Programmable USB Hub

A USB hub with per-port power and data disconnects + dev board + I²C, GPIO, and SPI bridges

Oct 21, 2019

Production Update

With the campaign successful (woohoo!) and concluded, our efforts have now transitioned to preparing-for and starting production.

Overall, this consists of:

As I said previously, we’ll be keeping you all informed as we progress through the above.

Case Production : First Articles

The first articles of the production case have arrived! They look great, and all of the thru features line up perfectly with the corresponding connections & light pipes.

Case First Article : Front

Case First Article : Rear

These samples were produced to validate the design of the production tooling prior to the production run itself. If there was a mistake in the design or in the fabrication of the punch tool or silk-screen, it’s much better to find out prior to making hundreds of bad parts.

Cases that you receive will have one additional change not pictured here. For CE Mark compliance purposes (and because it’s a good idea), the connector polarity, voltage range, and maximum power draw will all be printed next to the PWR connector.

The production run of cases started last week and should be arriving here in early December.

Production-Intent PCB Design

As mentioned at the start of the campaign, a final revision to the PCB design was expected prior to the production run of boards.

Production PCB Design

Testing of the prototypes resulted in these changes being made:

All of these changes have been validated, and the next step was FCC & CE laboratory testing.

A 10 Ohm Resistor on MISO net?

The USB4715 senses the loading of the MISO net during power-on to determine which ports support fast battery charging (e.g., which ports should advertise high current capability.) And unfortunately, there is no way to change this setting via the register.

A 10 ohm resistor is needed to tell the hub chip that all ports supply high currents, but a 10 ohm pull-down would prevent the SPI bus from working properly. The solution is for the MCU to switch in the resistor, bring up the USB Hub chip, and then disable the resistor, so the SPI bus works properly.

FCC & CE Compliance Testing

Over the last week the Programmable USB Hub has gone through a successful battery of tests to ensure that it does not cause interference with other equipment and that it operates as expected when subjected to interference. This specifically means the following tests:

GroupStandardResult
Conducted EmissionsFCC Part 15.107 (a)/ICES-003, Para. 6.1 , EN 55032 Table A.8Pass Class A limits
Radiated EmissionsFCC Part 15.109 (a)/ICES-003, Para. 6.2 , EN 55032 Table A.2 , EN 55032 Table A.3Pass Class A limits
Conducted ImmunityEN 61000-4-3 , 3 Vrms, 0.15 to 80 MHz on AC and Signal LinesPass
Radiated ImmunityEN 61000-4-3 , 3 V/M, 80 MHz to 5 GHzPass. See note below about ground loops.
Electrical Fast Transient BurstEN 61000-4-4 , 0.5 kV level on USB lines , 1.0 kV level on AC linesPass. Recovered from faults.
Surges, Voltage Dips, Voltage InterruptionsEN 61000-4-5 , EN 61000-4-11Pass
Electrostatic DischargeEN 61000-4-2 , 4 kV Contact, 8 kV Air Discharges on all ports.Pass after firmware change. Recovered from faults.

Below are a few sample photos taking during the EMC testing.

ESD & RF Emissions Tests

RF Immunity Testing with low & high frequency antennas

Electrical Fast Transient & Conducted Immunity Testing

During all tests, the Hub was being exercised and monitored for faults. This included:

General Notes & Recommendations on EMC Testing

There is a wealth of information online about the proper design of electronics to meet EMC requirements, and no need to repeat that here. However, I do have the following recommendations for the act of testing itself:

Radiated Immunity & Ground Loops

During all of the tests, both the upstream USB port and the MCU USB port were connected over 2 meter cables to systems that were exercising the Hub (described above).

During the Radiated Immunity Testing (where the device under test is subjected to strong RF fields), one of the two exercising systems sometimes experienced a fault in the USB link when both the host and MCU USB cables were connected — the other exercising system had zero issues with both USB cables attached. When only the upstream USB port was connected (and the MCU port disconnected), no faults were ever observed on either exercising system.

In a discussion with the EMC Lab’s engineers, I learned they’ve seen similar issues with that particular embedded computing system in the past and do not recommend using that system as support equipment during compliance testing. It is known to be both RF-susceptible and RF-noisy when cables are attached.

During this test, the area between the two upstream USB cables created a large ground loop between the Hub and this exercising system. This loop acted as an antenna and picked up the RF energy being emitted during the test — enough RF energy to occasionally disrupt the USB data signals of one of the exercising systems.

Electrostatic Discharge Faults

The Hub was not damaged during any of the ESD testing, but the EN specification says that the product must self-recover from any faults observed during ESD testing. This means that user intervention (e.g., restart software, reboot hardware) is not allowed.

Two different lock-up conditions were occasionally observed:

The first lock-up was solved by a small change to the MCU CircuitPython firmware. When the upstream port connection is lost for more than a few seconds, the MCU resets and reconfigures the Hub IC.

The second lock-up didn’t impact data functionality but meant the MCU was sometimes locked-up and, therefore unable to reset the Hub IC if it locked-up. This was solved by enabling and configuring the watchdog functionality of the SAMD51 MCU. If the CircuitPython code stops executing, then it stops feeding the watchdog, which results in the watchdog resetting the MCU.

Enabling and configuring the watchdog required some additional functionality in the C-based CircuitPython runtime. In the coming days, this patch will be sent upstream to Adafruit for integration into the mainline CircuitPython release.

Thanks!

Thanks to each and every one of you who has helped us get here! Without your support, the Programmable USB Hub would not have become a reality, and we are so grateful!


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Programmable USB Hub is part of the Microchip Get Launched design competition!


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