to investigate. Decabling this will take a while...

I wish it was a solder defect. The truth is worse.

Not sure how this got through design review...

Looking at the layout, bodging this is going to be fuuuun.

g10, g8, and g4 have pair D routed on layer 6 of 8. Getting to them (assuming I come from the back of the PCB to avoid desoldering the connector) will mean drilling down 250 μm - annoying but not too bad.

g13, g6, g2, and g0, all have pair D router on layer 3 of 8. Getting to *this* from the back side will mean drilling down almost 1.3 mm. That will be decidedly less fun.

And worst case, this isn't a fatal issue for a prototype. Having half the ports only run in 100baseTX mode, or even not work at all, would surely be annoying. But it wouldn't prevent me from using the board as a development platform for the full scale 24 port switch, which was the real goal.

But I'd like to make it fully functional if I can.

Not happening tonight, though. I've got too much else on my plate with time constraints.

Actually I might try some fixturing work and a preliminary cut while waiting for stuff to run on another project.

My microscope ring light was too fat to clear so I bodged up an LED headlamp with some tape.

First connector (on the DP83867s) bodged. Not attempting the rest (on the VSC8512) until I've brought it up.

Ended up milling all the way down and cutting the track then reconnecting on the surface. there's a small stub off a via which isn't great but it'll probably be fine on a prototype.

I'll save the other six for later. If the phy doesn't work, no point spending time reworking the RJ45s.

Initial signs of life out of the QSGMII PHY!

It's responding to MDIO with the correct address, but twice (?) and at 8 addresses (this is a 12 port PHY). Suspecting a timing issue related to the level shifters on the MDIO bus, but not sure yet. Dropping the MDIO clock frequency by 10x from 2.5 MHz to 250 kHz didn't fix it.

The actual PHY side seems OK, it links up with my laptop on every port I've tried (aside from the known pair D issue on the upper row of ports).

Also whoops I misspoke. The Ethernet test fixture is 16 dB couplers not 10. The directional coupler I use for TDR stuff is 10 dB and I mixed them up.

Too much RF hardware :p

Reading the programming guide in the VSC8512 datasheet.

Why??? IEEE has a perfectly well defined way to access up to 2^16 extended registers. You don't need to roll your own way to do it.

Loaded an FPGA bitstream that instantiates the QSGMII transceivers on the FPGA.

Power consumption climbed to 12.7W and the FPGA die temperature is up to 48.5C.

The 1V0 rail for the GTXes is sagging to 975.5 mV under load, since it's just pi filtered off of the main FPGA 1V0 rail without an independent remote sense. This is within spec... barely. But definitely something I will want to work on in the future. The full LATENTRED switch (with eight transceivers) will definitely need a dedicated SERDES power rail with independent regulation.

The FPGA 1V0 rail is doing just fine, 1.0015V at the test point and 0.996V measured by the on die ADC.