Teensy 4.1 Suggestions?

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donperryjm

Well-known member
I'm a big fan of the teensy and I've used them in a lot of low volume products I've made.

I'm now looking on a challenge I've always had with the teensies and that's the back-feed diode protection.
For those unfamiliar with this, a diode is required on the teensy board to prevent backfeed into USB port.
The PCB trace circled in the photo is also required to be cut for this to work. This will make the device fully USB compliant.

The arduino boards already have such a diode. I'm wondering if there is a reason the teensy doesn't come with this diode from the factory?
And if possible for future teensy to have this diode.


diode.png
 
Yes T_4.1 batch one built and sold out and restocked already.

When is backfeed an issue? Only when powered on the VIN pin?

When power from USB not needed cutting the trace prevents that - but then USB only works with external power - or a jumper to the internal edge VUSB pin, or on the fly with an added diode, or two.
 
I use two diodes as suggested by Paul in one of the guides. I used an TH diode last time, but will try the equivalent SMD version for a “cleaner” look
 
defragster said:
Yes T_4.1 batch one built and sold out and restocked already.

When is backfeed an issue? Only when powered on the VIN pin?

When power from USB not needed cutting the trace prevents that - but then USB only works with external power - or a jumper to the internal edge VUSB pin, or on the fly with an added diode, or two.
Click to expand...

Yes, when powered with the VIN pin and then you have to do some serial communication with it powered up on the main system. Think automotive, machinery.
My question is why not put a diode there and get rid of the trace and potentially back-feed issue and also be fully or more USB compliant? I know cost wouldn't be an issue, and i see a number of smd components on the underside already. The pad is so small and close to the pin that's it's not so easy to hot air solder.
 
donperryjm said:
My question is why not put a diode there
Click to expand...

One reason would be the 0.5 to 0.7 volt loss due to the diode's forward voltage drop. Many people connect other boards needing 5V power, even if they use 3.3V signals. On Teensy 4.1 we also use the 5V power for USB host, to try to power up other USB devices.

Unfortunately there isn't any perfect answer which fits everyone's needs.
 
Yes, but there are also schottky that allows only a 300mv drop, that leaves with 4.7v. Usually sufficient for most 5V circuit needs.
even so, they can do external power - eventually most will need to do, and then they'll have no backfeed issue.

If you run a poll to see how many have a need for the requirement you just mentioned vs backfeed prevention, I have a hunch more will have the back-feed prevention issue.
 
"Yes, but there are also schottky that allows only a 300mv drop, that leaves with 4.7v. Usually sufficient for most 5V circuit needs"

If your reasoning is valid the next device will also have a diode - that leaves it with 4.4v supply to the third device not 4.7v
How do you decide which one to fit with a diode and which can do without it?
 
donperryjm said:
Yes, but there are also schottky that allows only a 300mv drop, that leaves with 4.7v. Usually sufficient for most 5V circuit needs.
even so, they can do external power - eventually most will need to do, and then they'll have no backfeed issue.

If you run a poll to see how many have a need for the requirement you just mentioned vs backfeed prevention, I have a hunch more will have the back-feed prevention issue.
Click to expand...

Not sure I agree that more will want it. There are all sorts of side effects of a voltage drop. I ran into an issue where someone was using PWM to control the speed of a spindle with a teensy 4.1. The spindle was running slower than expected. Turns out their USB power was low (4.4V - spec say 4.75 is min) and the spindle controller was just doing a simple LPF to convert PWM to voltage and expecting 5V. Even with correct USB voltage level a schottky drop would be an issue. I set up my boards to allow switching to external 5V when it's important.
 
Saibot said:
"Yes, but there are also schottky that allows only a 300mv drop, that leaves with 4.7v. Usually sufficient for most 5V circuit needs"

If your reasoning is valid the next device will also have a diode - that leaves it with 4.4v supply to the third device not 4.7v
How do you decide which one to fit with a diode and which can do without it?
Click to expand...

Simply A VIN pin separate from the 5V pin like most other dev boards provide. So if you want a naked 5V you have that pin straight from USB, but to power the teensy through its schottky you also have that VIN input pin that won't backfeed to USB.
Here is an example with the Nano. https://i.postimg.cc/Xq96LJP2/nano.png
 
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