Power connection question

Discuss KerberosSDR - 4x Coherent RTL-SDR
kd7mrx
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Power connection question

Post by kd7mrx » Thu May 02, 2019 9:04 pm

The quickstart guide says that the KerberosSDR requires 5V at 3A which can be provided through the USB-C port. It also says "Note that the USB-C port will not activate for high voltage "power delivery" (PD) USB-C power supplies, such as those used with laptops. It will only work on normal 5V power supplies."

Does the KerberosSDR use the USB-C CC line to determine the power capabilities of the power supply/cable?
The USB-C standard describes three possible values for the Rp pullup resistor on the CC (power sensing) line:
56k (or greater) -> 500mA max
22k -> 1.5A max
10k (or less) -> 3A max

Most of the USB-A to USB-C charge/sync cables I've seen claim to have a 56k resistor even when the manufacturer also says "up to 2A." I have seen a few cables specifically advertised as supporting "3A fast charge" but without specifying the pullup resistor used. It's not clear if the manufacturer is really doing the right thing or is just being over-optimistic in the marketing claims.

Does it matter to the KerberosSDR?

rtlsdrblog
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Re: Power connection question

Post by rtlsdrblog » Sun May 05, 2019 1:06 am

The USB-C is used in legacy USB2.0 mode only. So it will NOT activate for PD supplies that use the resistor to sense.

The USB-C port is only used for the power port because it's design offers significantly less voltage drop compared to microUSB.

Stromeko
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Re: Power connection question

Post by Stromeko » Sat May 11, 2019 5:57 pm

Whether it's used for USB2.0 legacy (so there is another hub port behind it?) or not is immaterial to the question of how power gets there and whether a standard compliant PSU recognizes the port. The high voltage PD modes can only be used after active negotiation and a PD-PSU will always be limited to supply 5V if that doesn't take place. If the port acts as a power sink for 3.0A, then both CC lines in the receptable should be terminated with 5.1kOhm to ground, otherwise the VBus of the source will not come on at all if it is standard compliant. PD supplies should recognize that as a sink port that doesn't use negotiation and provide 5V at max. 3A ("USB Type-C current"). A 3A source will advertise by injecting a nominal 330µA into the termination, resulting in a sink-side termination voltage above 1.23V.

Non-compliant PSU (i.e just having a USB-C plug attached to the output voltage) are getting quite rare. In particular I want to use a PSU that also has USB-A outputs so I can power several rasPi or TinkerBoard from those and most of them have PD outputs (which should have been tested for compliance, so require proper termination). I may have found one that doesn't switch off VBus if CC is not properly terminated, but then again the description might just not mention that.

StandingWave
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Re: Power connection question

Post by StandingWave » Sun May 12, 2019 5:38 pm

I'm also having problems with choosing the right kind of USB3-PSU as it would seem that the USB power sources I had are simply not giving enough juice for the KSDR and hence I'm having troubles getting it to work properly. I posted this question on another thread that was about the EEPROM, but I guess the question is valid in this one, too:
Also, I'd need to ask what would be a suitable USB power source, i.e. would this one from Belkin be appropriate for the KSDR? I just looked into this model, and it has received only good reviews from its users: https://www.belkin.com/us/p/P-F8M990/

At a Glance:

Universal compatibility
4 USB ports
10 ft. cable
5.4A power
Automatic power adjust for connected devices
Wall-mountable
Elegant design

The bottom line being the highlighted row - is having a USB power source with automatic power adjust such as in the example model above bad idea with the KSDR? Belkin does make otherwise some decent stuff ...
I'd like to hear your take on what constitutes as a decent USB PSU for the KSDR - I'm in northern EU, we're running on 230VAC with an European "Schuko" type AC plugs&sockets.

rtlsdrblog
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Re: Power connection question

Post by rtlsdrblog » Sun May 12, 2019 11:38 pm

If it's not a PD supply it should work fine with the KSDR. The KSDR only really needs 2A, but if you're using single board computer like a Pi3 or Tinkerboard with it, then the singleboard computer absolutely must be powered with a 3A+ supply.

Stromeko
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Re: Power connection question

Post by Stromeko » Wed May 15, 2019 6:55 pm

It would really be helpful if you could state whether or not the pulldown resistors on VCONN are present and what values they have. Again, this has nothing at all to do with whether or not the PSU can do PD, but the fact you're using a USB-C receptable. The pulldown resistors are required to figure out that the port is a power sink and how much power it is going to use. There is no "legacy USB2.0 mode" on these ports, but a cable that connects to a USB-A plug would require pullup resistors on the USB-A side in order to power the pulldown resistors on the USB-C side correctly. These are the 56k resistors that are often mentioned for charger cables. With such a cable you could also target USB-BC (battery charger), but that's resistors in other places and only good for 1.5A officially.

rtlsdrblog
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Re: Power connection question

Post by rtlsdrblog » Thu May 16, 2019 2:54 am

This is what the electronics hardware designer of KerberosSDR said:
We are not using the USB-C CC lines pull up resistors. Kerberos using the USB-C connector in simply USB 2.0 legacy mode, waiting for a USB-C to USB-A cable, using classic 5V chargers. So there is no pullup resitor on the CC line. And I think need to emphasize that which type of cable is needed (again USB-C to USB-A male)

Might this confusing to the users, but the only reason we are using USB-C socket is that's introduces less series resistance on the GND and VDD lines, thus less voltage drop and losses compared to the terrible micro USB.

Stromeko
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Re: Power connection question

Post by Stromeko » Wed May 22, 2019 7:35 pm

OK, that reply confirms my suspicion. Please have them read the USB spec to come up with a spec compliant way to do a power sink with a USB-C connector. Again, you need pull-down resistors there, not pull-up.

So in summary the only option of powering through this connector is to try and get a non-compliant PSU that isn't probing VCONN and always powers VBUS without current limit, which excludes almost anything you can buy in the EU. The best bet are the PSU that have a captive cable ending in a USB-C, especially if that same model is available with other plugs or even with a set of adapters. It looks like all of these I can find are direct shipping from China.

rtlsdrblog
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Re: Power connection question

Post by rtlsdrblog » Wed May 22, 2019 10:30 pm

Stromeko wrote:
Wed May 22, 2019 7:35 pm
OK, that reply confirms my suspicion. Please have them read the USB spec to come up with a spec compliant way to do a power sink with a USB-C connector. Again, you need pull-down resistors there, not pull-up.

So in summary the only option of powering through this connector is to try and get a non-compliant PSU that isn't probing VCONN and always powers VBUS without current limit, which excludes almost anything you can buy in the EU. The best bet are the PSU that have a captive cable ending in a USB-C, especially if that same model is available with other plugs or even with a set of adapters. It looks like all of these I can find are direct shipping from China.
They don't sell standard 5V, 3A supplies with USB-A out in the EU? The idea is to simply use a USB-A to USB-C cable with a standard USB-A out supply. If a microUSB or miniUSB port was used on the Kerberos would you have the same problem? You can also use a microUSB to USB-C adapter in a pinch.

The electronics designer from Othernet is based in the EU, and he didn't seem to have have any issues finding supplies that work during the prototyping stages.

In the next batch we'll implement the resistors so that PD supplies can be used. It wasn't done in the first batch because apparently we need to go to a higher level of PCB manufacturing technology in order to create the micro vias that USB-C needs to get the traces out to the resistors.

Stromeko
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Re: Power connection question

Post by Stromeko » Fri May 24, 2019 6:36 pm

USB-A doesn't (officially) allow for 3A, so most PSU only supply a maximum of 2.4A per port (I have a bunch of those). The obvious non-USB option would have been a barrel plug, but I recognize that these may be more difficult to fit onto the board unless you're going to solder them separately.

Again, finding a supply that just puts out 5V/3A isn't a problem, and a passive adapter to USB-C should then just work. Beware of active adaptors, though; those have switches integrated into the plug that would again remove VBUS if the USB-C end is not properly terminated. I was specifically hoping to use a supply that has a USB-C 3A and three or four USB-A 2.4A outlets, but since that is unlikely to work with the Kerberos I'll have to switch to using multiple PSU for now.

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